Add revert_initialize_conversion_of_integer_to_bigint migration helper

for easier revert of `initialize_conversion_of_integer_to_bigint`.

See https://gitlab.com/gitlab-org/gitlab/-/issues/328600.

lib/gitlab/database/migration_helpers.rb

@@ -969,6 +969,20 @@ module Gitlab
         end
       end
 
+      # Reverts `initialize_conversion_of_integer_to_bigint`
+      #
+      # table - The name of the database table containing the columns
+      # columns - The name, or array of names, of the column(s) that we're converting to bigint.
+      def revert_initialize_conversion_of_integer_to_bigint(table, columns)
+        columns = Array.wrap(columns)
+        temporary_columns = columns.map { |column| "#{column}_convert_to_bigint" }
+
+        trigger_name = rename_trigger_name(table, columns, temporary_columns)
+        remove_rename_triggers_for_postgresql(table, trigger_name)
+
+        temporary_columns.each { |column| remove_column(table, column) }
+      end
+
       # Backfills the new columns used in an integer-to-bigint conversion using background migrations.
       #
       # - This helper should be called from a post-deployment migration.

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -4,6 +4,7 @@ require 'spec_helper'
 
 RSpec.describe Gitlab::Database::MigrationHelpers do
   include Database::TableSchemaHelpers
+  include Database::TriggerHelpers
 
   let(:model) do
     ActiveRecord::Migration.new.extend(described_class)
@@ -1791,6 +1792,48 @@ RSpec.describe Gitlab::Database::MigrationHelpers do
     end
   end
 
+  describe '#revert_initialize_conversion_of_integer_to_bigint' do
+    let(:table) { :test_table }
+
+    before do
+      model.create_table table, id: false do |t|
+        t.integer :id, primary_key: true
+        t.integer :other_id
+      end
+
+      model.initialize_conversion_of_integer_to_bigint(table, columns)
+    end
+
+    context 'when single column is given' do
+      let(:columns) { :id }
+
+      it 'removes column, trigger, and function' do
+        trigger_name = model.rename_trigger_name(table, :id, :id_convert_to_bigint)
+
+        model.revert_initialize_conversion_of_integer_to_bigint(table, columns)
+
+        expect(model.column_exists?(table, :id_convert_to_bigint)).to eq(false)
+        expect_trigger_not_to_exist(table, trigger_name)
+        expect_function_not_to_exist(trigger_name)
+      end
+    end
+
+    context 'when multiple columns are given' do
+      let(:columns) { [:id, :other_id] }
+
+      it 'removes column, trigger, and function' do
+        trigger_name = model.rename_trigger_name(table, columns, [:id_convert_to_bigint, :other_id_convert_to_bigint])
+
+        model.revert_initialize_conversion_of_integer_to_bigint(table, columns)
+
+        expect(model.column_exists?(table, :id_convert_to_bigint)).to eq(false)
+        expect(model.column_exists?(table, :other_id_convert_to_bigint)).to eq(false)
+        expect_trigger_not_to_exist(table, trigger_name)
+        expect_function_not_to_exist(trigger_name)
+      end
+    end
+  end
+
   describe '#backfill_conversion_of_integer_to_bigint' do
     let(:table) { :_test_backfill_table }
     let(:column) { :id }