Rewrite the GitHub importer from scratch

Prior to this MR there were two GitHub related importers:

* Github::Import: the main importer used for GitHub projects
* Gitlab::GithubImport: importer that's somewhat confusingly used for
  importing Gitea projects (apparently they have a compatible API)

This MR renames the Gitea importer to Gitlab::LegacyGithubImport and
introduces a new GitHub importer in the Gitlab::GithubImport namespace.
This new GitHub importer uses Sidekiq for importing multiple resources
in parallel, though it also has the ability to import data sequentially
should this be necessary.

The new code is spread across the following directories:

* lib/gitlab/github_import: this directory contains most of the importer
  code such as the classes used for importing resources.
* app/workers/gitlab/github_import: this directory contains the Sidekiq
  workers, most of which simply use the code from the directory above.
* app/workers/concerns/gitlab/github_import: this directory provides a
  few modules that are included in every GitHub importer worker.

== Stages

The import work is divided into separate stages, with each stage
importing a specific set of data. Stages will schedule the work that
needs to be performed, followed by scheduling a job for the
"AdvanceStageWorker" worker. This worker will periodically check if all
work is completed and schedule the next stage if this is the case. If
work is not yet completed this worker will reschedule itself.

Using this approach we don't have to block threads by calling `sleep()`,
as doing so for large projects could block the thread from doing any
work for many hours.

== Retrying Work

Workers will reschedule themselves whenever necessary. For example,
hitting the GitHub API's rate limit will result in jobs rescheduling
themselves. These jobs are not processed until the rate limit has been
reset.

== User Lookups

Part of the importing process involves looking up user details in the
GitHub API so we can map them to GitLab users. The old importer used
an in-memory cache, but this obviously doesn't work when the work is
spread across different threads.

The new importer uses a Redis cache and makes sure we only perform
API/database calls if absolutely necessary.  Frequently used keys are
refreshed, and lookup misses are also cached; removing the need for
performing API/database calls if we know we don't have the data we're
looking for.

== Performance & Models

The new importer in various places uses raw INSERT statements (as
generated by `Gitlab::Database.bulk_insert`) instead of using Rails
models. This allows us to bypass any validations and callbacks,
drastically reducing the number of SQL queries and Gitaly RPC calls
necessary to import projects.

To ensure the code produces valid data the corresponding tests check if
the produced rows are valid according to the model validation rules.

app/controllers/import/github_controller.rb

@@ -43,7 +43,7 @@ class Import::GithubController < Import::BaseController
     @target_namespace = find_or_create_namespace(namespace_path, current_user.namespace_path)
 
     if can?(current_user, :create_projects, @target_namespace)
-      @project = Gitlab::GithubImport::ProjectCreator.new(repo, @project_name, @target_namespace, current_user, access_params, type: provider).execute
+      @project = Gitlab::LegacyGithubImport::ProjectCreator.new(repo, @project_name, @target_namespace, current_user, access_params, type: provider).execute
     else
       render 'unauthorized'
     end
@@ -52,7 +52,7 @@ class Import::GithubController < Import::BaseController
   private
 
   def client
-    @client ||= Gitlab::GithubImport::Client.new(session[access_token_key], client_options)
+    @client ||= Gitlab::LegacyGithubImport::Client.new(session[access_token_key], client_options)
   end
 
   def verify_import_enabled

app/models/project.rb

@@ -365,6 +365,7 @@ class Project < ActiveRecord::Base
   scope :abandoned, -> { where('projects.last_activity_at < ?', 6.months.ago) }
 
   scope :excluding_project, ->(project) { where.not(id: project) }
+  scope :import_started, -> { where(import_status: 'started') }
 
   state_machine :import_status, initial: :none do
     event :import_schedule do
@@ -1190,6 +1191,10 @@ class Project < ActiveRecord::Base
     !!repository.exists?
   end
 
+  def wiki_repository_exists?
+    wiki.repository_exists?
+  end
+
   # update visibility_level of forks
   def update_forks_visibility_level
     return unless visibility_level < visibility_level_was
@@ -1433,6 +1438,31 @@ class Project < ActiveRecord::Base
     reload_repository!
   end
 
+  def after_import
+    repository.after_import
+    import_finish
+    remove_import_jid
+    update_project_counter_caches
+  end
+
+  def update_project_counter_caches
+    classes = [
+      Projects::OpenIssuesCountService,
+      Projects::OpenMergeRequestsCountService
+    ]
+
+    classes.each do |klass|
+      klass.new(self).refresh_cache
+    end
+  end
+
+  def remove_import_jid
+    return unless import_jid
+
+    Gitlab::SidekiqStatus.unset(import_jid)
+    update_column(:import_jid, nil)
+  end
+
   def running_or_pending_build_count(force: false)
     Rails.cache.fetch(['projects', id, 'running_or_pending_build_count'], force: force) do
       builds.running_or_pending.count(:all)
@@ -1690,6 +1720,17 @@ class Project < ActiveRecord::Base
     Gitlab::ReferenceCounter.new(gl_repository(is_wiki: wiki))
   end
 
+  # Refreshes the expiration time of the associated import job ID.
+  #
+  # This method can be used by asynchronous importers to refresh the status,
+  # preventing the StuckImportJobsWorker from marking the import as failed.
+  def refresh_import_jid_expiration
+    return unless import_jid
+
+    Gitlab::SidekiqStatus
+      .set(import_jid, StuckImportJobsWorker::IMPORT_JOBS_EXPIRATION)
+  end
+
   private
 
   def storage

app/models/repository.rb

@@ -973,6 +973,10 @@ class Repository
     raw_repository.fetch_source_branch!(source_repository.raw_repository, source_branch, local_ref)
   end
 
+  def remote_exists?(name)
+    raw_repository.remote_exists?(name)
+  end
+
   def compare_source_branch(target_branch_name, source_repository, source_branch_name, straight:)
     raw_repository.compare_source_branch(target_branch_name, source_repository.raw_repository, source_branch_name, straight: straight)
   end

app/models/user.rb

@@ -267,6 +267,11 @@ class User < ActiveRecord::Base
       end
     end
 
+    def for_github_id(id)
+      joins(:identities)
+        .where(identities: { provider: :github, extern_uid: id.to_s })
+    end
+
     # Find a User by their primary email or any associated secondary email
     def find_by_any_email(email)
       by_any_email(email).take

app/services/projects/import_service.rb

@@ -4,6 +4,18 @@ module Projects
 
     Error = Class.new(StandardError)
 
+    # Returns true if this importer is supposed to perform its work in the
+    # background.
+    #
+    # This method will only return `true` if async importing is explicitly
+    # supported by an importer class (`Gitlab::GithubImport::ParallelImporter`
+    # for example).
+    def async?
+      return false unless has_importer?
+
+      !!importer_class.try(:async?)
+    end
+
     def execute
       add_repository_to_project unless project.gitlab_project_import?
 
@@ -80,7 +92,11 @@ module Projects
     end
 
     def importer
-      Gitlab::ImportSources.importer(project.import_type).new(project)
+      importer_class.new(project)
+    end
+
+    def importer_class
+      Gitlab::ImportSources.importer(project.import_type)
     end
 
     def unknown_url?

app/workers/concerns/gitlab/github_import/notify_upon_death.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # NotifyUponDeath can be included into a GitHub worker class if it should
+    # notify any JobWaiter instances upon being moved to the Sidekiq dead queue.
+    #
+    # Note that this will only notify the waiter upon graceful termination, a
+    # SIGKILL will still result in the waiter _not_ being notified.
+    #
+    # Workers including this module must have jobs passed where the last
+    # argument is the key to notify, as a String.
+    module NotifyUponDeath
+      extend ActiveSupport::Concern
+
+      included do
+        # If a job is being exhausted we still want to notify the
+        # AdvanceStageWorker. This prevents the entire import from getting stuck
+        # just because 1 job threw too many errors.
+        sidekiq_retries_exhausted do |job|
+          args = job['args']
+          jid = job['jid']
+
+          if args.length == 3 && (key = args.last) && key.is_a?(String)
+            JobWaiter.notify(key, jid)
+          end
+        end
+      end
+    end
+  end
+end

app/workers/concerns/gitlab/github_import/object_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # ObjectImporter defines the base behaviour for every Sidekiq worker that
+    # imports a single resource such as a note or pull request.
+    module ObjectImporter
+      extend ActiveSupport::Concern
+
+      included do
+        include Sidekiq::Worker
+        include GithubImport::Queue
+        include ReschedulingMethods
+        include NotifyUponDeath
+      end
+
+      # project - An instance of `Project` to import the data into.
+      # client - An instance of `Gitlab::GithubImport::Client`
+      # hash - A Hash containing the details of the object to import.
+      def import(project, client, hash)
+        object = representation_class.from_json_hash(hash)
+
+        importer_class.new(object, project, client).execute
+
+        counter.increment(project: project.path_with_namespace)
+      end
+
+      def counter
+        @counter ||= Gitlab::Metrics.counter(counter_name, counter_description)
+      end
+
+      # Returns the representation class to use for the object. This class must
+      # define the class method `from_json_hash`.
+      def representation_class
+        raise NotImplementedError
+      end
+
+      # Returns the class to use for importing the object.
+      def importer_class
+        raise NotImplementedError
+      end
+
+      # Returns the name (as a Symbol) of the Prometheus counter.
+      def counter_name
+        raise NotImplementedError
+      end
+
+      # Returns the description (as a String) of the Prometheus counter.
+      def counter_description
+        raise NotImplementedError
+      end
+    end
+  end
+end

app/workers/concerns/gitlab/github_import/queue.rb

+module Gitlab
+  module GithubImport
+    module Queue
+      extend ActiveSupport::Concern
+
+      included do
+        # If a job produces an error it may block a stage from advancing
+        # forever. To prevent this from happening we prevent jobs from going to
+        # the dead queue. This does mean some resources may not be imported, but
+        # this is better than a project being stuck in the "import" state
+        # forever.
+        sidekiq_options queue: 'github_importer', dead: false, retry: 5
+      end
+    end
+  end
+end

app/workers/concerns/gitlab/github_import/rescheduling_methods.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # Module that provides methods shared by the various workers used for
+    # importing GitHub projects.
+    module ReschedulingMethods
+      # project_id - The ID of the GitLab project to import the note into.
+      # hash - A Hash containing the details of the GitHub object to imoprt.
+      # notify_key - The Redis key to notify upon completion, if any.
+      def perform(project_id, hash, notify_key = nil)
+        project = Project.find_by(id: project_id)
+
+        return notify_waiter(notify_key) unless project
+
+        client = GithubImport.new_client_for(project, parallel: true)
+
+        if try_import(project, client, hash)
+          notify_waiter(notify_key)
+        else
+          # In the event of hitting the rate limit we want to reschedule the job
+          # so its retried after our rate limit has been reset.
+          self.class
+            .perform_in(client.rate_limit_resets_in, project.id, hash, notify_key)
+        end
+      end
+
+      def try_import(*args)
+        import(*args)
+        true
+      rescue RateLimitError
+        false
+      end
+
+      def notify_waiter(key = nil)
+        JobWaiter.notify(key, jid) if key
+      end
+    end
+  end
+end

app/workers/concerns/gitlab/github_import/stage_methods.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module StageMethods
+      # project_id - The ID of the GitLab project to import the data into.
+      def perform(project_id)
+        return unless (project = find_project(project_id))
+
+        client = GithubImport.new_client_for(project)
+
+        try_import(client, project)
+      end
+
+      # client - An instance of Gitlab::GithubImport::Client.
+      # project - An instance of Project.
+      def try_import(client, project)
+        import(client, project)
+      rescue RateLimitError
+        self.class.perform_in(client.rate_limit_resets_in, project.id)
+      end
+
+      def find_project(id)
+        # If the project has been marked as failed we want to bail out
+        # automatically.
+        Project.import_started.find_by(id: id)
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/advance_stage_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # AdvanceStageWorker is a worker used by the GitHub importer to wait for a
+    # number of jobs to complete, without blocking a thread. Once all jobs have
+    # been completed this worker will advance the import process to the next
+    # stage.
+    class AdvanceStageWorker
+      include Sidekiq::Worker
+
+      sidekiq_options queue: 'github_importer_advance_stage', dead: false
+
+      INTERVAL = 30.seconds.to_i
+
+      # The number of seconds to wait (while blocking the thread) before
+      # continueing to the next waiter.
+      BLOCKING_WAIT_TIME = 5
+
+      # The known importer stages and their corresponding Sidekiq workers.
+      STAGES = {
+        issues_and_diff_notes: Stage::ImportIssuesAndDiffNotesWorker,
+        notes: Stage::ImportNotesWorker,
+        finish: Stage::FinishImportWorker
+      }.freeze
+
+      # project_id - The ID of the project being imported.
+      # waiters - A Hash mapping Gitlab::JobWaiter keys to the number of
+      #           remaining jobs.
+      # next_stage - The name of the next stage to start when all jobs have been
+      #              completed.
+      def perform(project_id, waiters, next_stage)
+        return unless (project = find_project(project_id))
+
+        new_waiters = wait_for_jobs(waiters)
+
+        if new_waiters.empty?
+          # We refresh the import JID here so workers importing individual
+          # resources (e.g. notes) don't have to do this all the time, reducing
+          # the pressure on Redis. We _only_ do this once all jobs are done so
+          # we don't get stuck forever if one or more jobs failed to notify the
+          # JobWaiter.
+          project.refresh_import_jid_expiration
+
+          STAGES.fetch(next_stage.to_sym).perform_async(project_id)
+        else
+          self.class.perform_in(INTERVAL, project_id, new_waiters, next_stage)
+        end
+      end
+
+      def wait_for_jobs(waiters)
+        waiters.each_with_object({}) do |(key, remaining), new_waiters|
+          waiter = JobWaiter.new(remaining, key)
+
+          # We wait for a brief moment of time so we don't reschedule if we can
+          # complete the work fast enough.
+          waiter.wait(BLOCKING_WAIT_TIME)
+
+          next unless waiter.jobs_remaining.positive?
+
+          new_waiters[waiter.key] = waiter.jobs_remaining
+        end
+      end
+
+      def find_project(id)
+        # We only care about the import JID so we can refresh it. We also only
+        # want the project if it hasn't been marked as failed yet. It's possible
+        # the import gets marked as stuck when jobs of the current stage failed
+        # somehow.
+        Project.select(:import_jid).import_started.find_by(id: id)
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/import_diff_note_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    class ImportDiffNoteWorker
+      include ObjectImporter
+
+      def representation_class
+        Representation::DiffNote
+      end
+
+      def importer_class
+        Importer::DiffNoteImporter
+      end
+
+      def counter_name
+        :github_importer_imported_diff_notes
+      end
+
+      def counter_description
+        'The number of imported GitHub pull request review comments'
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/import_issue_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    class ImportIssueWorker
+      include ObjectImporter
+
+      def representation_class
+        Representation::Issue
+      end
+
+      def importer_class
+        Importer::IssueAndLabelLinksImporter
+      end
+
+      def counter_name
+        :github_importer_imported_issues
+      end
+
+      def counter_description
+        'The number of imported GitHub issues'
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/import_note_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    class ImportNoteWorker
+      include ObjectImporter
+
+      def representation_class
+        Representation::Note
+      end
+
+      def importer_class
+        Importer::NoteImporter
+      end
+
+      def counter_name
+        :github_importer_imported_notes
+      end
+
+      def counter_description
+        'The number of imported GitHub comments'
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/import_pull_request_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    class ImportPullRequestWorker
+      include ObjectImporter
+
+      def representation_class
+        Representation::PullRequest
+      end
+
+      def importer_class
+        Importer::PullRequestImporter
+      end
+
+      def counter_name
+        :github_importer_imported_pull_requests
+      end
+
+      def counter_description
+        'The number of imported GitHub pull requests'
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/refresh_import_jid_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    class RefreshImportJidWorker
+      include Sidekiq::Worker
+      include GithubImport::Queue
+
+      # The interval to schedule new instances of this job at.
+      INTERVAL = 1.minute.to_i
+
+      def self.perform_in_the_future(*args)
+        perform_in(INTERVAL, *args)
+      end
+
+      # project_id - The ID of the project that is being imported.
+      # check_job_id - The ID of the job for which to check the status.
+      def perform(project_id, check_job_id)
+        return unless (project = find_project(project_id))
+
+        if SidekiqStatus.running?(check_job_id)
+          # As long as the repository is being cloned we want to keep refreshing
+          # the import JID status.
+          project.refresh_import_jid_expiration
+          self.class.perform_in_the_future(project_id, check_job_id)
+        end
+
+        # If the job is no longer running there's nothing else we need to do. If
+        # the clone job completed successfully it will have scheduled the next
+        # stage, if it died there's nothing we can do anyway.
+      end
+
+      def find_project(id)
+        Project.select(:import_jid).import_started.find_by(id: id)
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/stage/finish_import_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Stage
+      class FinishImportWorker
+        include Sidekiq::Worker
+        include GithubImport::Queue
+        include StageMethods
+
+        # project - An instance of Project.
+        def import(_, project)
+          project.after_import
+          report_import_time(project)
+        end
+
+        def report_import_time(project)
+          duration = Time.zone.now - project.created_at
+          path = project.path_with_namespace
+
+          histogram.observe({ project: path }, duration)
+          counter.increment
+
+          logger.info("GitHub importer finished for #{path} in #{duration.round(2)} seconds")
+        end
+
+        def histogram
+          @histogram ||= Gitlab::Metrics.histogram(
+            :github_importer_total_duration_seconds,
+            'Total time spent importing GitHub projects, in seconds'
+          )
+        end
+
+        def counter
+          @counter ||= Gitlab::Metrics.counter(
+            :github_importer_imported_projects,
+            'The number of imported GitHub projects'
+          )
+        end
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/stage/import_base_data_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Stage
+      class ImportBaseDataWorker
+        include Sidekiq::Worker
+        include GithubImport::Queue
+        include StageMethods
+
+        # These importers are fast enough that we can just run them in the same
+        # thread.
+        IMPORTERS = [
+          Importer::LabelsImporter,
+          Importer::MilestonesImporter,
+          Importer::ReleasesImporter
+        ].freeze
+
+        # client - An instance of Gitlab::GithubImport::Client.
+        # project - An instance of Project.
+        def import(client, project)
+          IMPORTERS.each do |klass|
+            klass.new(project, client).execute
+          end
+
+          project.refresh_import_jid_expiration
+
+          ImportPullRequestsWorker.perform_async(project.id)
+        end
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/stage/import_issues_and_diff_notes_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Stage
+      class ImportIssuesAndDiffNotesWorker
+        include Sidekiq::Worker
+        include GithubImport::Queue
+        include StageMethods
+
+        # The importers to run in this stage. Issues can't be imported earlier
+        # on as we also use these to enrich pull requests with assigned labels.
+        IMPORTERS = [
+          Importer::IssuesImporter,
+          Importer::DiffNotesImporter
+        ].freeze
+
+        # client - An instance of Gitlab::GithubImport::Client.
+        # project - An instance of Project.
+        def import(client, project)
+          waiters = IMPORTERS.each_with_object({}) do |klass, hash|
+            waiter = klass.new(project, client).execute
+            hash[waiter.key] = waiter.jobs_remaining
+          end
+
+          AdvanceStageWorker.perform_async(project.id, waiters, :notes)
+        end
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/stage/import_notes_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Stage
+      class ImportNotesWorker
+        include Sidekiq::Worker
+        include GithubImport::Queue
+        include StageMethods
+
+        # client - An instance of Gitlab::GithubImport::Client.
+        # project - An instance of Project.
+        def import(client, project)
+          waiter = Importer::NotesImporter
+            .new(project, client)
+            .execute
+
+          AdvanceStageWorker.perform_async(
+            project.id,
+            { waiter.key => waiter.jobs_remaining },
+            :finish
+          )
+        end
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/stage/import_pull_requests_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Stage
+      class ImportPullRequestsWorker
+        include Sidekiq::Worker
+        include GithubImport::Queue
+        include StageMethods
+
+        # client - An instance of Gitlab::GithubImport::Client.
+        # project - An instance of Project.
+        def import(client, project)
+          waiter = Importer::PullRequestsImporter
+            .new(project, client)
+            .execute
+
+          project.refresh_import_jid_expiration
+
+          AdvanceStageWorker.perform_async(
+            project.id,
+            { waiter.key => waiter.jobs_remaining },
+            :issues_and_diff_notes
+          )
+        end
+      end
+    end
+  end
+end

