Remove unexpected FK prior to swapping columns

See https://gitlab.com/gitlab-org/gitlab/-/issues/341822

db/post_migrate/20210907211557_finalize_ci_builds_bigint_conversion.rb

@@ -103,6 +103,11 @@ class FinalizeCiBuildsBigintConversion < Gitlab::Database::Migration[1.0]
       remove_foreign_key(:ci_resources, TABLE_NAME, column: :build_id, name: 'fk_rails_e169a8e3d5')
     end
 
+    # Remove this unexpected FK if it exists - https://gitlab.com/gitlab-org/gitlab/-/issues/341822
+    if foreign_key_exists?(:ci_sources_pipelines, TABLE_NAME, column: :source_job_id, name: 'fk_3f0c88d7dc')
+      remove_foreign_key(:ci_sources_pipelines, TABLE_NAME, column: :source_job_id, name: 'fk_3f0c88d7dc')
+    end
+
     swap_columns
   end
 

spec/migrations/20210907211557_finalize_ci_builds_bigint_conversion_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require_migration!('finalize_ci_builds_bigint_conversion')
+
+RSpec.describe FinalizeCiBuildsBigintConversion, :migration, schema: 20210907182359 do
+  context 'with an unexpected FK fk_3f0c88d7dc' do
+    it 'removes the FK and migrates successfully' do
+      # Add the unexpected FK
+      subject.add_foreign_key(:ci_sources_pipelines, :ci_builds, column: :source_job_id, name: 'fk_3f0c88d7dc')
+
+      expect { migrate! }.to change { subject.foreign_key_exists?(:ci_sources_pipelines, :ci_builds, column: :source_job_id, name: 'fk_3f0c88d7dc') }.from(true).to(false)
+
+      # Additional check: The actually expected FK should still exist
+      expect(subject.foreign_key_exists?(:ci_sources_pipelines, :ci_builds, column: :source_job_id, name: 'fk_be5624bf37')).to be_truthy
+    end
+  end
+end

spec/support/database/cross-join-allowlist.yml

@@ -63,6 +63,7 @@
 - "./spec/lib/api/entities/package_spec.rb"
 - "./spec/lib/gitlab/background_migration/migrate_legacy_artifacts_spec.rb"
 - "./spec/mailers/emails/pipelines_spec.rb"
+- "./spec/migrations/20210907211557_finalize_ci_builds_bigint_conversion_spec.rb"
 - "./spec/migrations/cleanup_legacy_artifact_migration_spec.rb"
 - "./spec/migrations/migrate_protected_attribute_to_pending_builds_spec.rb"
 - "./spec/migrations/re_schedule_latest_pipeline_id_population_with_all_security_related_artifact_types_spec.rb"