Update prepare_async_index migration helper to avoid subtransaction

`safe_find_or_create_by!` creates a savepoint if already in transaction,
which is usually the case for migrations. This updates the helper to
use 'find_or_create_by!` instead in order to avoid subtransactions.

Also changes the helper to update index definition if the record already
exists and there is new definition, something we state in the method
comment but were not actually doing.

https://gitlab.com/gitlab-org/gitlab/-/issues/339667

lib/gitlab/database/async_indexes/migration_helpers.rb

@@ -55,11 +55,14 @@ module Gitlab
           schema_creation = ActiveRecord::ConnectionAdapters::PostgreSQL::SchemaCreation.new(ApplicationRecord.connection)
           definition = schema_creation.accept(create_index)
 
-          async_index = PostgresAsyncIndex.safe_find_or_create_by!(name: index_name) do |rec|
+          async_index = PostgresAsyncIndex.find_or_create_by!(name: index_name) do |rec|
             rec.table_name = table_name
             rec.definition = definition
           end
 
+          async_index.definition = definition
+          async_index.save! # No-op if definition is not changed
+
           Gitlab::AppLogger.info(
             message: 'Prepared index for async creation',
             table_name: async_index.table_name,

spec/lib/gitlab/database/async_indexes/migration_helpers_spec.rb

@@ -150,6 +150,23 @@ RSpec.describe Gitlab::Database::AsyncIndexes::MigrationHelpers do
             migration.prepare_async_index(table_name, 'id')
           end.not_to change { index_model.where(name: index_name).count }
         end
+
+        it 'updates definition if changed' do
+          index = create(:postgres_async_index, table_name: table_name, name: index_name, definition: '...')
+
+          expect do
+            migration.prepare_async_index(table_name, 'id', name: index_name)
+          end.to change { index.reload.definition }
+        end
+
+        it 'does not update definition if not changed' do
+          definition = "CREATE INDEX CONCURRENTLY \"index_#{table_name}_on_id\" ON \"#{table_name}\" (\"id\")"
+          index = create(:postgres_async_index, table_name: table_name, name: index_name, definition: definition)
+
+          expect do
+            migration.prepare_async_index(table_name, 'id', name: index_name)
+          end.not_to change { index.reload.updated_at }
+        end
       end
 
       context 'when the async index table does not exist' do