app/workers/gitlab/github_import/stage/import_repository_worker.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Stage
+      class ImportRepositoryWorker
+        include Sidekiq::Worker
+        include GithubImport::Queue
+        include StageMethods
+
+        # client - An instance of Gitlab::GithubImport::Client.
+        # project - An instance of Project.
+        def import(client, project)
+          # In extreme cases it's possible for a clone to take more than the
+          # import job expiration time. To work around this we schedule a
+          # separate job that will periodically run and refresh the import
+          # expiration time.
+          RefreshImportJidWorker.perform_in_the_future(project.id, jid)
+
+          importer = Importer::RepositoryImporter.new(project, client)
+
+          return unless importer.execute
+
+          counter.increment
+
+          ImportBaseDataWorker.perform_async(project.id)
+        end
+
+        def counter
+          Gitlab::Metrics.counter(
+            :github_importer_imported_repositories,
+            'The number of imported GitHub repositories'
+          )
+        end
+      end
+    end
+  end
+end

app/workers/repository_import_worker.rb

@@ -17,11 +17,16 @@ class RepositoryImportWorker
                               import_url: project.import_url,
                               path: project.full_path)
 
-    result = Projects::ImportService.new(project, project.creator).execute
+    service = Projects::ImportService.new(project, project.creator)
+    result = service.execute
+
+    # Some importers may perform their work asynchronously. In this case it's up
+    # to those importers to mark the import process as complete.
+    return if service.async?
+
     raise ImportError, result[:message] if result[:status] == :error
 
-    project.repository.after_import
-    project.import_finish
+    project.after_import
   rescue ImportError => ex
     fail_import(project, ex.message)
     raise

config/sidekiq_queues.yml

@@ -40,6 +40,8 @@
   - [upload_checksum, 1]
   - [repository_fork, 1]
   - [repository_import, 1]
+  - [github_importer, 1]
+  - [github_importer_advance_stage, 1]
   - [project_service, 1]
   - [delete_user, 1]
   - [delete_merged_branches, 1]

doc/README.md

@@ -171,6 +171,7 @@ have access to GitLab administration tools and settings.
 - [GitLab performance monitoring with InfluxDB](administration/monitoring/performance/introduction.md): Configure GitLab and InfluxDB for measuring performance metrics.
 - [GitLab performance monitoring with Prometheus](administration/monitoring/prometheus/index.md): Configure GitLab and Prometheus for measuring performance metrics.
 - [Monitoring uptime](user/admin_area/monitoring/health_check.md): Check the server status using the health check endpoint.
+- [Monitoring GitHub imports](administration/monitoring/github_imports.md)
 
 ### Performance
 

doc/administration/monitoring/github_imports.md

+# Monitoring GitHub imports
+
+>**Note:**
+Available since [GitLab 10.2][14731].
+
+The GitHub importer exposes various Prometheus metrics that you can use to
+monitor the health and progress of the importer.
+
+## Import Duration Times
+
+| Name                                     | Type      |
+|------------------------------------------|-----------|
+| `github_importer_total_duration_seconds` | histogram |
+
+This metric tracks the total time spent (in seconds) importing a project (from
+project creation until the import process finishes), for every imported project.
+
+The name of the project is stored in the `project` label in the format
+`namespace/name` (e.g. `gitlab-org/gitlab-ce`).
+
+## Number of imported projects
+
+| Name                                | Type    |
+|-------------------------------------|---------|
+| `github_importer_imported_projects` | counter |
+
+This metric tracks the total number of projects imported over time. This metric
+does not expose any labels.
+
+## Number of GitHub API calls
+
+| Name                            | Type    |
+|---------------------------------|---------|
+| `github_importer_request_count` | counter |
+
+This metric tracks the total number of GitHub API calls performed over time, for
+all projects. This metric does not expose any labels.
+
+## Rate limit errors
+
+| Name                              | Type    |
+|-----------------------------------|---------|
+| `github_importer_rate_limit_hits` | counter |
+
+This metric tracks the number of times we hit the GitHub rate limit, for all
+projects. This metric does not expose any labels.
+
+## Number of imported issues
+
+| Name                              | Type    |
+|-----------------------------------|---------|
+| `github_importer_imported_issues` | counter |
+
+This metric tracks the number of imported issues across all projects.
+
+The name of the project is stored in the `project` label in the format
+`namespace/name` (e.g. `gitlab-org/gitlab-ce`).
+
+## Number of imported pull requests
+
+| Name                                     | Type    |
+|------------------------------------------|---------|
+| `github_importer_imported_pull_requests` | counter |
+
+This metric tracks the number of imported pull requests across all projects.
+
+The name of the project is stored in the `project` label in the format
+`namespace/name` (e.g. `gitlab-org/gitlab-ce`).
+
+## Number of imported comments
+
+| Name                             | Type    |
+|----------------------------------|---------|
+| `github_importer_imported_notes` | counter |
+
+This metric tracks the number of imported comments across all projects.
+
+The name of the project is stored in the `project` label in the format
+`namespace/name` (e.g. `gitlab-org/gitlab-ce`).
+
+## Number of imported pull request review comments
+
+| Name                                  | Type    |
+|---------------------------------------|---------|
+| `github_importer_imported_diff_notes` | counter |
+
+This metric tracks the number of imported comments across all projects.
+
+The name of the project is stored in the `project` label in the format
+`namespace/name` (e.g. `gitlab-org/gitlab-ce`).
+
+## Number of imported repositories
+
+| Name                                    | Type    |
+|-----------------------------------------|---------|
+| `github_importer_imported_repositories` | counter |
+
+This metric tracks the number of imported repositories across all projects. This
+metric does not expose any labels.
+
+[14731]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14731

doc/development/README.md

@@ -37,6 +37,7 @@ comments: false
 - [Gotchas](gotchas.md) to avoid
 - [Issue and merge requests state models](object_state_models.md)
 - [How to dump production data to staging](db_dump.md)
+- [Working with the GitHub importer](github_importer.md)
 
 ## Performance guides
 

doc/development/github_importer.md

+# Working with the GitHub importer
+
+In GitLab 10.2 a new version of the GitHub importer was introduced. This new
+importer performs its work in parallel using Sidekiq, greatly reducing the time
+necessary to import GitHub projects into a GitLab instance.
+
+The GitHub importer offers two different types of importers: a sequential
+importer and a parallel importer. The Rake task `import:github` uses the
+sequential importer, while everything else uses the parallel importer. The
+difference between these two importers is quite simple: the sequential importer
+does all work in a single thread, making it more useful for debugging purposes
+or Rake tasks. The parallel importer on the other hand uses Sidekiq.
+
+## Requirements
+
+* GitLab CE 10.2.0 or newer.
+* Sidekiq workers that process the `github_importer` and
+  `github_importer_advance_stage` queues (this is enabled by default).
+* Octokit (used for interacting with the GitHub API)
+
+## Code structure
+
+The importer's codebase is broken up into the following directories:
+
+* `lib/gitlab/github_import`: this directory contains most of the code such as
+  the classes used for importing resources.
+* `app/workers/gitlab/github_import`: this directory contains the Sidekiq
+  workers.
+* `app/workers/concerns/gitlab/github_import`: this directory contains a few
+  modules reused by the various Sidekiq workers.
+
+## Architecture overview
+
+When a GitHub project is imported we schedule and execute a job for the
+`RepositoryImportworker` worker as all other importers. However, unlike other
+importers we don't immediately perform the work necessary. Instead work is
+divided into separate stages, with each stage consisting out of a set of Sidekiq
+jobs that are executed. Between every stage a job is scheduled that periodically
+checks if all work of the current stage is completed, advancing the import
+process to the next stage when this is the case. The worker handling this is
+called `Gitlab::GithubImport::AdvanceStageWorker`.
+
+## Stages
+
+### 1. RepositoryImportWorker
+
+This worker will kick off the import process by simply scheduling a job for the
+next worker.
+
+### 2. Stage::ImportRepositoryWorker
+
+This worker will import the repository and wiki, scheduling the next stage when
+done.
+
+### 3. Stage::ImportBaseDataWorker
+
+This worker will import base data such as labels, milestones, and releases. This
+work is done in a single thread since it can be performed fast enough that we
+don't need to perform this work in parallel.
+
+### 4. Stage::ImportPullRequestsWorker
+
+This worker will import all pull requests. For every pull request a job for the
+`Gitlab::GithubImport::ImportPullRequestWorker` worker is scheduled.
+
+### 5. Stage::ImportIssuesAndDiffNotesWorker
+
+This worker will import all issues and pull request comments. For every issue we
+schedule a job for the `Gitlab::GithubImport::ImportIssueWorker` worker. For
+pull request comments we instead schedule jobs for the
+`Gitlab::GithubImport::DiffNoteImporter` worker.
+
+This worker processes both issues and diff notes in parallel so we don't need to
+schedule a separate stage and wait for the previous one to complete.
+
+Issues are imported separately from pull requests because only the "issues" API
+includes labels for both issue and pull requests. Importing issues and setting
+label links in the same worker removes the need for performing a separate crawl
+through the API data, reducing the number of API calls necessary to import a
+project.
+
+### 6. Stage::ImportNotesWorker
+
+This worker imports regular comments for both issues and pull requests. For
+every comment we schedule a job for the
+`Gitlab::GithubImport::ImportNoteWorker` worker.
+
+Regular comments have to be imported at the end since the GitHub API used
+returns comments for both issues and pull requests. This means we have to wait
+for all issues and pull requests to be imported before we can import regular
+comments.
+
+### 7. Stage::FinishImportWorker
+
+This worker will wrap up the import process by performing some housekeeping
+(such as flushing any caches) and by marking the import as completed.
+
+## Advancing stages
+
+Advancing stages is done in one of two ways:
+
+1. Scheduling the worker for the next stage directly.
+2. Scheduling a job for `Gitlab::GithubImport::AdvanceStageWorker` which will
+   advance the stage when all work of the current stage has been completed.
+
+The first approach should only be used by workers that perform all their work in
+a single thread, while `AdvanceStageWorker` should be used for everything else.
+
+The way `AdvanceStageWorker` works is fairly simple. When scheduling a job it
+will be given a project ID, a list of Redis keys, and the name of the next
+stage. The Redis keys (produced by `Gitlab::JobWaiter`) are used to check if the
+currently running stage has been completed or not. If the stage has not yet been
+completed `AdvanceStageWorker` will reschedule itself. Once a stage finishes
+`AdvanceStageworker` will refresh the import JID (more on this below) and
+schedule the worker of the next stage.
+
+To reduce the number of `AdvanceStageWorker` jobs scheduled this worker will
+briefly wait for jobs to complete before deciding what the next action should
+be. For small projects this may slow down the import process a bit, but it will
+also reduce pressure on the system as a whole.
+
+## Refreshing import JIDs
+
+GitLab includes a worker called `StuckImportJobsWorker` that will periodically
+run and mark project imports as failed if they have been running for more than
+15 hours. For GitHub projects this poses a bit of a problem: importing large
+projects could take several hours depending on how often we hit the GitHub rate
+limit (more on this below), but we don't want `StuckImportJobsWorker` to mark
+our import as failed because of this.
+
+To prevent this from happening we periodically refresh the expiration time of
+the import process. This works by storing the JID of the import job in the
+database, then refreshing this JID's TTL at various stages throughout the import
+process. This is done by calling `Project#refresh_import_jid_expiration`. By
+refreshing this TTL we can ensure our import does not get marked as failed so
+long we're still performing work.
+
+## GitHub rate limit
+
+GitHub has a rate limit of 5 000 API calls per hour. The number of requests
+necessary to import a project is largely dominated by the number of unique users
+involved in a project (e.g. issue authors). Other data such as issue pages
+and comments typically only requires a few dozen requests to import.  This is
+because we need the Email address of users in order to map them to GitLab users.
+
+We handle this by doing the following:
+
+1. Once we hit the rate limit all jobs will automatically reschedule themselves
+   in such a way that they are not executed until the rate limit has been reset.
+2. We cache the mapping of GitHub users to GitLab users in Redis.
+
+More information on user caching can be found below.
+
+## Caching user lookups
+
+When mapping GitHub users to GitLab users we need to (in the worst case)
+perform:
+
+1. One API call to get the user's Email address.
+2. Two database queries to see if a corresponding GitLab user exists. One query
+   will try to find the user based on the GitHub user ID, while the second query
+   is used to find the user using their GitHub Email address.
+
+Because this process is quite expensive we cache the result of these lookups in
+Redis. For every user looked up we store three keys:
+
+1. A Redis key mapping GitHub usernames to their Email addresses.
+2. A Redis key mapping a GitHub Email addresses to a GitLab user ID.
+3. A Redis key mapping a GitHub user ID to GitLab user ID.
+
+There are two types of lookups we cache:
+
+1. A positive lookup, meaning we found a GitLab user ID.
+2. A negative lookup, meaning we didn't find a GitLab user ID. Caching this
+   prevents us from performing the same work for users that we know don't exist
+   in our GitLab database.
+
+The expiration time of these keys is 24 hours. When retrieving the cache of a
+positive lookups we refresh the TTL automatically. The TTL of false lookups is
+never refreshed.
+
+Because of this caching layer it's possible newly registered GitLab accounts
+won't be linked to their corresponding GitHub accounts. This however will sort
+itself out once the cached keys expire.
+
+The user cache lookup is shared across projects. This means that the more
+projects get imported the fewer GitHub API calls will be needed.
+
+The code for this resides in:
+
+* `lib/gitlab/github_import/user_finder.rb`
+* `lib/gitlab/github_import/caching.rb`
+
+## Mapping labels and milestones
+
+To reduce pressure on the database we do not query it when setting labels and
+milestones on issues and merge requests. Instead we cache this data when we
+import labels and milestones, then we reuse this cache when assigning them to
+issues/merge requests. Similar to the user lookups these cache keys are expired
+automatically after 24 hours of not being used.
+
+Unlike the user lookup caches these label and milestone caches are scoped to the
+project that is being imported.
+
+The code for this resides in:
+
+* `lib/gitlab/github_import/label_finder.rb`
+* `lib/gitlab/github_import/milestone_finder.rb`
+* `lib/gitlab/github_import/caching.rb`

doc/user/project/import/github.md

@@ -24,6 +24,8 @@ constrains of a Sidekiq worker.
   - the milestones (GitLab 8.7+)
   - the labels (GitLab 8.7+)
   - the release note descriptions (GitLab 8.12+)
+  - the pull request review comments (GitLab 10.2+)
+  - the regular issue and pull request comments
 - References to pull requests and issues are preserved (GitLab 8.7+)
 - Repository public access is retained. If a repository is private in GitHub
   it will be created as private in GitLab as well.
@@ -43,10 +45,13 @@ the case the namespace is taken, the repository will be imported under the user'
 namespace that started the import process.
 
