Initialize bigint conversion for merge_trains

This is the first step to convert pipeline_id related to bigint for:
- merge_trains.pipeline_id

See https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#initialize-the-conversion-and-start-migrating-existing-data-release-n

Changelog: added

db/post_migrate/20240401074104_init_conversion_for_merge_trains_pipeline_id_bigint.rb

+# frozen_string_literal: true
+
+class InitConversionForMergeTrainsPipelineIdBigint < Gitlab::Database::Migration[2.2]
+  disable_ddl_transaction!
+  milestone '16.11'
+
+  TABLE = :merge_trains
+  COLUMN = :pipeline_id
+
+  def up
+    initialize_conversion_of_integer_to_bigint(TABLE, COLUMN)
+  end
+
+  def down
+    revert_initialize_conversion_of_integer_to_bigint(TABLE, COLUMN)
+  end
+end

db/post_migrate/20240404084104_backfill_merge_trains_pipeline_id_bigint.rb

+# frozen_string_literal: true
+
+class BackfillMergeTrainsPipelineIdBigint < Gitlab::Database::Migration[2.2]
+  restrict_gitlab_migration gitlab_schema: :gitlab_main
+  milestone '16.11'
+
+  TABLE = :merge_trains
+  COLUMN = :pipeline_id
+
+  def up
+    backfill_conversion_of_integer_to_bigint(TABLE, COLUMN)
+  end
+
+  def down
+    revert_backfill_conversion_of_integer_to_bigint(TABLE, COLUMN)
+  end
+end

db/schema_migrations/20240401074104

+db237e66d5a4e215c5959b14815fe658ce2f0f10894f1a052b0b6946b5455670
\ No newline at end of file

db/schema_migrations/20240404084104

+9941d3b5393aefbe47607af58f1b39c2eb7951ff01ccc91da51c37af6d8b8a31
\ No newline at end of file

db/structure.sql

@@ -668,6 +668,15 @@ BEGIN
 END;
 $$;
+CREATE FUNCTION trigger_3857ca5ea4af() RETURNS trigger
+    LANGUAGE plpgsql
+    AS $$
+BEGIN
+  NEW."pipeline_id_convert_to_bigint" := NEW."pipeline_id";
+  RETURN NEW;
+END;
+$$;
+
 CREATE FUNCTION trigger_388e93f88fdd() RETURNS trigger
     LANGUAGE plpgsql
     AS $$
@@ -11345,7 +11354,8 @@ CREATE TABLE merge_trains (
     target_branch text NOT NULL,
     status smallint DEFAULT 0 NOT NULL,
     merged_at timestamp with time zone,
-    duration integer
+    duration integer,
+    pipeline_id_convert_to_bigint bigint
 );
 CREATE SEQUENCE merge_trains_id_seq
@@ -29614,6 +29624,8 @@ CREATE TRIGGER trigger_10ee1357e825 BEFORE INSERT OR UPDATE ON p_ci_builds FOR E
 CREATE TRIGGER trigger_2428b5519042 BEFORE INSERT OR UPDATE ON vulnerability_feedback FOR EACH ROW EXECUTE FUNCTION trigger_2428b5519042();
+CREATE TRIGGER trigger_3857ca5ea4af BEFORE INSERT OR UPDATE ON merge_trains FOR EACH ROW EXECUTE FUNCTION trigger_3857ca5ea4af();
+
 CREATE TRIGGER trigger_388e93f88fdd BEFORE INSERT OR UPDATE ON packages_build_infos FOR EACH ROW EXECUTE FUNCTION trigger_388e93f88fdd();
 CREATE TRIGGER trigger_b2d852e1e2cb BEFORE INSERT OR UPDATE ON ci_pipelines FOR EACH ROW EXECUTE FUNCTION trigger_b2d852e1e2cb();

ee/app/models/merge_trains/car.rb

@@ -5,6 +5,9 @@ module MergeTrains
   class Car < ApplicationRecord
     include Gitlab::Utils::StrongMemoize
     include AfterCommitQueue
+    include IgnorableColumns
+
+    ignore_columns :pipeline_id_convert_to_bigint, remove_with: '17.1', remove_after: '2024-06-14'
 
     # For legacy reasons, each row is a merge train in the database
     self.table_name = 'merge_trains'