Add spec to validate we do not add more cross-database foreign keys

We are trying to remove all cross-database foreign keys in
https://gitlab.com/groups/gitlab-org/-/epics/7249 . Cross-database
foreign keys are foreign keys that span the future decomposed `ci` and
`main` databases. In an effort to avoid adding more while this work is
ongoing it will help to have a spec in place to prevent it. This spec
should also be kept after we finish that work to ensure nobody adds more
cross-database foreign keys in future.

spec/lib/gitlab/database/no_cross_db_foreign_keys_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'cross-database foreign keys' do
+  # TODO: We are trying to empty out this list in
+  # https://gitlab.com/groups/gitlab-org/-/epics/7249 . Once we are done we can
+  # keep this test and assert that there are no cross-db foreign keys. We
+  # should not be adding anything to this list but should instead only add new
+  # loose foreign keys
+  # https://docs.gitlab.com/ee/development/database/loose_foreign_keys.html .
+  let(:allowed_cross_database_foreign_keys) do
+    %w(
+      ci_build_report_results.project_id
+      ci_builds.project_id
+      ci_builds_metadata.project_id
+      ci_daily_build_group_report_results.group_id
+      ci_daily_build_group_report_results.project_id
+      ci_freeze_periods.project_id
+      ci_job_artifacts.project_id
+      ci_job_token_project_scope_links.added_by_id
+      ci_job_token_project_scope_links.source_project_id
+      ci_job_token_project_scope_links.target_project_id
+      ci_pending_builds.namespace_id
+      ci_pending_builds.project_id
+      ci_pipeline_artifacts.project_id
+      ci_pipeline_schedules.owner_id
+      ci_pipeline_schedules.project_id
+      ci_pipelines.merge_request_id
+      ci_pipelines.project_id
+      ci_project_monthly_usages.project_id
+      ci_refs.project_id
+      ci_resource_groups.project_id
+      ci_runner_namespaces.namespace_id
+      ci_runner_projects.project_id
+      ci_running_builds.project_id
+      ci_sources_pipelines.project_id
+      ci_sources_pipelines.source_project_id
+      ci_sources_projects.source_project_id
+      ci_stages.project_id
+      ci_subscriptions_projects.downstream_project_id
+      ci_subscriptions_projects.upstream_project_id
+      ci_triggers.owner_id
+      ci_triggers.project_id
+      ci_unit_tests.project_id
+      ci_variables.project_id
+      dast_profiles_pipelines.ci_pipeline_id
+      dast_scanner_profiles_builds.ci_build_id
+      dast_site_profiles_builds.ci_build_id
+      dast_site_profiles_pipelines.ci_pipeline_id
+      external_pull_requests.project_id
+      merge_requests.head_pipeline_id
+      merge_trains.pipeline_id
+      requirements_management_test_reports.build_id
+      security_scans.build_id
+      vulnerability_feedback.pipeline_id
+      vulnerability_occurrence_pipelines.pipeline_id
+      vulnerability_statistics.latest_pipeline_id
+    ).freeze
+  end
+
+  def fks_query(table)
+    <<~SQL
+      SELECT
+        tc.constraint_name,
+        tc.table_name,
+        kcu.column_name,
+        ccu.table_name AS foreign_table_name
+      FROM information_schema.table_constraints AS tc
+        JOIN information_schema.key_column_usage AS kcu
+          ON tc.constraint_name = kcu.constraint_name
+          AND tc.table_schema = kcu.table_schema
+        JOIN information_schema.constraint_column_usage AS ccu
+          ON ccu.constraint_name = tc.constraint_name
+          AND ccu.table_schema = tc.table_schema
+        JOIN information_schema.referential_constraints as rc
+          ON tc.constraint_name = rc.constraint_name
+      WHERE tc.constraint_type = 'FOREIGN KEY' AND tc.table_name='#{table}';
+    SQL
+  end
+
+  def is_cross_db?(fk_record)
+    Gitlab::Database::GitlabSchema.table_schemas([fk_record['table_name'], fk_record['foreign_table_name']]).many?
+  end
+
+  it 'onlies have allowed list of cross-database foreign keys', :aggregate_failures do
+    all_tables = ApplicationRecord.connection.data_sources
+
+    cross_db_fks = []
+    all_tables.each do |table|
+      ::ApplicationRecord.connection.execute(fks_query(table)).each do |row|
+        if is_cross_db?(row)
+          column = "#{row['table_name']}.#{row['column_name']}"
+          cross_db_fks << column
+          expect(allowed_cross_database_foreign_keys).to include(column), "Found extra cross-database foreign key #{column} referencing #{row['foreign_table_name']} with constraint name #{row['constraint_name']}. When a foreign key references another database you must use a Loose Foreign Key instead https://docs.gitlab.com/ee/development/database/loose_foreign_keys.html ."
+        end
+      end
+    end
+
+    # Ensure that ALLOWED_CROSS_DATABASE_FOREIGN_KEYS does not contain extra
+    # foreign keys that have since been removed
+    expect(cross_db_fks).to match_array(allowed_cross_database_foreign_keys)
+  end
+end