 The importer will also import branches on forks of projects related to open pull
-requests. These branches will be imported with a naming scheume similar to 
+requests. These branches will be imported with a naming scheme similar to
 GH-SHA-Username/Pull-Request-number/fork-name/branch. This may lead to a discrepency
 in branches compared to the GitHub Repository.
 
+For a more technical description and an overview of the architecture you can
+refer to [Working with the GitHub importer][gh-import-dev-docs].
+
 ## Importing your GitHub repositories
 
 The importer page is visible when you create a new project.
@@ -121,7 +126,29 @@ If you want, you can import all your GitHub projects in one go by hitting
 You can also choose a different name for the project and a different namespace,
 if you have the privileges to do so.
 
+## Making the import process go faster
+
+For large projects it may take a while to import all data. To reduce the time
+necessary you can increase the number of Sidekiq workers that process the
+following queues:
+
+* `github_importer`
+* `github_importer_advance_stage`
+
+For an optimal experience we recommend having at least 4 Sidekiq processes (each
+running a number of threads equal to the number of CPU cores) that _only_
+process these queues. We also recommend that these processes run on separate
+servers. For 4 servers with 8 cores this means you can import up to 32 objects
+(e.g. issues) in parallel.
+
+Reducing the time spent in cloning a repository can be done by increasing
+network throughput, CPU capacity, and disk performance (e.g.  by using high
+performance SSDs) of the disks that store the Git repositories (for your GitLab
+instance). Increasing the number of Sidekiq workers will _not_ reduce the time
+spent cloning repositories.
+
 [gh-import]: ../../../integration/github.md "GitHub integration"
 [gh-rake]: ../../../administration/raketasks/github_import.md "GitHub rake task"
 [gh-integration]: #authorize-access-to-your-repositories-using-the-github-integration
 [gh-token]: #authorize-access-to-your-repositories-using-a-personal-access-token
+[gh-import-dev-docs]: ../../../development/github_importer.md "Working with the GitHub importer"

lib/gitlab/git/repository.rb

@@ -920,6 +920,11 @@ module Gitlab
         false
       end
 
+      # Returns true if a remote exists.
+      def remote_exists?(name)
+        rugged.remotes[name].present?
+      end
+
       # Update the specified remote using the values in the +options+ hash
       #
       # Example

lib/gitlab/github_import.rb

+module Gitlab
+  module GithubImport
+    def self.new_client_for(project, token: nil, parallel: true)
+      token_to_use = token || project.import_data&.credentials&.fetch(:user)
+
+      Client.new(token_to_use, parallel: parallel)
+    end
+
+    # Inserts a raw row and returns the ID of the inserted row.
+    #
+    # attributes - The attributes/columns to set.
+    # relation - An ActiveRecord::Relation to use for finding the ID of the row
+    #            when using MySQL.
+    def self.insert_and_return_id(attributes, relation)
+      # We use bulk_insert here so we can bypass any queries executed by
+      # callbacks or validation rules, as doing this wouldn't scale when
+      # importing very large projects.
+      result = Gitlab::Database
+        .bulk_insert(relation.table_name, [attributes], return_ids: true)
+
+      # MySQL doesn't support returning the IDs of a bulk insert in a way that
+      # is not a pain, so in this case we'll issue an extra query instead.
+      result.first ||
+        relation.where(iid: attributes[:iid]).limit(1).pluck(:id).first
+    end
+
+    # Returns the ID of the ghost user.
+    def self.ghost_user_id
+      key = 'github-import/ghost-user-id'
+
+      Caching.read_integer(key) || Caching.write(key, User.select(:id).ghost.id)
+    end
+  end
+end

lib/gitlab/github_import/bulk_importing.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module BulkImporting
+      # Builds and returns an Array of objects to bulk insert into the
+      # database.
+      #
+      # enum - An Enumerable that returns the objects to turn into database
+      #        rows.
+      def build_database_rows(enum)
+        enum.each_with_object([]) do |(object, _), rows|
+          rows << build(object) unless already_imported?(object)
+        end
+      end
+
+      # Bulk inserts the given rows into the database.
+      def bulk_insert(model, rows, batch_size: 100)
+        rows.each_slice(batch_size) do |slice|
+          Gitlab::Database.bulk_insert(model.table_name, slice)
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/caching.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Caching
+      # The default timeout of the cache keys.
+      TIMEOUT = 24.hours.to_i
+
+      WRITE_IF_GREATER_SCRIPT = <<-EOF.strip_heredoc.freeze
+      local key, value, ttl = KEYS[1], tonumber(ARGV[1]), ARGV[2]
+      local existing = tonumber(redis.call("get", key))
+
+      if existing == nil or value > existing then
+        redis.call("set", key, value)
+        redis.call("expire", key, ttl)
+        return true
+      else
+        return false
+      end
+      EOF
+
+      # Reads a cache key.
+      #
+      # If the key exists and has a non-empty value its TTL is refreshed
+      # automatically.
+      #
+      # raw_key - The cache key to read.
+      # timeout - The new timeout of the key if the key is to be refreshed.
+      def self.read(raw_key, timeout: TIMEOUT)
+        key = cache_key_for(raw_key)
+        value = Redis::Cache.with { |redis| redis.get(key) }
+
+        if value.present?
+          # We refresh the expiration time so frequently used keys stick
+          # around, removing the need for querying the database as much as
+          # possible.
+          #
+          # A key may be empty when we looked up a GitHub user (for example) but
+          # did not find a matching GitLab user. In that case we _don't_ want to
+          # refresh the TTL so we automatically pick up the right data when said
+          # user were to register themselves on the GitLab instance.
+          Redis::Cache.with { |redis| redis.expire(key, timeout) }
+        end
+
+        value
+      end
+
+      # Reads an integer from the cache, or returns nil if no value was found.
+      #
+      # See Caching.read for more information.
+      def self.read_integer(raw_key, timeout: TIMEOUT)
+        value = read(raw_key, timeout: timeout)
+
+        value.to_i if value.present?
+      end
+
+      # Sets a cache key to the given value.
+      #
+      # key - The cache key to write.
+      # value - The value to set.
+      # timeout - The time after which the cache key should expire.
+      def self.write(raw_key, value, timeout: TIMEOUT)
+        key = cache_key_for(raw_key)
+
+        Redis::Cache.with do |redis|
+          redis.set(key, value, ex: timeout)
+        end
+
+        value
+      end
+
+      # Adds a value to a set.
+      #
+      # raw_key - The key of the set to add the value to.
+      # value - The value to add to the set.
+      # timeout - The new timeout of the key.
+      def self.set_add(raw_key, value, timeout: TIMEOUT)
+        key = cache_key_for(raw_key)
+
+        Redis::Cache.with do |redis|
+          redis.multi do |m|
+            m.sadd(key, value)
+            m.expire(key, timeout)
+          end
+        end
+      end
+
+      # Returns true if the given value is present in the set.
+      #
+      # raw_key - The key of the set to check.
+      # value - The value to check for.
+      def self.set_includes?(raw_key, value)
+        key = cache_key_for(raw_key)
+
+        Redis::Cache.with do |redis|
+          redis.sismember(key, value)
+        end
+      end
+
+      # Sets multiple keys to a given value.
+      #
+      # mapping - A Hash mapping the cache keys to their values.
+      # timeout - The time after which the cache key should expire.
+      def self.write_multiple(mapping, timeout: TIMEOUT)
+        Redis::Cache.with do |redis|
+          redis.multi do |multi|
+            mapping.each do |raw_key, value|
+              multi.set(cache_key_for(raw_key), value, ex: timeout)
+            end
+          end
+        end
+      end
+
+      # Sets the expiration time of a key.
+      #
+      # raw_key - The key for which to change the timeout.
+      # timeout - The new timeout.
+      def self.expire(raw_key, timeout)
+        key = cache_key_for(raw_key)
+
+        Redis::Cache.with do |redis|
+          redis.expire(key, timeout)
+        end
+      end
+
+      # Sets a key to the given integer but only if the existing value is
+      # smaller than the given value.
+      #
+      # This method uses a Lua script to ensure the read and write are atomic.
+      #
+      # raw_key - The key to set.
+      # value - The new value for the key.
+      # timeout - The key timeout in seconds.
+      #
+      # Returns true when the key was overwritten, false otherwise.
+      def self.write_if_greater(raw_key, value, timeout: TIMEOUT)
+        key = cache_key_for(raw_key)
+        val = Redis::Cache.with do |redis|
+          redis
+            .eval(WRITE_IF_GREATER_SCRIPT, keys: [key], argv: [value, timeout])
+        end
+
+        val ? true : false
+      end
+
+      def self.cache_key_for(raw_key)
+        "#{Redis::Cache::CACHE_NAMESPACE}:#{raw_key}"
+      end
+    end
+  end
+end

lib/gitlab/github_import/client.rb

+# frozen_string_literal: true
+
 module Gitlab
   module GithubImport
+    # HTTP client for interacting with the GitHub API.
+    #
+    # This class is basically a fancy wrapped around Octokit while adding some
+    # functionality to deal with rate limiting and parallel imports. Usage is
+    # mostly the same as Octokit, for example:
+    #
+    #     client = GithubImport::Client.new('hunter2')
+    #
+    #     client.labels.each do |label|
+    #       puts label.name
+    #     end
     class Client
-      GITHUB_SAFE_REMAINING_REQUESTS = 100
-      GITHUB_SAFE_SLEEP_TIME = 500
+      attr_reader :octokit
 
-      attr_reader :access_token, :host, :api_version
+      # A single page of data and the corresponding page number.
+      Page = Struct.new(:objects, :number)
 
-      def initialize(access_token, host: nil, api_version: 'v3')
-        @access_token = access_token
-        @host = host.to_s.sub(%r{/+\z}, '')
-        @api_version = api_version
-        @users = {}
+      # The minimum number of requests we want to keep available.
+      #
+      # We don't use a value of 0 as multiple threads may be using the same
+      # token in parallel. This could result in all of them hitting the GitHub
+      # rate limit at once. The threshold is put in place to not hit the limit
+      # in most cases.
+      RATE_LIMIT_THRESHOLD = 50
 
-        if access_token
-          ::Octokit.auto_paginate = false
-        end
+      # token - The GitHub API token to use.
+      #
+      # per_page - The number of objects that should be displayed per page.
+      #
+      # parallel - When set to true hitting the rate limit will result in a
+      #            dedicated error being raised. When set to `false` we will
+      #            instead just `sleep()` until the rate limit is reset. Setting
+      #            this value to `true` for parallel importing is crucial as
+      #            otherwise hitting the rate limit will result in a thread
+      #            being blocked in a `sleep()` call for up to an hour.
+      def initialize(token, per_page: 100, parallel: true)
+        @octokit = Octokit::Client.new(access_token: token, per_page: per_page)
+        @parallel = parallel
       end
 
-      def api
-        @api ||= ::Octokit::Client.new(
-          access_token: access_token,
-          api_endpoint: api_endpoint,
-          # If there is no config, we're connecting to github.com and we
-          # should verify ssl.
-          connection_options: {
-            ssl: { verify: config ? config['verify_ssl'] : true }
-          }
-        )
+      def parallel?
+        @parallel
       end
 
-      def client
-        unless config
-          raise Projects::ImportService::Error,
-            'OAuth configuration for GitHub missing.'
+      # Returns the details of a GitHub user.
+      #
+      # username - The username of the user.
+      def user(username)
+        with_rate_limit { octokit.user(username) }
       end
 
-        @client ||= ::OAuth2::Client.new(
-          config.app_id,
-          config.app_secret,
-          github_options.merge(ssl: { verify: config['verify_ssl'] })
-        )
+      # Returns the details of a GitHub repository.
+      #
+      # name - The path (in the form `owner/repository`) of the repository.
+      def repository(name)
+        with_rate_limit { octokit.repo(name) }
       end
 
-      def authorize_url(redirect_uri)
-        client.auth_code.authorize_url({
-          redirect_uri: redirect_uri,
-          scope: "repo, user, user:email"
-        })
+      def labels(*args)
+        each_object(:labels, *args)
       end
 
-      def get_token(code)
-        client.auth_code.get_token(code).token
+      def milestones(*args)
+        each_object(:milestones, *args)
       end
 
-      def method_missing(method, *args, &block)
-        if api.respond_to?(method)
-          request(method, *args, &block)
-        else
-          super(method, *args, &block)
-        end
+      def releases(*args)
+        each_object(:releases, *args)
       end
 
-      def respond_to?(method)
-        api.respond_to?(method) || super
+      # Fetches data from the GitHub API and yields a Page object for every page
+      # of data, without loading all of them into memory.
+      #
+      # method - The Octokit method to use for getting the data.
+      # args - Arguments to pass to the Octokit method.
+      #
+      # rubocop: disable GitlabSecurity/PublicSend
+      def each_page(method, *args, &block)
+        return to_enum(__method__, method, *args) unless block_given?
+
+        page =
+          if args.last.is_a?(Hash) && args.last[:page]
+            args.last[:page]
+          else
+            1
           end
 
-      def user(login)
-        return nil unless login.present?
-        return @users[login] if @users.key?(login)
+        collection = with_rate_limit { octokit.public_send(method, *args) }
+        next_url = octokit.last_response.rels[:next]
 
-        @users[login] = api.user(login)
-      end
+        yield Page.new(collection, page)
 
-      private
+        while next_url
+          response = with_rate_limit { next_url.get }
+          next_url = response.rels[:next]
 
-      def api_endpoint
-        if host.present? && api_version.present?
-          "#{host}/api/#{api_version}"
-        else
-          github_options[:site]
+          yield Page.new(response.data, page += 1)
         end
       end
 
-      def config
-        Gitlab.config.omniauth.providers.find { |provider| provider.name == "github" }
-      end
+      # Iterates over all of the objects for the given method (e.g. `:labels`).
+      #
+      # method - The method to send to Octokit for querying data.
+      # args - Any arguments to pass to the Octokit method.
+      def each_object(method, *args, &block)
+        return to_enum(__method__, method, *args) unless block_given?
 
-      def github_options
-        if config
-          config["args"]["client_options"].deep_symbolize_keys
-        else
-          OmniAuth::Strategies::GitHub.default_options[:client_options].symbolize_keys
+        each_page(method, *args) do |page|
+          page.objects.each do |object|
+            yield object
           end
         end
-
-      def rate_limit
-        api.rate_limit!
-      # GitHub Rate Limit API returns 404 when the rate limit is
-      # disabled. In this case we just want to return gracefully
-      # instead of spitting out an error.
-      rescue Octokit::NotFound
-        nil
       end
 
-      def has_rate_limit?
-        return @has_rate_limit if defined?(@has_rate_limit)
+      # Yields the supplied block, responding to any rate limit errors.
+      #
+      # The exact strategy used for handling rate limiting errors depends on
+      # whether we are running in parallel mode or not. For more information see
+      # `#rate_or_wait_for_rate_limit`.
+      def with_rate_limit
+        request_count_counter.increment
 
-        @has_rate_limit = rate_limit.present?
-      end
+        raise_or_wait_for_rate_limit unless requests_remaining?
 
