Fix project imports for pipelines for merge requests

When a pipeline for a merge request was exported, the merge request was
not properly mapped, which caused a validation error, "Validation
failed: Merge request can't be blank".

Since neither merge requests nor CI pipelines have an explicit
relationship in `import_export.yml`, the full merge_request hash was
never exported. To make these older exports work, we now process
`merge_request_id` by replacing it with the newly-imported ID.

Closes https://gitlab.com/gitlab-org/gitlab/issues/31414

changelogs/unreleased/sh-fix-project-export-for-pipelines-for-mrs.yml

+---
+title: Fix project imports for pipelines for merge requests
+merge_request: 17799
+author:
+type: fixed

lib/gitlab/import_export/project_tree_restorer.rb

@@ -52,6 +52,11 @@ module Gitlab
                                                                     project: restored_project)
       end
 
+      # A Hash of the imported merge request ID -> imported ID.
+      def merge_requests_mapping
+        @merge_requests_mapping ||= {}
+      end
+
       # Loops through the tree of models defined in import_export.yml and
       # finds them in the imported JSON so they can be instantiated and saved
       # in the DB. The structure and relationships between models are guessed from
@@ -80,10 +85,26 @@ module Gitlab
 
         @saved = false unless restored_project.append_or_update_attribute(relation_key, relation_hash)
 
+        save_id_mappings(relation_key, relation_hash_batch, relation_hash)
+
         # Restore the project again, extra query that skips holding the AR objects in memory
         @restored_project = Project.find(@project_id)
       end
 
+      # Older, serialized CI pipeline exports may only have a
+      # merge_request_id and not the full hash of the merge request. To
+      # import these pipelines, we need to preserve the mapping between
+      # the old and new the merge request ID.
+      def save_id_mappings(relation_key, relation_hash_batch, relation_hash)
+        return unless relation_key == 'merge_requests'
+
+        relation_hash = Array(relation_hash)
+
+        Array(relation_hash_batch).each_with_index do |raw_data, index|
+          merge_requests_mapping[raw_data['id']] = relation_hash[index]['id']
+        end
+      end
+
       # Remove project models that became group models as we found them at group level.
       # This no longer required saving them at the root project level.
       # For example, in the case of an existing group label that matched the title.
@@ -222,6 +243,7 @@ module Gitlab
             relation_sym: relation_key.to_sym,
             relation_hash: relation_hash,
             members_mapper: members_mapper,
+            merge_requests_mapping: merge_requests_mapping,
             user: @user,
             project: @restored_project,
             excluded_keys: excluded_keys_for_relation(relation_key))

lib/gitlab/import_export/relation_factory.rb

@@ -55,10 +55,11 @@ module Gitlab
         relation_name.to_s.constantize
       end
 
-      def initialize(relation_sym:, relation_hash:, members_mapper:, user:, project:, excluded_keys: [])
+      def initialize(relation_sym:, relation_hash:, members_mapper:, merge_requests_mapping:, user:, project:, excluded_keys: [])
         @relation_name = self.class.overrides[relation_sym]&.to_sym || relation_sym
         @relation_hash = relation_hash.except('noteable_id')
         @members_mapper = members_mapper
+        @merge_requests_mapping = merge_requests_mapping
         @user = user
         @project = project
         @imported_object_retries = 0
@@ -109,7 +110,10 @@ module Gitlab
         update_group_references
         remove_duplicate_assignees
 
-        setup_pipeline if @relation_name == :'Ci::Pipeline'
+        if @relation_name == :'Ci::Pipeline'
+          update_merge_request_references
+          setup_pipeline
+        end
 
         reset_tokens!
         remove_encrypted_attributes!
@@ -194,6 +198,28 @@ module Gitlab
         @relation_hash['group_id'] = @project.namespace_id
       end
 
+      # This code is a workaround for broken project exports that don't
+      # export merge requests with CI pipelines (i.e. exports that were
+      # generated from
+      # https://gitlab.com/gitlab-org/gitlab/merge_requests/17844).
+      # This method can be removed in GitLab 12.6.
+      def update_merge_request_references
+        # If a merge request was properly created, we don't need to fix
+        # up this export.
+        return if @relation_hash['merge_request']
+
+        merge_request_id = @relation_hash['merge_request_id']
+
+        return unless merge_request_id
+
+        new_merge_request_id = @merge_requests_mapping[merge_request_id]
+
+        return unless new_merge_request_id
+
+        @relation_hash['merge_request_id'] = new_merge_request_id
+        parsed_relation_hash['merge_request_id'] = new_merge_request_id
+      end
+
       def reset_tokens!
         return unless Gitlab::ImportExport.reset_tokens? && TOKEN_RESET_MODELS.include?(@relation_name)
 

spec/fixtures/lib/gitlab/import_export/project.json

@@ -6175,6 +6175,8 @@
       "finished_at": null,
       "user_id": 9999,
       "duration": null,
+      "source": "push",
+      "merge_request_id": null,
       "notes": [
         {
           "id": 999,

spec/lib/gitlab/import_export/project_tree_restorer_spec.rb

@@ -96,6 +96,17 @@ describe Gitlab::ImportExport::ProjectTreeRestorer do
         expect(Ci::Pipeline.where(ref: nil)).not_to be_empty
       end
 
+      it 'restores pipeline for merge request' do
+        pipeline = Ci::Pipeline.find_by_sha('048721d90c449b244b7b4c53a9186b04330174ec')
+
+        expect(pipeline).to be_valid
+        expect(pipeline.tag).to be_falsey
+        expect(pipeline.source).to eq('merge_request_event')
+        expect(pipeline.merge_request.id).to be > 0
+        expect(pipeline.merge_request.target_branch).to eq('feature')
+        expect(pipeline.merge_request.source_branch).to eq('feature_conflict')
+      end
+
       it 'preserves updated_at on issues' do
         issue = Issue.where(description: 'Aliquam enim illo et possimus.').first
 

spec/lib/gitlab/import_export/relation_factory_spec.rb

@@ -3,12 +3,14 @@ require 'spec_helper'
 describe Gitlab::ImportExport::RelationFactory do
   let(:project) { create(:project) }
   let(:members_mapper) { double('members_mapper').as_null_object }
+  let(:merge_requests_mapping) { {} }
   let(:user) { create(:admin) }
   let(:excluded_keys) { [] }
   let(:created_object) do
     described_class.create(relation_sym: relation_sym,
                            relation_hash: relation_hash,
                            members_mapper: members_mapper,
+                           merge_requests_mapping: merge_requests_mapping,
                            user: user,
                            project: project,
                            excluded_keys: excluded_keys)