Fix null source_project_id in pool_repositories

Due to a bug, some pool_repositories in production have a null
source_project_id column. This migration aims to fix those rows.

changelogs/unreleased/jc-migration-for-source-project-id.yml

+---
+title: Fix null source_project_id in pool_repositories
+merge_request: 29157
+author:
+type: other

db/migrate/20190604184643_fix_pool_repository_source_project_id.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class FixPoolRepositorySourceProjectId < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  def up
+    execute "UPDATE pool_repositories SET source_project_id = (SELECT MIN(id) FROM projects WHERE pool_repository_id = pool_repositories.id) WHERE pool_repositories.source_project_id IS NULL"
+  end
+
+  def down
+    # nothing to do her
+  end
+end

spec/migrations/fix_pool_repository_source_project_id_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20190604184643_fix_pool_repository_source_project_id.rb')
+
+describe FixPoolRepositorySourceProjectId, :migration do
+  let(:projects) { table(:projects) }
+  let(:pool_repositories) { table(:pool_repositories) }
+  let(:shards) { table(:shards) }
+
+  it 'fills in source_project_ids' do
+    shard = shards.create!(name: 'default')
+
+    # gitaly is a project with a pool repository that has a source_project_id
+    gitaly = projects.create!(name: 'gitaly', path: 'gitlab-org/gitaly', namespace_id: 1)
+    pool_repository = pool_repositories.create(shard_id: shard.id, source_project_id: gitaly.id)
+    gitaly.update_column(:pool_repository_id, pool_repository.id)
+
+    # gitlab is a project with a pool repository that's missing a source_project_id
+    pool_repository_without_source_project = pool_repositories.create(shard_id: shard.id, source_project_id: nil)
+    gitlab = projects.create!(name: 'gitlab', path: 'gitlab-org/gitlab-ce', namespace_id: 1, pool_repository_id: pool_repository_without_source_project.id)
+    projects.create!(name: 'gitlab-fork-1', path: 'my-org-1/gitlab-ce', namespace_id: 1, pool_repository_id: pool_repository_without_source_project.id)
+
+    migrate!
+
+    expect(pool_repositories.find(pool_repository_without_source_project.id).source_project_id).to eq(gitlab.id)
+    expect(pool_repositories.find(pool_repository.id).source_project_id).to eq(gitaly.id)
+  end
+end