-      def rate_limit_exceed?
-        has_rate_limit? && rate_limit.remaining <= GITHUB_SAFE_REMAINING_REQUESTS
-      end
+        begin
+          yield
+        rescue Octokit::TooManyRequests
+          raise_or_wait_for_rate_limit
 
-      def rate_limit_sleep_time
-        rate_limit.resets_in + GITHUB_SAFE_SLEEP_TIME
+          # This retry will only happen when running in sequential mode as we'll
+          # raise an error in parallel mode.
+          retry
+        end
       end
 
-      def request(method, *args, &block)
-        sleep rate_limit_sleep_time if rate_limit_exceed?
+      # Returns `true` if we're still allowed to perform API calls.
+      def requests_remaining?
+        remaining_requests > RATE_LIMIT_THRESHOLD
+      end
 
-        data = api.__send__(method, *args) # rubocop:disable GitlabSecurity/PublicSend
-        return data unless data.is_a?(Array)
+      def remaining_requests
+        octokit.rate_limit.remaining
+      end
 
-        last_response = api.last_response
+      def raise_or_wait_for_rate_limit
+        rate_limit_counter.increment
 
-        if block_given?
-          yield data
-          # api.last_response could change while we're yielding (e.g. fetching labels for each PR)
-          # so we cache our own last response
-          each_response_page(last_response, &block)
+        if parallel?
+          raise RateLimitError
         else
-          each_response_page(last_response) { |page| data.concat(page) }
-          data
+          sleep(rate_limit_resets_in)
+        end
       end
+
+      def rate_limit_resets_in
+        # We add a few seconds to the rate limit so we don't _immediately_
+        # resume when the rate limit resets as this may result in us performing
+        # a request before GitHub has a chance to reset the limit.
+        octokit.rate_limit.resets_in + 5
+      end
+
+      def respond_to_missing?(method, include_private = false)
+        octokit.respond_to?(method, include_private)
       end
 
-      def each_response_page(last_response)
-        while last_response.rels[:next]
-          sleep rate_limit_sleep_time if rate_limit_exceed?
-          last_response = last_response.rels[:next].get
-          yield last_response.data if last_response.data.is_a?(Array)
+      def rate_limit_counter
+        @rate_limit_counter ||= Gitlab::Metrics.counter(
+          :github_importer_rate_limit_hits,
+          'The number of times we hit the GitHub rate limit when importing projects'
+        )
       end
+
+      def request_count_counter
+        @request_counter ||= Gitlab::Metrics.counter(
+          :github_importer_request_count,
+          'The number of GitHub API calls performed when importing projects'
+        )
       end
     end
   end

lib/gitlab/github_import/importer/diff_note_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class DiffNoteImporter
+        attr_reader :note, :project, :client, :user_finder
+
+        # note - An instance of `Gitlab::GithubImport::Representation::DiffNote`.
+        # project - An instance of `Project`.
+        # client - An instance of `Gitlab::GithubImport::Client`.
+        def initialize(note, project, client)
+          @note = note
+          @project = project
+          @client = client
+          @user_finder = UserFinder.new(project, client)
+        end
+
+        def execute
+          return unless (mr_id = find_merge_request_id)
+
+          author_id, author_found = user_finder.author_id_for(note)
+
+          note_body =
+            MarkdownText.format(note.note, note.author, author_found)
+
+          attributes = {
+            noteable_type: 'MergeRequest',
+            noteable_id: mr_id,
+            project_id: project.id,
+            author_id: author_id,
+            note: note_body,
+            system: false,
+            commit_id: note.commit_id,
+            line_code: note.line_code,
+            type: 'LegacyDiffNote',
+            created_at: note.created_at,
+            updated_at: note.updated_at,
+            st_diff: note.diff_hash.to_yaml
+          }
+
+          # It's possible that during an import we'll insert tens of thousands
+          # of diff notes. If we were to use the Note/LegacyDiffNote model here
+          # we'd also have to run additional queries for both validations and
+          # callbacks, putting a lot of pressure on the database.
+          #
+          # To work around this we're using bulk_insert with a single row. This
+          # allows us to efficiently insert data (even if it's just 1 row)
+          # without having to use all sorts of hacks to disable callbacks.
+          Gitlab::Database.bulk_insert(LegacyDiffNote.table_name, [attributes])
+        rescue ActiveRecord::InvalidForeignKey
+          # It's possible the project and the issue have been deleted since
+          # scheduling this job. In this case we'll just skip creating the note.
+        end
+
+        # Returns the ID of the merge request this note belongs to.
+        def find_merge_request_id
+          GithubImport::IssuableFinder.new(project, note).database_id
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/diff_notes_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class DiffNotesImporter
+        include ParallelScheduling
+
+        def representation_class
+          Representation::DiffNote
+        end
+
+        def importer_class
+          DiffNoteImporter
+        end
+
+        def sidekiq_worker_class
+          ImportDiffNoteWorker
+        end
+
+        def collection_method
+          :pull_requests_comments
+        end
+
+        def id_for_already_imported_cache(note)
+          note.id
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/issue_and_label_links_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class IssueAndLabelLinksImporter
+        attr_reader :issue, :project, :client
+
+        # issue - An instance of `Gitlab::GithubImport::Representation::Issue`.
+        # project - An instance of `Project`
+        # client - An instance of `Gitlab::GithubImport::Client`
+        def initialize(issue, project, client)
+          @issue = issue
+          @project = project
+          @client = client
+        end
+
+        def execute
+          IssueImporter.import_if_issue(issue, project, client)
+          LabelLinksImporter.new(issue, project, client).execute
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/issue_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class IssueImporter
+        attr_reader :project, :issue, :client, :user_finder, :milestone_finder,
+                    :issuable_finder
+
+        # Imports an issue if it's a regular issue and not a pull request.
+        def self.import_if_issue(issue, project, client)
+          new(issue, project, client).execute unless issue.pull_request?
+        end
+
+        # issue - An instance of `Gitlab::GithubImport::Representation::Issue`.
+        # project - An instance of `Project`
+        # client - An instance of `Gitlab::GithubImport::Client`
+        def initialize(issue, project, client)
+          @issue = issue
+          @project = project
+          @client = client
+          @user_finder = UserFinder.new(project, client)
+          @milestone_finder = MilestoneFinder.new(project)
+          @issuable_finder = GithubImport::IssuableFinder.new(project, issue)
+        end
+
+        def execute
+          Issue.transaction do
+            if (issue_id = create_issue)
+              create_assignees(issue_id)
+              issuable_finder.cache_database_id(issue_id)
+            end
+          end
+        end
+
+        # Creates a new GitLab issue for the current GitHub issue.
+        #
+        # Returns the ID of the created issue as an Integer. If the issue
+        # couldn't be created this method will return `nil` instead.
+        def create_issue
+          author_id, author_found = user_finder.author_id_for(issue)
+
+          description =
+            MarkdownText.format(issue.description, issue.author, author_found)
+
+          attributes = {
+            iid: issue.iid,
+            title: issue.truncated_title,
+            author_id: author_id,
+            project_id: project.id,
+            description: description,
+            milestone_id: milestone_finder.id_for(issue),
+            state: issue.state,
+            created_at: issue.created_at,
+            updated_at: issue.updated_at
+          }
+
+          GithubImport.insert_and_return_id(attributes, project.issues)
+        rescue ActiveRecord::InvalidForeignKey
+          # It's possible the project has been deleted since scheduling this
+          # job. In this case we'll just skip creating the issue.
+        end
+
+        # Stores all issue assignees in the database.
+        #
+        # issue_id - The ID of the created issue.
+        def create_assignees(issue_id)
+          assignees = []
+
+          issue.assignees.each do |assignee|
+            if (user_id = user_finder.user_id_for(assignee))
+              assignees << { issue_id: issue_id, user_id: user_id }
+            end
+          end
+
+          Gitlab::Database.bulk_insert(IssueAssignee.table_name, assignees)
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/issues_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class IssuesImporter
+        include ParallelScheduling
+
+        def importer_class
+          IssueAndLabelLinksImporter
+        end
+
+        def representation_class
+          Representation::Issue
+        end
+
+        def sidekiq_worker_class
+          ImportIssueWorker
+        end
+
+        def collection_method
+          :issues
+        end
+
+        def id_for_already_imported_cache(issue)
+          issue.number
+        end
+
+        def collection_options
+          { state: 'all', sort: 'created', direction: 'asc' }
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/label_links_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class LabelLinksImporter
+        attr_reader :issue, :project, :client, :label_finder
+
+        # issue - An instance of `Gitlab::GithubImport::Representation::Issue`
+        # project - An instance of `Project`
+        # client - An instance of `Gitlab::GithubImport::Client`
+        def initialize(issue, project, client)
+          @issue = issue
+          @project = project
+          @client = client
+          @label_finder = LabelFinder.new(project)
+        end
+
+        def execute
+          create_labels
+        end
+
+        def create_labels
+          time = Time.zone.now
+          rows = []
+          target_id = find_target_id
+
+          issue.label_names.each do |label_name|
+            # Although unlikely it's technically possible for an issue to be
+            # given a label that was created and assigned after we imported all
+            # the project's labels.
+            next unless (label_id = label_finder.id_for(label_name))
+
+            rows << {
+              label_id: label_id,
+              target_id: target_id,
+              target_type: issue.issuable_type,
+              created_at: time,
+              updated_at: time
+            }
+          end
+
+          Gitlab::Database.bulk_insert(LabelLink.table_name, rows)
+        end
+
+        def find_target_id
+          GithubImport::IssuableFinder.new(project, issue).database_id
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/labels_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class LabelsImporter
+        include BulkImporting
+
+        attr_reader :project, :client, :existing_labels
+
+        # project - An instance of `Project`.
+        # client - An instance of `Gitlab::GithubImport::Client`.
+        def initialize(project, client)
+          @project = project
+          @client = client
+          @existing_labels = project.labels.pluck(:title).to_set
+        end
+
+        def execute
+          bulk_insert(Label, build_labels)
+          build_labels_cache
+        end
+
+        def build_labels
+          build_database_rows(each_label)
+        end
+
+        def already_imported?(label)
+          existing_labels.include?(label.name)
+        end
+
+        def build_labels_cache
+          LabelFinder.new(project).build_cache
+        end
+
+        def build(label)
+          time = Time.zone.now
+
+          {
+            title: label.name,
+            color: '#' + label.color,
+            project_id: project.id,
+            type: 'ProjectLabel',
+            created_at: time,
+            updated_at: time
+          }
+        end
+
+        def each_label
+          client.labels(project.import_source)
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/milestones_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class MilestonesImporter
+        include BulkImporting
+
+        attr_reader :project, :client, :existing_milestones
+
+        # project - An instance of `Project`
+        # client - An instance of `Gitlab::GithubImport::Client`
+        def initialize(project, client)
+          @project = project
+          @client = client
+          @existing_milestones = project.milestones.pluck(:iid).to_set
+        end
+
+        def execute
+          bulk_insert(Milestone, build_milestones)
+          build_milestones_cache
+        end
+
+        def build_milestones
+          build_database_rows(each_milestone)
+        end
+
+        def already_imported?(milestone)
+          existing_milestones.include?(milestone.number)
+        end
+
+        def build_milestones_cache
+          MilestoneFinder.new(project).build_cache
+        end
+
+        def build(milestone)
+          {
+            iid: milestone.number,
+            title: milestone.title,
+            description: milestone.description,
+            project_id: project.id,
+            state: state_for(milestone),
+            created_at: milestone.created_at,
+            updated_at: milestone.updated_at
+          }
+        end
+
+        def state_for(milestone)
+          milestone.state == 'open' ? :active : :closed
+        end
+
+        def each_milestone
+          client.milestones(project.import_source, state: 'all')
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/note_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class NoteImporter
+        attr_reader :note, :project, :client, :user_finder
+
+        # note - An instance of `Gitlab::GithubImport::Representation::Note`.
+        # project - An instance of `Project`.
+        # client - An instance of `Gitlab::GithubImport::Client`.
+        def initialize(note, project, client)
+          @note = note
+          @project = project
+          @client = client
+          @user_finder = UserFinder.new(project, client)
+        end
+
+        def execute
+          return unless (noteable_id = find_noteable_id)
+
+          author_id, author_found = user_finder.author_id_for(note)
+
+          note_body =
+            MarkdownText.format(note.note, note.author, author_found)
+
+          attributes = {
+            noteable_type: note.noteable_type,
+            noteable_id: noteable_id,
+            project_id: project.id,
+            author_id: author_id,
+            note: note_body,
+            system: false,
+            created_at: note.created_at,
+            updated_at: note.updated_at
+          }
+
+          # We're using bulk_insert here so we can bypass any validations and
+          # callbacks. Running these would result in a lot of unnecessary SQL
+          # queries being executed when importing large projects.
+          Gitlab::Database.bulk_insert(Note.table_name, [attributes])
+        rescue ActiveRecord::InvalidForeignKey
+          # It's possible the project and the issue have been deleted since
+          # scheduling this job. In this case we'll just skip creating the note.
+        end
+
+        # Returns the ID of the issue or merge request to create the note for.
+        def find_noteable_id
+          GithubImport::IssuableFinder.new(project, note).database_id
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/notes_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class NotesImporter
+        include ParallelScheduling
+
+        def importer_class
+          NoteImporter
+        end
+
+        def representation_class
+          Representation::Note
+        end
+
+        def sidekiq_worker_class
+          ImportNoteWorker
+        end
+
+        def collection_method
+          :issues_comments
+        end
+
+        def id_for_already_imported_cache(note)
+          note.id
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/pull_request_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class PullRequestImporter
+        attr_reader :pull_request, :project, :client, :user_finder,
+                    :milestone_finder, :issuable_finder
+
+        # pull_request - An instance of
+        #                `Gitlab::GithubImport::Representation::PullRequest`.
+        # project - An instance of `Project`
+        # client - An instance of `Gitlab::GithubImport::Client`
+        def initialize(pull_request, project, client)
+          @pull_request = pull_request
+          @project = project
+          @client = client
+          @user_finder = UserFinder.new(project, client)
+          @milestone_finder = MilestoneFinder.new(project)
+          @issuable_finder =
+            GithubImport::IssuableFinder.new(project, pull_request)
+        end
+
+        def execute
+          if (mr_id = create_merge_request)
+            issuable_finder.cache_database_id(mr_id)
+          end
+        end
+
+        # Creates the merge request and returns its ID.
+        #
+        # This method will return `nil` if the merge request could not be
+        # created.
+        def create_merge_request
+          author_id, author_found = user_finder.author_id_for(pull_request)
+
+          description = MarkdownText
+            .format(pull_request.description, pull_request.author, author_found)
+
+          # This work must be wrapped in a transaction as otherwise we can leave
+          # behind incomplete data in the event of an error. This can then lead
+          # to duplicate key errors when jobs are retried.
+          MergeRequest.transaction do
+            attributes = {
+              iid: pull_request.iid,
+              title: pull_request.truncated_title,
+              description: description,
+              source_project_id: project.id,
+              target_project_id: project.id,
+              source_branch: pull_request.formatted_source_branch,
+              target_branch: pull_request.target_branch,
+              state: pull_request.state,
+              milestone_id: milestone_finder.id_for(pull_request),
+              author_id: author_id,
+              assignee_id: user_finder.assignee_id_for(pull_request),
+              created_at: pull_request.created_at,
+              updated_at: pull_request.updated_at
+            }
+
+            # When creating merge requests there are a lot of hooks that may
+            # run, for many different reasons. Many of these hooks (e.g. the
+            # ones used for rendering Markdown) are completely unnecessary and
+            # may even lead to transaction timeouts.
+            #
+            # To ensure importing pull requests has a minimal impact and can
+            # complete in a reasonable time we bypass all the hooks by inserting
+            # the row and then retrieving it. We then only perform the
+            # additional work that is strictly necessary.
+            merge_request_id = GithubImport
+              .insert_and_return_id(attributes, project.merge_requests)
+
+            merge_request = project.merge_requests.find(merge_request_id)
+
+            # These fields are set so we can create the correct merge request
+            # diffs.
+            merge_request.source_branch_sha = pull_request.source_branch_sha
+            merge_request.target_branch_sha = pull_request.target_branch_sha
+
+            merge_request.keep_around_commit
+            merge_request.merge_request_diffs.create
+
+            merge_request.id
+          end
+        rescue ActiveRecord::InvalidForeignKey
+          # It's possible the project has been deleted since scheduling this
+          # job. In this case we'll just skip creating the merge request.
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/pull_requests_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class PullRequestsImporter
+        include ParallelScheduling
+
+        def importer_class
+          PullRequestImporter
+        end
+
+        def representation_class
+          Representation::PullRequest
+        end
+
+        def sidekiq_worker_class
+          ImportPullRequestWorker
+        end
+
+        def id_for_already_imported_cache(pr)
+          pr.number
+        end
+
+        def each_object_to_import
+          super do |pr|
+            update_repository if update_repository?(pr)
+            yield pr
+          end
+        end
+
+        def update_repository
+          # We set this column _before_ fetching the repository, and this is
+          # deliberate. If we were to update this column after the fetch we may
+          # miss out on changes pushed during the fetch or between the fetch and
+          # updating the timestamp.
+          project.update_column(:last_repository_updated_at, Time.zone.now)
+
+          project.repository.fetch_remote('github', forced: false)
+
+          pname = project.path_with_namespace
+
+          Rails.logger
+            .info("GitHub importer finished updating repository for #{pname}")
+
+          repository_updates_counter.increment(project: pname)
+        end
+
+        def update_repository?(pr)
+          last_update = project.last_repository_updated_at || project.created_at
+
+          return false if pr.updated_at < last_update
+
+          # PRs may be updated without there actually being new commits, thus we
+          # check to make sure we only re-fetch if truly necessary.
+          !(commit_exists?(pr.head.sha) && commit_exists?(pr.base.sha))
+        end
+
+        def commit_exists?(sha)
+          project.repository.lookup(sha)
+          true
+        rescue Rugged::Error
+          false
+        end
+
+        def collection_method
+          :pull_requests
+        end
+
+        def collection_options
+          { state: 'all', sort: 'created', direction: 'asc' }
+        end
+
+        def repository_updates_counter
+          @repository_updates_counter ||= Gitlab::Metrics.counter(
+            :github_importer_repository_updates,
+            'The number of times repositories have to be updated again'
+          )
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/releases_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class ReleasesImporter
+        include BulkImporting
+
+        attr_reader :project, :client, :existing_tags
+
+        # project - An instance of `Project`
+        # client - An instance of `Gitlab::GithubImport::Client`
+        def initialize(project, client)
+          @project = project
+          @client = client
+          @existing_tags = project.releases.pluck(:tag).to_set
+        end
+
+        def execute
+          bulk_insert(Release, build_releases)
+        end
+
+        def build_releases
+          build_database_rows(each_release)
+        end
+
+        def already_imported?(release)
+          existing_tags.include?(release.tag_name)
+        end
+
+        def build(release)
+          {
+            tag: release.tag_name,
+            description: description_for(release),
+            created_at: release.created_at,
+            updated_at: release.updated_at,
+            project_id: project.id
+          }
+        end
+
+        def each_release
+          client.releases(project.import_source)
+        end
+
+        def description_for(release)
+          if release.body.present?
+            release.body
+          else
+            "Release for tag #{release.tag_name}"
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/importer/repository_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Importer
+      class RepositoryImporter
+        include Gitlab::ShellAdapter
+
+        attr_reader :project, :client
+
+        def initialize(project, client)
+          @project = project
+          @client = client
+        end
+
+        # Returns true if we should import the wiki for the project.
+        def import_wiki?
+          client.repository(project.import_source)&.has_wiki &&
+            !project.wiki_repository_exists?
+        end
+
+        # Imports the repository data.
+        #
+        # This method will return true if the data was imported successfully or
+        # the repository had already been imported before.
+        def execute
+          imported =
+            # It's possible a repository has already been imported when running
+            # this code, e.g. because we had to retry this job after
+            # `import_wiki?` raised a rate limit error. In this case we'll skip
+            # re-importing the main repository.
+            if project.repository.empty_repo?
+              import_repository
+            else
+              true
+            end
+
+          update_clone_time if imported
+
+          imported = import_wiki_repository if import_wiki? && imported
+
+          imported
+        end
+
+        def import_repository
+          project.ensure_repository
+
+          configure_repository_remote
+
+          project.repository.fetch_remote('github', forced: true)
+
+          true
+        rescue Gitlab::Git::Repository::NoRepository, Gitlab::Shell::Error => e
+          fail_import("Failed to import the repository: #{e.message}")
+        end
+
+        def configure_repository_remote
+          return if project.repository.remote_exists?('github')
+
+          project.repository.add_remote('github', project.import_url)
+          project.repository.set_import_remote_as_mirror('github')
+
+          project.repository.add_remote_fetch_config(
+            'github',
+            '+refs/pull/*/head:refs/merge-requests/*/head'
+          )
+        end
+
+        def import_wiki_repository
+          wiki_path = "#{project.disk_path}.wiki"
+          wiki_url = project.import_url.sub(/\.git\z/, '.wiki.git')
+          storage_path = project.repository_storage_path
+
+          gitlab_shell.import_repository(storage_path, wiki_path, wiki_url)
+
+          true
+        rescue Gitlab::Shell::Error => e
+          if e.message !~ /repository not exported/
+            fail_import("Failed to import the wiki: #{e.message}")
+          else
+            true
+          end
+        end
+
+        def update_clone_time
+          project.update_column(:last_repository_updated_at, Time.zone.now)
+        end
+
+        def fail_import(message)
+          project.mark_import_as_failed(message)
+          false
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/issuable_finder.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # IssuableFinder can be used for caching and retrieving database IDs for
+    # issuable objects such as issues and pull requests. By caching these IDs we
+    # remove the need for running a lot of database queries when importing
+    # GitHub projects.
+    class IssuableFinder
+      attr_reader :project, :object
+
+      # The base cache key to use for storing/retrieving issuable IDs.
+      CACHE_KEY = 'github-import/issuable-finder/%{project}/%{type}/%{iid}'.freeze
+
+      # project - An instance of `Project`.
+      # object - The object to look up or set a database ID for.
+      def initialize(project, object)
+        @project = project
+        @object = object
+      end
+
+      # Returns the database ID for the object.
+      #
+      # This method will return `nil` if no ID could be found.
+      def database_id
+        val = Caching.read(cache_key)
+
+        val.to_i if val.present?
+      end
+
+      # Associates the given database ID with the current object.
+      #
+      # database_id - The ID of the corresponding database row.
+      def cache_database_id(database_id)
+        Caching.write(cache_key, database_id)
+      end
+
+      private
+
+      def cache_key
+        CACHE_KEY % {
+          project: project.id,
+          type: cache_key_type,
+          iid: cache_key_iid
+        }
+      end
+
+      # Returns the identifier to use for cache keys.
+      #
+      # For issues and pull requests this will be "Issue" or "MergeRequest"
+      # respectively. For diff notes this will return "MergeRequest", for
+      # regular notes it will either return "Issue" or "MergeRequest" depending
+      # on what type of object the note belongs to.
+      def cache_key_type
+        if object.respond_to?(:issuable_type)
+          object.issuable_type
+        elsif object.respond_to?(:noteable_type)
+          object.noteable_type
+        else
+          raise(
+            TypeError,
+            "Instances of #{object.class} are not supported"
+          )
+        end
+      end
+
+      def cache_key_iid
+        if object.respond_to?(:noteable_id)
+          object.noteable_id
+        elsif object.respond_to?(:iid)
+          object.iid
+        else
+          raise(
+            TypeError,
+            "Instances of #{object.class} are not supported"
+          )
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/label_finder.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    class LabelFinder
+      attr_reader :project
+
+      # The base cache key to use for storing/retrieving label IDs.
+      CACHE_KEY = 'github-import/label-finder/%{project}/%{name}'.freeze
+
+      # project - An instance of `Project`.
+      def initialize(project)
+        @project = project
+      end
+
+      # Returns the label ID for the given name.
+      def id_for(name)
+        Caching.read_integer(cache_key_for(name))
+      end
+
+      def build_cache
+        mapping = @project
+          .labels
+          .pluck(:id, :name)
+          .each_with_object({}) do |(id, name), hash|
+            hash[cache_key_for(name)] = id
+          end
+
+        Caching.write_multiple(mapping)
+      end
+
+      def cache_key_for(name)
+        CACHE_KEY % { project: project.id, name: name }
+      end
+    end
+  end
+end

