Extract swapping of column names in a migration helper

to be used for `integer` to `bigint` conversion.

https://gitlab.com/gitlab-org/gitlab/-/issues/288005.

db/post_migrate/20210622041846_finalize_push_event_payloads_bigint_conversion.rb

@@ -32,10 +32,7 @@ class FinalizePushEventPayloadsBigintConversion < ActiveRecord::Migration[6.1]
     add_concurrent_foreign_key TABLE_NAME, :events, column: :event_id_convert_to_bigint, on_delete: :cascade
 
     with_lock_retries(safe: true) do
-      # Swap column names
-      rename_column TABLE_NAME, :event_id, :event_id_convert_to_bigint_tmp
-      rename_column TABLE_NAME, :event_id_convert_to_bigint, :event_id
-      rename_column TABLE_NAME, :event_id_convert_to_bigint_tmp, :event_id_convert_to_bigint
+      swap_column_names TABLE_NAME, :event_id, :event_id_convert_to_bigint # rubocop:disable Migration/WithLockRetriesDisallowedMethod
 
       # Swap defaults
       change_column_default TABLE_NAME, :event_id, nil

lib/gitlab/database/migration_helpers.rb

@@ -1616,6 +1616,17 @@ into similar problems in the future (e.g. when new tables are created).
         raise
       end
 
+      def swap_column_names(table_name, column_1, column_2)
+        unless transaction_open?
+          raise 'Cannot call swap_column_names without a transaction open or outside of a transaction block.'
+        end
+
+        temp_name = "#{column_1}_tmp"
+        rename_column(table_name, column_1, temp_name)
+        rename_column(table_name, column_2, column_1)
+        rename_column(table_name, temp_name, column_2)
+      end
+
       private
 
       def validate_check_constraint_name!(constraint_name)

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -2980,4 +2980,42 @@ RSpec.describe Gitlab::Database::MigrationHelpers do
       end
     end
   end
+
+  describe '#swap_column_names' do
+    let(:table) { :_test_table }
+    let(:ar_model) do
+      Class.new(ActiveRecord::Base) do
+        self.table_name = :_test_table
+      end
+    end
+
+    before do
+      model.drop_table table, if_exists: true
+      model.create_table table, id: false do |t|
+        t.integer :column_1
+        t.bigint  :column_2
+      end
+    end
+
+    after do
+      model.drop_table table, if_exists: true
+    end
+
+    it 'swaps column names as requested' do
+      model.swap_column_names(table, :column_1, :column_2)
+
+      column_1 = ar_model.column_for_attribute(:column_1)
+      column_2 = ar_model.column_for_attribute(:column_2)
+
+      expect(column_1.sql_type).to eq('bigint')
+      expect(column_2.sql_type).to eq('integer')
+    end
+
+    it 'raises an error when not in transaction' do
+      expect(model).to receive(:transaction_open?).and_return(false)
+
+      expect { model.swap_column_names(table, :column_1, :column_2) }
+        .to raise_error 'Cannot call swap_column_names without a transaction open or outside of a transaction block.'
+    end
+  end
 end