lib/gitlab/github_import/markdown_text.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    class MarkdownText
+      attr_reader :text, :author, :exists
+
+      def self.format(*args)
+        new(*args).to_s
+      end
+
+      # text - The Markdown text as a String.
+      # author - An instance of `Gitlab::GithubImport::Representation::User`
+      # exists - Boolean that indicates the user exists in the GitLab database.
+      def initialize(text, author, exists = false)
+        @text = text
+        @author = author
+        @exists = exists
+      end
+
+      def to_s
+        if exists
+          text
+        else
+          "*Created by: #{author.login}*\n\n#{text}"
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/milestone_finder.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    class MilestoneFinder
+      attr_reader :project
+
+      # The base cache key to use for storing/retrieving milestone IDs.
+      CACHE_KEY = 'github-import/milestone-finder/%{project}/%{iid}'.freeze
+
+      # project - An instance of `Project`
+      def initialize(project)
+        @project = project
+      end
+
+      # issuable - An instance of `Gitlab::GithubImport::Representation::Issue`
+      #            or `Gitlab::GithubImport::Representation::PullRequest`.
+      def id_for(issuable)
+        return unless issuable.milestone_number
+
+        Caching.read_integer(cache_key_for(issuable.milestone_number))
+      end
+
+      def build_cache
+        mapping = @project
+          .milestones
+          .pluck(:id, :iid)
+          .each_with_object({}) do |(id, iid), hash|
+            hash[cache_key_for(iid)] = id
+          end
+
+        Caching.write_multiple(mapping)
+      end
+
+      def cache_key_for(iid)
+        CACHE_KEY % { project: project.id, iid: iid }
+      end
+    end
+  end
+end

lib/gitlab/github_import/page_counter.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # PageCounter can be used to keep track of the last imported page of a
+    # collection, allowing workers to resume where they left off in the event of
+    # an error.
+    class PageCounter
+      attr_reader :cache_key
+
+      # The base cache key to use for storing the last page number.
+      CACHE_KEY = 'github-importer/page-counter/%{project}/%{collection}'.freeze
+
+      def initialize(project, collection)
+        @cache_key = CACHE_KEY % { project: project.id, collection: collection }
+      end
+
+      # Sets the page number to the given value.
+      #
+      # Returns true if the page number was overwritten, false otherwise.
+      def set(page)
+        Caching.write_if_greater(cache_key, page)
+      end
+
+      # Returns the current value from the cache.
+      def current
+        Caching.read_integer(cache_key) || 1
+      end
+    end
+  end
+end

lib/gitlab/github_import/parallel_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # The ParallelImporter schedules the importing of a GitHub project using
+    # Sidekiq.
+    class ParallelImporter
+      attr_reader :project
+
+      def self.async?
+        true
+      end
+
+      def initialize(project)
+        @project = project
+      end
+
+      def execute
+        jid = generate_jid
+
+        # The original import JID is the JID of the RepositoryImportWorker job,
+        # which will be removed once that job completes. Reusing that JID could
+        # result in StuckImportJobsWorker marking the job as stuck before we get
+        # to running Stage::ImportRepositoryWorker.
+        #
+        # We work around this by setting the JID to a custom generated one, then
+        # refreshing it in the various stages whenever necessary.
+        Gitlab::SidekiqStatus
+          .set(jid, StuckImportJobsWorker::IMPORT_JOBS_EXPIRATION)
+
+        project.update_column(:import_jid, jid)
+
+        Stage::ImportRepositoryWorker
+          .perform_async(project.id)
+
+        true
+      end
+
+      def generate_jid
+        "github-importer/#{project.id}"
+      end
+    end
+  end
+end

lib/gitlab/github_import/parallel_scheduling.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module ParallelScheduling
+      attr_reader :project, :client, :page_counter, :already_imported_cache_key
+
+      # The base cache key to use for tracking already imported objects.
+      ALREADY_IMPORTED_CACHE_KEY =
+        'github-importer/already-imported/%{project}/%{collection}'.freeze
+
+      # project - An instance of `Project`.
+      # client - An instance of `Gitlab::GithubImport::Client`.
+      # parallel - When set to true the objects will be imported in parallel.
+      def initialize(project, client, parallel: true)
+        @project = project
+        @client = client
+        @parallel = parallel
+        @page_counter = PageCounter.new(project, collection_method)
+        @already_imported_cache_key = ALREADY_IMPORTED_CACHE_KEY %
+          { project: project.id, collection: collection_method }
+      end
+
+      def parallel?
+        @parallel
+      end
+
+      def execute
+        retval =
+          if parallel?
+            parallel_import
+          else
+            sequential_import
+          end
+
+        # Once we have completed all work we can remove our "already exists"
+        # cache so we don't put too much pressure on Redis.
+        #
+        # We don't immediately remove it since it's technically possible for
+        # other instances of this job to still run, instead we set the
+        # expiration time to a lower value. This prevents the other jobs from
+        # still scheduling duplicates while. Since all work has already been
+        # completed those jobs will just cycle through any remaining pages while
+        # not scheduling anything.
+        Caching.expire(already_imported_cache_key, 15.minutes.to_i)
+
+        retval
+      end
+
+      # Imports all the objects in sequence in the current thread.
+      def sequential_import
+        each_object_to_import do |object|
+          repr = representation_class.from_api_response(object)
+
+          importer_class.new(repr, project, client).execute
+        end
+      end
+
+      # Imports all objects in parallel by scheduling a Sidekiq job for every
+      # individual object.
+      def parallel_import
+        waiter = JobWaiter.new
+
+        each_object_to_import do |object|
+          repr = representation_class.from_api_response(object)
+
+          sidekiq_worker_class
+            .perform_async(project.id, repr.to_hash, waiter.key)
+
+          waiter.jobs_remaining += 1
+        end
+
+        waiter
+      end
+
+      # The method that will be called for traversing through all the objects to
+      # import, yielding them to the supplied block.
+      def each_object_to_import
+        repo = project.import_source
+
+        # We inject the page number here to make sure that all importers always
+        # start where they left off. Simply starting over wouldn't work for
+        # repositories with a lot of data (e.g. tens of thousands of comments).
+        options = collection_options.merge(page: page_counter.current)
+
+        client.each_page(collection_method, repo, options) do |page|
+          # Technically it's possible that the same work is performed multiple
+          # times, as Sidekiq doesn't guarantee there will ever only be one
+          # instance of a job. In such a scenario it's possible for one job to
+          # have a lower page number (e.g. 5) compared to another (e.g. 10). In
+          # this case we skip over all the objects until we have caught up,
+          # reducing the number of duplicate jobs scheduled by the provided
+          # block.
+          next unless page_counter.set(page.number)
+
+          page.objects.each do |object|
+            next if already_imported?(object)
+
+            yield object
+
+            # We mark the object as imported immediately so we don't end up
+            # scheduling it multiple times.
+            mark_as_imported(object)
+          end
+        end
+      end
+
+      # Returns true if the given object has already been imported, false
+      # otherwise.
+      #
+      # object - The object to check.
+      def already_imported?(object)
+        id = id_for_already_imported_cache(object)
+
+        Caching.set_includes?(already_imported_cache_key, id)
+      end
+
+      # Marks the given object as "already imported".
+      def mark_as_imported(object)
+        id = id_for_already_imported_cache(object)
+
+        Caching.set_add(already_imported_cache_key, id)
+      end
+
+      # Returns the ID to use for the cache used for checking if an object has
+      # already been imported or not.
+      #
+      # object - The object we may want to import.
+      def id_for_already_imported_cache(object)
+        raise NotImplementedError
+      end
+
+      # The class used for converting API responses to Hashes when performing
+      # the import.
+      def representation_class
+        raise NotImplementedError
+      end
+
+      # The class to use for importing objects when importing them sequentially.
+      def importer_class
+        raise NotImplementedError
+      end
+
+      # The Sidekiq worker class used for scheduling the importing of objects in
+      # parallel.
+      def sidekiq_worker_class
+        raise NotImplementedError
+      end
+
+      # The name of the method to call to retrieve the data to import.
+      def collection_method
+        raise NotImplementedError
+      end
+
+      # Any options to be passed to the method used for retrieving the data to
+      # import.
+      def collection_options
+        {}
+      end
+    end
+  end
+end

lib/gitlab/github_import/rate_limit_error.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # Error that will be raised when we're about to reach (or have reached) the
+    # GitHub API's rate limit.
+    RateLimitError = Class.new(StandardError)
+  end
+end

lib/gitlab/github_import/representation.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Representation
+      TIMESTAMP_KEYS = %i[created_at updated_at merged_at].freeze
+
+      # Converts a Hash with String based keys to one that can be used by the
+      # various Representation classes.
+      #
+      # Example:
+      #
+      #     Representation.symbolize_hash('number' => 10) # => { number: 10 }
+      def self.symbolize_hash(raw_hash = nil)
+        hash = raw_hash.deep_symbolize_keys
+
+        TIMESTAMP_KEYS.each do |key|
+          hash[key] = Time.parse(hash[key]) if hash[key].is_a?(String)
+        end
+
+        hash
+      end
+    end
+  end
+end

lib/gitlab/github_import/representation/diff_note.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Representation
+      class DiffNote
+        include ToHash
+        include ExposeAttribute
+
+        attr_reader :attributes
+
+        expose_attribute :noteable_type, :noteable_id, :commit_id, :file_path,
+                         :diff_hunk, :author, :note, :created_at, :updated_at,
+                         :github_id
+
+        NOTEABLE_ID_REGEX = /\/pull\/(?<iid>\d+)/i
+
+        # Builds a diff note from a GitHub API response.
+        #
+        # note - An instance of `Sawyer::Resource` containing the note details.
+        def self.from_api_response(note)
+          matches = note.html_url.match(NOTEABLE_ID_REGEX)
+
+          unless matches
+            raise(
+              ArgumentError,
+              "The note URL #{note.html_url.inspect} is not supported"
+            )
+          end
+
+          user = Representation::User.from_api_response(note.user) if note.user
+          hash = {
+            noteable_type: 'MergeRequest',
+            noteable_id: matches[:iid].to_i,
+            file_path: note.path,
+            commit_id: note.commit_id,
+            diff_hunk: note.diff_hunk,
+            author: user,
+            note: note.body,
+            created_at: note.created_at,
+            updated_at: note.updated_at,
+            github_id: note.id
+          }
+
+          new(hash)
+        end
+
+        # Builds a new note using a Hash that was built from a JSON payload.
+        def self.from_json_hash(raw_hash)
+          hash = Representation.symbolize_hash(raw_hash)
+          hash[:author] &&= Representation::User.from_json_hash(hash[:author])
+
+          new(hash)
+        end
+
+        # attributes - A Hash containing the raw note details. The keys of this
+        #              Hash must be Symbols.
+        def initialize(attributes)
+          @attributes = attributes
+        end
+
+        def line_code
+          diff_line = Gitlab::Diff::Parser.new.parse(diff_hunk.lines).to_a.last
+
+          Gitlab::Git
+            .diff_line_code(file_path, diff_line.new_pos, diff_line.old_pos)
+        end
+
+        # Returns a Hash that can be used to populate `notes.st_diff`, removing
+        # the need for requesting Git data for every diff note.
+        def diff_hash
+          {
+            diff: diff_hunk,
+            new_path: file_path,
+            old_path: file_path,
+
+            # These fields are not displayed for LegacyDiffNote notes, so it
+            # doesn't really matter what we set them to.
+            a_mode: '100644',
+            b_mode: '100644',
+            new_file: false
+          }
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/representation/expose_attribute.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Representation
+      module ExposeAttribute
+        extend ActiveSupport::Concern
+
+        module ClassMethods
+          # Defines getter methods for the given attribute names.
+          #
+          # Example:
+          #
+          #     expose_attribute :iid, :title
+          def expose_attribute(*names)
+            names.each do |name|
+              name = name.to_sym
+
+              define_method(name) { attributes[name] }
+            end
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/representation/issue.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Representation
+      class Issue
+        include ToHash
+        include ExposeAttribute
+
+        attr_reader :attributes
+
+        expose_attribute :iid, :title, :description, :milestone_number,
+                         :created_at, :updated_at, :state, :assignees,
+                         :label_names, :author
+
+        # Builds an issue from a GitHub API response.
+        #
+        # issue - An instance of `Sawyer::Resource` containing the issue
+        #         details.
+        def self.from_api_response(issue)
+          user =
+            if issue.user
+              Representation::User.from_api_response(issue.user)
+            end
+
+          hash = {
+            iid: issue.number,
+            title: issue.title,
+            description: issue.body,
+            milestone_number: issue.milestone&.number,
+            state: issue.state == 'open' ? :opened : :closed,
+            assignees: issue.assignees.map do |u|
+              Representation::User.from_api_response(u)
+            end,
+            label_names: issue.labels.map(&:name),
+            author: user,
+            created_at: issue.created_at,
+            updated_at: issue.updated_at,
+            pull_request: issue.pull_request ? true : false
+          }
+
+          new(hash)
+        end
+
+        # Builds a new issue using a Hash that was built from a JSON payload.
+        def self.from_json_hash(raw_hash)
+          hash = Representation.symbolize_hash(raw_hash)
+
+          hash[:state] = hash[:state].to_sym
+          hash[:assignees].map! { |u| Representation::User.from_json_hash(u) }
+          hash[:author] &&= Representation::User.from_json_hash(hash[:author])
+
+          new(hash)
+        end
+
+        # attributes - A hash containing the raw issue details. The keys of this
+        #              Hash (and any nested hashes) must be symbols.
+        def initialize(attributes)
+          @attributes = attributes
+        end
+
+        def truncated_title
+          title.truncate(255)
+        end
+
+        def labels?
+          label_names && label_names.any?
+        end
+
+        def pull_request?
+          attributes[:pull_request]
+        end
+
+        def issuable_type
+          pull_request? ? 'MergeRequest' : 'Issue'
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/representation/note.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Representation
+      class Note
+        include ToHash
+        include ExposeAttribute
+
+        attr_reader :attributes
+
+        expose_attribute :noteable_id, :noteable_type, :author, :note,
+                         :created_at, :updated_at, :github_id
+
+        NOTEABLE_TYPE_REGEX = /\/(?<type>(pull|issues))\/(?<iid>\d+)/i
+
+        # Builds a note from a GitHub API response.
+        #
+        # note - An instance of `Sawyer::Resource` containing the note details.
+        def self.from_api_response(note)
+          matches = note.html_url.match(NOTEABLE_TYPE_REGEX)
+
+          if !matches || !matches[:type]
+            raise(
+              ArgumentError,
+              "The note URL #{note.html_url.inspect} is not supported"
+            )
+          end
+
+          noteable_type =
+            if matches[:type] == 'pull'
+              'MergeRequest'
+            else
+              'Issue'
+            end
+
+          user = Representation::User.from_api_response(note.user) if note.user
+          hash = {
+            noteable_type: noteable_type,
+            noteable_id: matches[:iid].to_i,
+            author: user,
+            note: note.body,
+            created_at: note.created_at,
+            updated_at: note.updated_at,
+            github_id: note.id
+          }
+
+          new(hash)
+        end
+
+        # Builds a new note using a Hash that was built from a JSON payload.
+        def self.from_json_hash(raw_hash)
+          hash = Representation.symbolize_hash(raw_hash)
+
+          hash[:author] &&= Representation::User.from_json_hash(hash[:author])
+
+          new(hash)
+        end
+
+        # attributes - A Hash containing the raw note details. The keys of this
+        #              Hash must be Symbols.
+        def initialize(attributes)
+          @attributes = attributes
+        end
+
+        alias_method :issuable_type, :noteable_type
+      end
+    end
+  end
+end

lib/gitlab/github_import/representation/pull_request.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Representation
+      class PullRequest
+        include ToHash
+        include ExposeAttribute
+
+        attr_reader :attributes
+
+        expose_attribute :iid, :title, :description, :source_branch,
+                         :source_branch_sha, :target_branch, :target_branch_sha,
+                         :milestone_number, :author, :assignee, :created_at,
+                         :updated_at, :merged_at, :source_repository_id,
+                         :target_repository_id, :source_repository_owner
+
+        # Builds a PR from a GitHub API response.
+        #
+        # issue - An instance of `Sawyer::Resource` containing the PR details.
+        def self.from_api_response(pr)
+          assignee =
+            if pr.assignee
+              Representation::User.from_api_response(pr.assignee)
+            end
+
+          user = Representation::User.from_api_response(pr.user) if pr.user
+          hash = {
+            iid: pr.number,
+            title: pr.title,
+            description: pr.body,
+            source_branch: pr.head.ref,
+            target_branch: pr.base.ref,
+            source_branch_sha: pr.head.sha,
+            target_branch_sha: pr.base.sha,
+            source_repository_id: pr.head&.repo&.id,
+            target_repository_id: pr.base&.repo&.id,
+            source_repository_owner: pr.head&.user&.login,
+            state: pr.state == 'open' ? :opened : :closed,
+            milestone_number: pr.milestone&.number,
+            author: user,
+            assignee: assignee,
+            created_at: pr.created_at,
+            updated_at: pr.updated_at,
+            merged_at: pr.merged_at
+          }
+
+          new(hash)
+        end
+
+        # Builds a new PR using a Hash that was built from a JSON payload.
+        def self.from_json_hash(raw_hash)
+          hash = Representation.symbolize_hash(raw_hash)
+
+          hash[:state] = hash[:state].to_sym
+          hash[:author] &&= Representation::User.from_json_hash(hash[:author])
+
+          # Assignees are optional so we only convert it from a Hash if one was
+          # set.
+          hash[:assignee] &&= Representation::User
+            .from_json_hash(hash[:assignee])
+
+          new(hash)
+        end
+
+        # attributes - A Hash containing the raw PR details. The keys of this
+        #              Hash (and any nested hashes) must be symbols.
+        def initialize(attributes)
+          @attributes = attributes
+        end
+
+        def truncated_title
+          title.truncate(255)
+        end
+
+        # Returns a formatted source branch.
+        #
+        # For cross-project pull requests the branch name will be in the format
+        # `owner-name:branch-name`.
+        def formatted_source_branch
+          if cross_project? && source_repository_owner
+            "#{source_repository_owner}:#{source_branch}"
+          elsif source_branch == target_branch
+            # Sometimes the source and target branch are the same, but GitLab
+            # doesn't support this. This can happen when both the user and
+            # source repository have been deleted, and the PR was submitted from
+            # the fork's master branch.
+            "#{source_branch}-#{iid}"
+          else
+            source_branch
+          end
+        end
+
+        def state
+          if merged_at
+            :merged
+          else
+            attributes[:state]
+          end
+        end
+
+        def cross_project?
+          return true unless source_repository_id
+
+          source_repository_id != target_repository_id
+        end
+
+        def issuable_type
+          'MergeRequest'
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/representation/to_hash.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Representation
+      module ToHash
+        # Converts the current representation to a Hash. The keys of this Hash
+        # will be Symbols.
+        def to_hash
+          hash = {}
+
+          attributes.each do |key, value|
+            hash[key] = convert_value_for_to_hash(value)
+          end
+
+          hash
+        end
+
+        def convert_value_for_to_hash(value)
+          if value.is_a?(Array)
+            value.map { |v| convert_value_for_to_hash(v) }
+          elsif value.respond_to?(:to_hash)
+            value.to_hash
+          else
+            value
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/representation/user.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    module Representation
+      class User
+        include ToHash
+        include ExposeAttribute
+
+        attr_reader :attributes
+
+        expose_attribute :id, :login
+
+        # Builds a user from a GitHub API response.
+        #
+        # user - An instance of `Sawyer::Resource` containing the user details.
+        def self.from_api_response(user)
+          new(id: user.id, login: user.login)
+        end
+
+        # Builds a user using a Hash that was built from a JSON payload.
+        def self.from_json_hash(raw_hash)
+          new(Representation.symbolize_hash(raw_hash))
+        end
+
+        # attributes - A Hash containing the user details. The keys of this
+        #              Hash (and any nested hashes) must be symbols.
+        def initialize(attributes)
+          @attributes = attributes
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/sequential_importer.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # The SequentialImporter imports a GitHub project in a single thread,
+    # without using Sidekiq. This makes it useful for testing purposes as well
+    # as Rake tasks, but it should be avoided for anything else in favour of the
+    # parallel importer.
+    class SequentialImporter
+      attr_reader :project, :client
+
+      SEQUENTIAL_IMPORTERS = [
+        Importer::LabelsImporter,
+        Importer::MilestonesImporter,
+        Importer::ReleasesImporter
+      ].freeze
+
+      PARALLEL_IMPORTERS = [
+        Importer::PullRequestsImporter,
+        Importer::IssuesImporter,
+        Importer::DiffNotesImporter,
+        Importer::NotesImporter
+      ].freeze
+
+      # project - The project to import the data into.
+      # token - The token to use for the GitHub API.
+      def initialize(project, token: nil)
+        @project = project
+        @client = GithubImport
+          .new_client_for(project, token: token, parallel: false)
+      end
+
+      def execute
+        Importer::RepositoryImporter.new(project, client).execute
+
+        SEQUENTIAL_IMPORTERS.each do |klass|
+          klass.new(project, client).execute
+        end
+
+        PARALLEL_IMPORTERS.each do |klass|
+          klass.new(project, client, parallel: false).execute
+        end
+
+        project.repository.after_import
+
+        true
+      end
+    end
+  end
+end

lib/gitlab/github_import/user_finder.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module GithubImport
+    # Class that can be used for finding a GitLab user ID based on a GitHub user
+    # ID or username.
+    #
+    # Any found user IDs are cached in Redis to reduce the number of SQL queries
+    # executed over time. Valid keys are refreshed upon access so frequently
+    # used keys stick around.
+    #
+    # Lookups are cached even if no ID was found to remove the need for querying
+    # the database when most queries are not going to return results anyway.
+    class UserFinder
+      attr_reader :project, :client
+
+      # The base cache key to use for caching user IDs for a given GitHub user
+      # ID.
+      ID_CACHE_KEY = 'github-import/user-finder/user-id/%s'.freeze
+
+      # The base cache key to use for caching user IDs for a given GitHub email
+      # address.
+      ID_FOR_EMAIL_CACHE_KEY =
+        'github-import/user-finder/id-for-email/%s'.freeze
+
+      # The base cache key to use for caching the Email addresses of GitHub
+      # usernames.
+      EMAIL_FOR_USERNAME_CACHE_KEY =
+        'github-import/user-finder/email-for-username/%s'.freeze
+
+      # project - An instance of `Project`
+      # client - An instance of `Gitlab::GithubImport::Client`
+      def initialize(project, client)
+        @project = project
+        @client = client
+      end
+
+      # Returns the GitLab user ID of an object's author.
+      #
+      # If the object has no author ID we'll use the ID of the GitLab ghost
+      # user.
+      def author_id_for(object)
+        id =
+          if object&.author
+            user_id_for(object.author)
+          else
+            GithubImport.ghost_user_id
+          end
+
+        if id
+          [id, true]
+        else
+          [project.creator_id, false]
+        end
+      end
+
+      # Returns the GitLab user ID of an issuable's assignee.
+      def assignee_id_for(issuable)
+        user_id_for(issuable.assignee) if issuable.assignee
+      end
+
+      # Returns the GitLab user ID for a GitHub user.
+      #
+      # user - An instance of `Gitlab::GithubImport::Representation::User`.
+      def user_id_for(user)
+        find(user.id, user.login)
+      end
+
+      # Returns the GitLab ID for the given GitHub ID or username.
+      #
+      # id - The ID of the GitHub user.
+      # username - The username of the GitHub user.
+      def find(id, username)
+        email = email_for_github_username(username)
+        cached, found_id = find_from_cache(id, email)
+
+        return found_id if found_id
+
+        # We only want to query the database if necessary. If previous lookups
+        # didn't yield a user ID we won't query the database again until the
+        # keys expire.
+        find_id_from_database(id, email) unless cached
+      end
+
+      # Finds a user ID from the cache for a given GitHub ID or Email.
+      def find_from_cache(id, email = nil)
+        id_exists, id_for_github_id = cached_id_for_github_id(id)
+
+        return [id_exists, id_for_github_id] if id_for_github_id
+
+        # Just in case no Email address could be retrieved (for whatever reason)
+        return [false] unless email
+
+        cached_id_for_github_email(email)
+      end
+
+      # Finds a GitLab user ID from the database for a given GitHub user ID or
+      # Email.
+      def find_id_from_database(id, email)
+        id_for_github_id(id) || id_for_github_email(email)
+      end
+
+      def email_for_github_username(username)
+        cache_key = EMAIL_FOR_USERNAME_CACHE_KEY % username
+        email = Caching.read(cache_key)
+
+        unless email
+          user = client.user(username)
+          email = Caching.write(cache_key, user.email) if user
+        end
+
+        email
+      end
+
+      def cached_id_for_github_id(id)
+        read_id_from_cache(ID_CACHE_KEY % id)
+      end
+
+      def cached_id_for_github_email(email)
+        read_id_from_cache(ID_FOR_EMAIL_CACHE_KEY % email)
+      end
+
+      # Queries and caches the GitLab user ID for a GitHub user ID, if one was
+      # found.
+      def id_for_github_id(id)
+        gitlab_id = query_id_for_github_id(id) || nil
+
+        Caching.write(ID_CACHE_KEY % id, gitlab_id)
+      end
+
+      # Queries and caches the GitLab user ID for a GitHub email, if one was
+      # found.
+      def id_for_github_email(email)
+        gitlab_id = query_id_for_github_email(email) || nil
+
+        Caching.write(ID_FOR_EMAIL_CACHE_KEY % email, gitlab_id)
+      end
+
+      def query_id_for_github_id(id)
+        User.for_github_id(id).pluck(:id).first
+      end
+
+      def query_id_for_github_email(email)
+        User.by_any_email(email).pluck(:id).first
+      end
+
+      # Reads an ID from the cache.
+      #
+      # The return value is an Array with two values:
+      #
+      # 1. A boolean indicating if the key was present or not.
+      # 2. The ID as an Integer, or nil in case no ID could be found.
+      def read_id_from_cache(key)
+        value = Caching.read(key)
+        exists = !value.nil?
+        number = value.to_i
+
+        # The cache key may be empty to indicate a previously looked up user for
+        # which we couldn't find an ID.
+        [exists, number.positive? ? number : nil]
+      end
+    end
+  end
+end

lib/gitlab/import_sources.rb

@@ -15,7 +15,7 @@ module Gitlab
       ImportSource.new('fogbugz',        'FogBugz',       Gitlab::FogbugzImport::Importer),
       ImportSource.new('git',            'Repo by URL',   nil),
       ImportSource.new('gitlab_project', 'GitLab export', Gitlab::ImportExport::Importer),
-      ImportSource.new('gitea',          'Gitea',         Gitlab::GithubImport::Importer)
+      ImportSource.new('gitea',          'Gitea',         Gitlab::LegacyGithubImport::Importer)
     ].freeze
 
     class << self

lib/gitlab/job_waiter.rb

@@ -19,11 +19,13 @@ module Gitlab
       Gitlab::Redis::SharedState.with { |redis| redis.lpush(key, jid) }
     end
 
-    attr_reader :key, :jobs_remaining, :finished
+    attr_reader :key, :finished
+    attr_accessor :jobs_remaining
 
     # jobs_remaining - the number of jobs left to wait for
-    def initialize(jobs_remaining)
-      @key = "gitlab:job_waiter:#{SecureRandom.uuid}"
+    # key - The key of this waiter.
+    def initialize(jobs_remaining = 0, key = "gitlab:job_waiter:#{SecureRandom.uuid}")
+      @key = key
       @jobs_remaining = jobs_remaining
       @finished = []
     end

lib/gitlab/github_import/base_formatter.rb → lib/gitlab/legacy_github_import/base_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class BaseFormatter
       attr_reader :client, :formatter, :project, :raw_data
 

lib/gitlab/github_import/branch_formatter.rb → lib/gitlab/legacy_github_import/branch_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class BranchFormatter < BaseFormatter
       delegate :repo, :sha, :ref, to: :raw_data
 

lib/gitlab/legacy_github_import/client.rb

+module Gitlab
+  module LegacyGithubImport
+    class Client
+      GITHUB_SAFE_REMAINING_REQUESTS = 100
+      GITHUB_SAFE_SLEEP_TIME = 500
+
+      attr_reader :access_token, :host, :api_version
+
+      def initialize(access_token, host: nil, api_version: 'v3')
+        @access_token = access_token
+        @host = host.to_s.sub(%r{/+\z}, '')
+        @api_version = api_version
+        @users = {}
+
+        if access_token
+          ::Octokit.auto_paginate = false
+        end
+      end
+
+      def api
+        @api ||= ::Octokit::Client.new(
+          access_token: access_token,
+          api_endpoint: api_endpoint,
+          # If there is no config, we're connecting to github.com and we
+          # should verify ssl.
+          connection_options: {
+            ssl: { verify: config ? config['verify_ssl'] : true }
+          }
+        )
+      end
+
+      def client
+        unless config
+          raise Projects::ImportService::Error,
+            'OAuth configuration for GitHub missing.'
+        end
+
+        @client ||= ::OAuth2::Client.new(
+          config.app_id,
+          config.app_secret,
+          github_options.merge(ssl: { verify: config['verify_ssl'] })
+        )
+      end
+
+      def authorize_url(redirect_uri)
+        client.auth_code.authorize_url({
+          redirect_uri: redirect_uri,
+          scope: "repo, user, user:email"
+        })
+      end
+
+      def get_token(code)
+        client.auth_code.get_token(code).token
+      end
+
+      def method_missing(method, *args, &block)
+        if api.respond_to?(method)
+          request(method, *args, &block)
+        else
+          super(method, *args, &block)
+        end
+      end
+
+      def respond_to?(method)
+        api.respond_to?(method) || super
+      end
+
+      def user(login)
+        return nil unless login.present?
+        return @users[login] if @users.key?(login)
+
+        @users[login] = api.user(login)
+      end
+
+      private
+
+      def api_endpoint
+        if host.present? && api_version.present?
+          "#{host}/api/#{api_version}"
+        else
+          github_options[:site]
+        end
+      end
+
+      def config
+        Gitlab.config.omniauth.providers.find { |provider| provider.name == "github" }
+      end
+
+      def github_options
+        if config
+          config["args"]["client_options"].deep_symbolize_keys
+        else
+          OmniAuth::Strategies::GitHub.default_options[:client_options].symbolize_keys
+        end
+      end
+
+      def rate_limit
+        api.rate_limit!
+      # GitHub Rate Limit API returns 404 when the rate limit is
+      # disabled. In this case we just want to return gracefully
+      # instead of spitting out an error.
+      rescue Octokit::NotFound
+        nil
+      end
+
+      def has_rate_limit?
+        return @has_rate_limit if defined?(@has_rate_limit)
+
+        @has_rate_limit = rate_limit.present?
+      end
+
+      def rate_limit_exceed?
+        has_rate_limit? && rate_limit.remaining <= GITHUB_SAFE_REMAINING_REQUESTS
+      end
+
+      def rate_limit_sleep_time
+        rate_limit.resets_in + GITHUB_SAFE_SLEEP_TIME
+      end
+
+      def request(method, *args, &block)
+        sleep rate_limit_sleep_time if rate_limit_exceed?
+
+        data = api.__send__(method, *args) # rubocop:disable GitlabSecurity/PublicSend
+        return data unless data.is_a?(Array)
+
+        last_response = api.last_response
+
+        if block_given?
+          yield data
+          # api.last_response could change while we're yielding (e.g. fetching labels for each PR)
+          # so we cache our own last response
+          each_response_page(last_response, &block)
+        else
+          each_response_page(last_response) { |page| data.concat(page) }
+          data
+        end
+      end
+
+      def each_response_page(last_response)
+        while last_response.rels[:next]
+          sleep rate_limit_sleep_time if rate_limit_exceed?
+          last_response = last_response.rels[:next].get
+          yield last_response.data if last_response.data.is_a?(Array)
+        end
+      end
+    end
+  end
+end

lib/gitlab/github_import/comment_formatter.rb → lib/gitlab/legacy_github_import/comment_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class CommentFormatter < BaseFormatter
       attr_writer :author_id
 

lib/gitlab/github_import/importer.rb → lib/gitlab/legacy_github_import/importer.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class Importer
       include Gitlab::ShellAdapter
 

lib/gitlab/github_import/issuable_formatter.rb → lib/gitlab/legacy_github_import/issuable_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class IssuableFormatter < BaseFormatter
       attr_writer :assignee_id, :author_id
 

lib/gitlab/github_import/issue_formatter.rb → lib/gitlab/legacy_github_import/issue_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class IssueFormatter < IssuableFormatter
       def attributes
         {

lib/gitlab/github_import/label_formatter.rb → lib/gitlab/legacy_github_import/label_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class LabelFormatter < BaseFormatter
       def attributes
         {

lib/gitlab/github_import/milestone_formatter.rb → lib/gitlab/legacy_github_import/milestone_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class MilestoneFormatter < BaseFormatter
       def attributes
         {

lib/gitlab/github_import/project_creator.rb → lib/gitlab/legacy_github_import/project_creator.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class ProjectCreator
       include Gitlab::CurrentSettings
 

lib/gitlab/github_import/pull_request_formatter.rb → lib/gitlab/legacy_github_import/pull_request_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class PullRequestFormatter < IssuableFormatter
       delegate :user, :project, :ref, :repo, :sha, to: :source_branch, prefix: true
       delegate :user, :exists?, :project, :ref, :repo, :sha, :short_sha, to: :target_branch, prefix: true

lib/gitlab/github_import/release_formatter.rb → lib/gitlab/legacy_github_import/release_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class ReleaseFormatter < BaseFormatter
       def attributes
         {

lib/gitlab/github_import/user_formatter.rb → lib/gitlab/legacy_github_import/user_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class UserFormatter
       attr_reader :client, :raw
 

lib/gitlab/github_import/wiki_formatter.rb → lib/gitlab/legacy_github_import/wiki_formatter.rb

 module Gitlab
-  module GithubImport
+  module LegacyGithubImport
     class WikiFormatter
       attr_reader :project
 

spec/controllers/import/github_controller_spec.rb

@@ -21,9 +21,9 @@ describe Import::GithubController do
   describe "GET callback" do
     it "updates access token" do
       token = "asdasd12345"
-      allow_any_instance_of(Gitlab::GithubImport::Client)
+      allow_any_instance_of(Gitlab::LegacyGithubImport::Client)
         .to receive(:get_token).and_return(token)
-      allow_any_instance_of(Gitlab::GithubImport::Client)
+      allow_any_instance_of(Gitlab::LegacyGithubImport::Client)
         .to receive(:github_options).and_return({})
       stub_omniauth_provider('github')
 

spec/lib/gitlab/git/repository_spec.rb

@@ -648,6 +648,21 @@ describe Gitlab::Git::Repository, seed_helper: true do
     end
   end
 
+  describe '#remote_exists?' do
+    before(:all) do
+      @repo = Gitlab::Git::Repository.new('default', TEST_MUTABLE_REPO_PATH, '')
+      @repo.add_remote("new_remote", SeedHelper::GITLAB_GIT_TEST_REPO_URL)
+    end
+
+    it 'returns true for an existing remote' do
+      expect(@repo.remote_exists?('new_remote')).to eq(true)
+    end
+
+    it 'returns false for a non-existing remote' do
+      expect(@repo.remote_exists?('foo')).to eq(false)
+    end
+  end
+
   describe "#log" do
     let(:commit_with_old_name) do
       Gitlab::Git::Commit.decorate(repository, @commit_with_old_name_id)

spec/lib/gitlab/github_import/bulk_importing_spec.rb

+require 'spec_helper'
+
+describe Gitlab::GithubImport::BulkImporting do
+  let(:importer) do
+    Class.new { include(Gitlab::GithubImport::BulkImporting) }.new
+  end
+
+  describe '#build_database_rows' do
+    it 'returns an Array containing the rows to insert' do
+      object = double(:object, title: 'Foo')
+
+      expect(importer)
+        .to receive(:build)
+        .with(object)
+        .and_return({ title: 'Foo' })
+
+      expect(importer)
+        .to receive(:already_imported?)
+        .with(object)
+        .and_return(false)
+
+      enum = [[object, 1]].to_enum
+
+      expect(importer.build_database_rows(enum)).to eq([{ title: 'Foo' }])
+    end
+
+    it 'does not import objects that have already been imported' do
+      object = double(:object, title: 'Foo')
+
+      expect(importer)
+        .not_to receive(:build)
+
+      expect(importer)
+        .to receive(:already_imported?)
+        .with(object)
+        .and_return(true)
+
+      enum = [[object, 1]].to_enum
+
+      expect(importer.build_database_rows(enum)).to be_empty
+    end
+  end
+
+  describe '#bulk_insert' do
+    it 'bulk inserts rows into the database' do
+      rows = [{ title: 'Foo' }] * 10
+      model = double(:model, table_name: 'kittens')
+
+      expect(Gitlab::Database)
+        .to receive(:bulk_insert)
+        .ordered
+        .with('kittens', rows.first(5))
+
+      expect(Gitlab::Database)
+        .to receive(:bulk_insert)
+        .ordered
+        .with('kittens', rows.last(5))
+
+      importer.bulk_insert(model, rows, batch_size: 5)
+    end
+  end
+end

spec/lib/gitlab/github_import/caching_spec.rb

+require 'spec_helper'
+
+describe Gitlab::GithubImport::Caching, :clean_gitlab_redis_cache do
+  describe '.read' do
+    it 'reads a value from the cache' do
+      described_class.write('foo', 'bar')
+
+      expect(described_class.read('foo')).to eq('bar')
+    end
+
+    it 'returns nil if the cache key does not exist' do
+      expect(described_class.read('foo')).to be_nil
+    end
+
+    it 'refreshes the cache key if a value is present' do
+      described_class.write('foo', 'bar')
+
+      redis = double(:redis)
+
+      expect(redis).to receive(:get).with(/foo/).and_return('bar')
+      expect(redis).to receive(:expire).with(/foo/, described_class::TIMEOUT)
+      expect(Gitlab::Redis::Cache).to receive(:with).twice.and_yield(redis)
+
+      described_class.read('foo')
+    end
+
+    it 'does not refresh the cache key if a value is empty' do
+      described_class.write('foo', nil)
+
+      redis = double(:redis)
+
+      expect(redis).to receive(:get).with(/foo/).and_return('')
+      expect(redis).not_to receive(:expire)
+      expect(Gitlab::Redis::Cache).to receive(:with).and_yield(redis)
+
+      described_class.read('foo')
+    end
+  end
+
+  describe '.read_integer' do
+    it 'returns an Integer' do
+      described_class.write('foo', '10')
+
+      expect(described_class.read_integer('foo')).to eq(10)
+    end
+
+    it 'returns nil if no value was found' do
+      expect(described_class.read_integer('foo')).to be_nil
+    end
+  end
+
+  describe '.write' do
+    it 'writes a value to the cache and returns the written value' do
+      expect(described_class.write('foo', 10)).to eq(10)
+      expect(described_class.read('foo')).to eq('10')
+    end
+  end
+
+  describe '.set_add' do
+    it 'adds a value to a set' do
+      described_class.set_add('foo', 10)
+      described_class.set_add('foo', 10)
+
+      key = described_class.cache_key_for('foo')
+      values = Gitlab::Redis::Cache.with { |r| r.smembers(key) }
+
+      expect(values).to eq(['10'])
+    end
+  end
+
+  describe '.set_includes?' do
+    it 'returns false when the key does not exist' do
+      expect(described_class.set_includes?('foo', 10)).to eq(false)
+    end
+
+    it 'returns false when the value is not present in the set' do
+      described_class.set_add('foo', 10)
+
+      expect(described_class.set_includes?('foo', 20)).to eq(false)
+    end
+
+    it 'returns true when the set includes the given value' do
+      described_class.set_add('foo', 10)
+
+      expect(described_class.set_includes?('foo', 10)).to eq(true)
+    end
+  end
+
+  describe '.write_multiple' do
+    it 'sets multiple keys' do
+      mapping = { 'foo' => 10, 'bar' => 20 }
+
+      described_class.write_multiple(mapping)
+
+      mapping.each do |key, value|
+        full_key = described_class.cache_key_for(key)
+        found = Gitlab::Redis::Cache.with { |r| r.get(full_key) }
+
+        expect(found).to eq(value.to_s)
+      end
+    end
+  end
+
+  describe '.expire' do
+    it 'sets the expiration time of a key' do
+      timeout = 1.hour.to_i
+
+      described_class.write('foo', 'bar', timeout: 2.hours.to_i)
+      described_class.expire('foo', timeout)
+
+      key = described_class.cache_key_for('foo')
+      found_ttl = Gitlab::Redis::Cache.with { |r| r.ttl(key) }
+
+      expect(found_ttl).to be <= timeout
+    end
+  end
+end

spec/lib/gitlab/github_import/importer/diff_note_importer_spec.rb

+require 'spec_helper'
+
+describe Gitlab::GithubImport::Importer::DiffNoteImporter do
+  let(:project) { create(:project) }
+  let(:client) { double(:client) }
+  let(:user) { create(:user) }
+  let(:created_at) { Time.new(2017, 1, 1, 12, 00) }
+  let(:updated_at) { Time.new(2017, 1, 1, 12, 15) }
+
+  let(:hunk) do
+    '@@ -1 +1 @@
+    -Hello
+    +Hello world'
+  end
+
+  let(:note) do
+    Gitlab::GithubImport::Representation::DiffNote.new(
+      noteable_type: 'MergeRequest',
+      noteable_id: 1,
+      commit_id: '123abc',
+      file_path: 'README.md',
+      diff_hunk: hunk,
+      author: Gitlab::GithubImport::Representation::User
+        .new(id: user.id, login: user.username),
+      note: 'Hello',
+      created_at: created_at,
+      updated_at: updated_at,
+      github_id: 1
+    )
+  end
+
+  let(:importer) { described_class.new(note, project, client) }
+
+  describe '#execute' do
+    context 'when the merge request no longer exists' do
+      it 'does not import anything' do
+        expect(Gitlab::Database).not_to receive(:bulk_insert)
+
+        importer.execute
+      end
+    end
+
+    context 'when the merge request exists' do
+      let!(:merge_request) do
+        create(:merge_request, source_project: project, target_project: project)
+      end
+
+      before do
+        allow(importer)
+          .to receive(:find_merge_request_id)
+          .and_return(merge_request.id)
+      end
+
+      it 'imports the note' do
+        allow(importer.user_finder)
+          .to receive(:author_id_for)
+          .and_return([user.id, true])
+
+        expect(Gitlab::Database)
+          .to receive(:bulk_insert)
+          .with(
+            LegacyDiffNote.table_name,
+            [
+              {
+                noteable_type: 'MergeRequest',
+                noteable_id: merge_request.id,
+                project_id: project.id,
+                author_id: user.id,
+                note: 'Hello',
+                system: false,
+                commit_id: '123abc',
+                line_code: note.line_code,
+                type: 'LegacyDiffNote',
+                created_at: created_at,
+                updated_at: updated_at,
+                st_diff: note.diff_hash.to_yaml
+              }
+            ]
+          )
+          .and_call_original
+
+        importer.execute
+      end
+
+      it 'imports the note when the author could not be found' do
+        allow(importer.user_finder)
+          .to receive(:author_id_for)
+          .and_return([project.creator_id, false])
+
+        expect(Gitlab::Database)
+          .to receive(:bulk_insert)
+          .with(
+            LegacyDiffNote.table_name,
+            [
+              {
+                noteable_type: 'MergeRequest',
+                noteable_id: merge_request.id,
+                project_id: project.id,
+                author_id: project.creator_id,
+                note: "*Created by: #{user.username}*\n\nHello",
+                system: false,
+                commit_id: '123abc',
+                line_code: note.line_code,
+                type: 'LegacyDiffNote',
+                created_at: created_at,
+                updated_at: updated_at,
+                st_diff: note.diff_hash.to_yaml
+              }
+            ]
+          )
+          .and_call_original
+
+        importer.execute
+      end
+
+      it 'produces a valid LegacyDiffNote' do
+        allow(importer.user_finder)
+          .to receive(:author_id_for)
+          .and_return([user.id, true])
+
+        importer.execute
+
+        note = project.notes.diff_notes.take
+
+        expect(note).to be_valid
+        expect(note.diff).to be_an_instance_of(Gitlab::Git::Diff)
+      end
+
+      it 'does not import the note when a foreign key error is raised' do
+        allow(importer.user_finder)
+          .to receive(:author_id_for)
+          .and_return([project.creator_id, false])
+
+        expect(Gitlab::Database)
+          .to receive(:bulk_insert)
+          .and_raise(ActiveRecord::InvalidForeignKey, 'invalid foreign key')
+
+        expect { importer.execute }.not_to raise_error
+      end
+    end
+  end
+
+  describe '#find_merge_request_id' do
+    it 'returns a merge request ID' do
+      expect_any_instance_of(Gitlab::GithubImport::IssuableFinder)
+        .to receive(:database_id)
+        .and_return(10)
+
+      expect(importer.find_merge_request_id).to eq(10)
+    end
+  end
+end

spec/lib/gitlab/github_import/importer/diff_notes_importer_spec.rb

+require 'spec_helper'
+
+describe Gitlab::GithubImport::Importer::DiffNotesImporter do
+  let(:project) { double(:project, id: 4, import_source: 'foo/bar') }
+  let(:client) { double(:client) }
+
+  let(:github_comment) do
+    double(
+      :response,
+      html_url: 'https://github.com/foo/bar/pull/42',
+      path: 'README.md',
+      commit_id: '123abc',
+      diff_hunk: "@@ -1 +1 @@\n-Hello\n+Hello world",
+      user: double(:user, id: 4, login: 'alice'),
+      body: 'Hello world',
+      created_at: Time.zone.now,
+      updated_at: Time.zone.now,
+      id: 1
+    )
+  end
+
+  describe '#parallel?' do
+    it 'returns true when running in parallel mode' do
+      importer = described_class.new(project, client)
+      expect(importer).to be_parallel
+    end
+
+    it 'returns false when running in sequential mode' do
+      importer = described_class.new(project, client, parallel: false)
+      expect(importer).not_to be_parallel
+    end
+  end
+
+  describe '#execute' do
+    context 'when running in parallel mode' do
+      it 'imports diff notes in parallel' do
+        importer = described_class.new(project, client)
+
+        expect(importer).to receive(:parallel_import)
+
+        importer.execute
+      end
+    end
+
+    context 'when running in sequential mode' do
+      it 'imports diff notes in sequence' do
+        importer = described_class.new(project, client, parallel: false)
+
+        expect(importer).to receive(:sequential_import)
+
+        importer.execute
+      end
+    end
+  end
+
+  describe '#sequential_import' do
+    it 'imports each diff note in sequence' do
+      importer = described_class.new(project, client, parallel: false)
+      diff_note_importer = double(:diff_note_importer)
+
+      allow(importer)
+        .to receive(:each_object_to_import)
+        .and_yield(github_comment)
+
+      expect(Gitlab::GithubImport::Importer::DiffNoteImporter)
+        .to receive(:new)
+        .with(
+          an_instance_of(Gitlab::GithubImport::Representation::DiffNote),
+          project,
+          client
+        )
+        .and_return(diff_note_importer)
+
+      expect(diff_note_importer).to receive(:execute)
+
+      importer.sequential_import
+    end
+  end
+
+  describe '#parallel_import' do
+    it 'imports each diff note in parallel' do
+      importer = described_class.new(project, client)
+
+      allow(importer)
+        .to receive(:each_object_to_import)
+        .and_yield(github_comment)
+
+      expect(Gitlab::GithubImport::ImportDiffNoteWorker)
+        .to receive(:perform_async)
+        .with(project.id, an_instance_of(Hash), an_instance_of(String))
+
+      waiter = importer.parallel_import
+
+      expect(waiter).to be_an_instance_of(Gitlab::JobWaiter)
+      expect(waiter.jobs_remaining).to eq(1)
+    end
+  end
+
+  describe '#id_for_already_imported_cache' do
+    it 'returns the ID of the given note' do
+      importer = described_class.new(project, client)
+
+      expect(importer.id_for_already_imported_cache(github_comment))
+        .to eq(1)
+    end
+  end
+
+  describe '#collection_options' do
+    it 'returns an empty Hash' do
+      # For large projects (e.g. kubernetes/kubernetes) GitHub's API may produce
+      # HTTP 500 errors when using explicit sorting options, regardless of what
+      # order you sort in. Not using any sorting options at all allows us to
+      # work around this.
+      importer = described_class.new(project, client)
+
+      expect(importer.collection_options).to eq({})
+    end
+  end
+end

spec/lib/gitlab/github_import/importer/issue_and_label_links_importer_spec.rb

+require 'spec_helper'
+
+describe Gitlab::GithubImport::Importer::IssueAndLabelLinksImporter do
+  describe '#execute' do
+    it 'imports an issue and its labels' do
+      issue = double(:issue)
+      project = double(:project)
+      client = double(:client)
+      label_links_instance = double(:label_links_importer)
+      importer = described_class.new(issue, project, client)
+
+      expect(Gitlab::GithubImport::Importer::IssueImporter)
+        .to receive(:import_if_issue)
+        .with(issue, project, client)
+
+      expect(Gitlab::GithubImport::Importer::LabelLinksImporter)
+        .to receive(:new)
+        .with(issue, project, client)
+        .and_return(label_links_instance)
+
+      expect(label_links_instance)
+        .to receive(:execute)
+
+      importer.execute
+    end
+  end
+end

spec/lib/gitlab/github_import/importer/issue_importer_spec.rb

+require 'spec_helper'
+
+describe Gitlab::GithubImport::Importer::IssueImporter, :clean_gitlab_redis_cache do
+  let(:project) { create(:project) }
+  let(:client) { double(:client) }
+  let(:user) { create(:user) }
+  let(:milestone) { create(:milestone, project: project) }
+  let(:created_at) { Time.new(2017, 1, 1, 12, 00) }
+  let(:updated_at) { Time.new(2017, 1, 1, 12, 15) }
+
+  let(:issue) do
+    Gitlab::GithubImport::Representation::Issue.new(
+      iid: 42,
+      title: 'My Issue',
+      description: 'This is my issue',
+      milestone_number: 1,
+      state: :opened,
+      assignees: [
+        Gitlab::GithubImport::Representation::User.new(id: 4, login: 'alice'),
+        Gitlab::GithubImport::Representation::User.new(id: 5, login: 'bob')
+      ],
+      label_names: %w[bug],
+      author: Gitlab::GithubImport::Representation::User.new(id: 4, login: 'alice'),
+      created_at: created_at,
+      updated_at: updated_at,
+      pull_request: false
+    )
+  end
+
+  describe '.import_if_issue' do
+    it 'imports an issuable if it is a regular issue' do
+      importer = double(:importer)
+
+      expect(described_class)
+        .to receive(:new)
+        .with(issue, project, client)
+        .and_return(importer)
+
+      expect(importer).to receive(:execute)
+
+      described_class.import_if_issue(issue, project, client)
+    end
+
+    it 'does not import the issuable if it is a pull request' do
+      expect(issue).to receive(:pull_request?).and_return(true)
+
+      expect(described_class).not_to receive(:new)
+
+      described_class.import_if_issue(issue, project, client)
+    end
+  end
+
+  describe '#execute' do
+    let(:importer) { described_class.new(issue, project, client) }
+
+    it 'creates the issue and assignees' do
+      expect(importer)
+        .to receive(:create_issue)
+        .and_return(10)
+
+      expect(importer)
+        .to receive(:create_assignees)
+        .with(10)
+
+      expect(importer.issuable_finder)
+        .to receive(:cache_database_id)
+        .with(10)
+
+      importer.execute
+    end
+  end
+
+  describe '#create_issue' do
+    let(:importer) { described_class.new(issue, project, client) }
+
+    before do
+      allow(importer.milestone_finder)
+        .to receive(:id_for)
+        .with(issue)
+        .and_return(milestone.id)
+    end
+
+    context 'when the issue author could be found' do
+      it 'creates the issue with the found author as the issue author' do
+        allow(importer.user_finder)
+          .to receive(:author_id_for)
+          .with(issue)
+          .and_return([user.id, true])
+
+        expect(Gitlab::GithubImport)
+          .to receive(:insert_and_return_id)
+          .with(
+            {
+              iid: 42,
+              title: 'My Issue',
+              author_id: user.id,
+              project_id: project.id,
+              description: 'This is my issue',
+              milestone_id: milestone.id,
+              state: :opened,
+              created_at: created_at,
+              updated_at: updated_at
+            },
+            project.issues
+          )
+          .and_call_original
+
+        importer.create_issue
+      end
+    end
+
+    context 'when the issue author could not be found' do
+      it 'creates the issue with the project creator as the issue author' do
+        allow(importer.user_finder)
+          .to receive(:author_id_for)
+          .with(issue)
+          .and_return([project.creator_id, false])
+
+        expect(Gitlab::GithubImport)
+          .to receive(:insert_and_return_id)
+          .with(
+            {
+              iid: 42,
+              title: 'My Issue',
+              author_id: project.creator_id,
+              project_id: project.id,
+              description: "*Created by: alice*\n\nThis is my issue",
+              milestone_id: milestone.id,
+              state: :opened,
+              created_at: created_at,
+              updated_at: updated_at
+            },
+            project.issues
+          )
+          .and_call_original
+
+        importer.create_issue
+      end
+    end
+
+    context 'when the import fails due to a foreign key error' do
+      it 'does not raise any errors' do
+        allow(importer.user_finder)
+          .to receive(:author_id_for)
+          .with(issue)
+          .and_return([user.id, true])
+
+        expect(Gitlab::GithubImport)
+          .to receive(:insert_and_return_id)
+          .and_raise(ActiveRecord::InvalidForeignKey, 'invalid foreign key')
+
+        expect { importer.create_issue }.not_to raise_error
+      end
+    end
+
+    it 'produces a valid Issue' do
+      allow(importer.user_finder)
+        .to receive(:author_id_for)
+        .with(issue)
+        .and_return([user.id, true])
+
+      importer.create_issue
+
+      expect(project.issues.take).to be_valid
+    end
+
+    it 'returns the ID of the created issue' do
+      allow(importer.user_finder)
+        .to receive(:author_id_for)
+        .with(issue)
+        .and_return([user.id, true])
+
+      expect(importer.create_issue).to be_a_kind_of(Numeric)
+    end
+  end
+
+  describe '#create_assignees' do
+    it 'inserts the issue assignees in bulk' do
+      importer = described_class.new(issue, project, client)
+
+      allow(importer.user_finder)
+        .to receive(:user_id_for)
+        .ordered.with(issue.assignees[0])
+        .and_return(4)
+
+      allow(importer.user_finder)
+        .to receive(:user_id_for)
+        .ordered.with(issue.assignees[1])
+        .and_return(5)
+
+      expect(Gitlab::Database)
+        .to receive(:bulk_insert)
+        .with(
+          IssueAssignee.table_name,
+          [{ issue_id: 1, user_id: 4 }, { issue_id: 1, user_id: 5 }]
+        )
+
+      importer.create_assignees(1)
+    end
+  end
+end

spec/lib/gitlab/github_import/importer/issues_importer_spec.rb

+require 'spec_helper'
+
+describe Gitlab::GithubImport::Importer::IssuesImporter do
+  let(:project) { double(:project, id: 4, import_source: 'foo/bar') }
+  let(:client) { double(:client) }
+  let(:created_at) { Time.new(2017, 1, 1, 12, 00) }
+  let(:updated_at) { Time.new(2017, 1, 1, 12, 15) }
+
+  let(:github_issue) do
+    double(
+      :response,
+      number: 42,
+      title: 'My Issue',
+      body: 'This is my issue',
+      milestone: double(:milestone, number: 4),
+      state: 'open',
+      assignees: [double(:user, id: 4, login: 'alice')],
+      labels: [double(:label, name: 'bug')],
+      user: double(:user, id: 4, login: 'alice'),
+      created_at: created_at,
+      updated_at: updated_at,
+      pull_request: false
+    )
+  end
+
+  describe '#parallel?' do
+    it 'returns true when running in parallel mode' do
+      importer = described_class.new(project, client)
+      expect(importer).to be_parallel
+    end
+
+    it 'returns false when running in sequential mode' do
+      importer = described_class.new(project, client, parallel: false)
+      expect(importer).not_to be_parallel
+    end
+  end
+
+  describe '#execute' do
+    context 'when running in parallel mode' do
+      it 'imports issues in parallel' do
+        importer = described_class.new(project, client)
+
+        expect(importer).to receive(:parallel_import)
+
+        importer.execute
+      end
+    end
+
+    context 'when running in sequential mode' do
+      it 'imports issues in sequence' do
+        importer = described_class.new(project, client, parallel: false)
+
+        expect(importer).to receive(:sequential_import)
+
+        importer.execute
+      end
+    end
+  end
+
+  describe '#sequential_import' do
+    it 'imports each issue in sequence' do
+      importer = described_class.new(project, client, parallel: false)
+      issue_importer = double(:importer)
+
+      allow(importer)
+        .to receive(:each_object_to_import)
+        .and_yield(github_issue)
+
+      expect(Gitlab::GithubImport::Importer::IssueAndLabelLinksImporter)
+        .to receive(:new)
+        .with(
+          an_instance_of(Gitlab::GithubImport::Representation::Issue),
+          project,
+          client
+        )
+        .and_return(issue_importer)
+
+      expect(issue_importer).to receive(:execute)
+
+      importer.sequential_import
+    end
+  end
+
+  describe '#parallel_import' do
+    it 'imports each issue in parallel' do
+      importer = described_class.new(project, client)
+
+      allow(importer)
+        .to receive(:each_object_to_import)
+        .and_yield(github_issue)
+
+      expect(Gitlab::GithubImport::ImportIssueWorker)
+        .to receive(:perform_async)
+        .with(project.id, an_instance_of(Hash), an_instance_of(String))
+
+      waiter = importer.parallel_import
+
+      expect(waiter).to be_an_instance_of(Gitlab::JobWaiter)
+      expect(waiter.jobs_remaining).to eq(1)
+    end
+  end
+
+  describe '#id_for_already_imported_cache' do
+    it 'returns the issue number of the given issue' do
+      importer = described_class.new(project, client)
+
+      expect(importer.id_for_already_imported_cache(github_issue))
+        .to eq(42)
+    end
+  end
+end