Capture total_tuple_count only if present

The column `total_tuple_count` has only been introduced after the
migration helper had been used for the first time. That is, there is a
preceding migration using the helper. This migration and its schema
version does not know about the later column addition and hence the code
path fails.

We check if the attribute exists here, so we can safely work with it.

This is only needed in code paths that are already being used in
existing migrations.

See https://gitlab.com/gitlab-org/gitlab/-/issues/327447

db/post_migrate/20210311120155_backfill_events_id_for_bigint_conversion.rb

@@ -6,13 +6,13 @@ class BackfillEventsIdForBigintConversion < ActiveRecord::Migration[6.0]
   DOWNTIME = false
 
   def up
-    return unless Gitlab.dev_env_or_com?
+    return unless should_run?
 
     backfill_conversion_of_integer_to_bigint :events, :id, batch_size: 15000, sub_batch_size: 100
   end
 
   def down
-    return unless Gitlab.dev_env_or_com?
+    return unless should_run?
 
     Gitlab::Database::BackgroundMigration::BatchedMigration
       .where(job_class_name: 'CopyColumnUsingBackgroundMigrationJob')
@@ -20,4 +20,10 @@ class BackfillEventsIdForBigintConversion < ActiveRecord::Migration[6.0]
       .where('job_arguments = ?', %w[id id_convert_to_bigint].to_json)
       .delete_all
   end
+
+  private
+
+  def should_run?
+    Gitlab.dev_or_test_env? || Gitlab.com?
+  end
 end

db/post_migrate/20210311120156_backfill_push_event_payload_event_id_for_bigint_conversion.rb

@@ -6,14 +6,14 @@ class BackfillPushEventPayloadEventIdForBigintConversion < ActiveRecord::Migrati
   DOWNTIME = false
 
   def up
-    return unless Gitlab.dev_env_or_com?
+    return unless should_run?
 
     backfill_conversion_of_integer_to_bigint :push_event_payloads, :event_id, primary_key: :event_id,
       batch_size: 15000, sub_batch_size: 100
   end
 
   def down
-    return unless Gitlab.dev_env_or_com?
+    return unless should_run?
 
     Gitlab::Database::BackgroundMigration::BatchedMigration
       .where(job_class_name: 'CopyColumnUsingBackgroundMigrationJob')
@@ -21,4 +21,10 @@ class BackfillPushEventPayloadEventIdForBigintConversion < ActiveRecord::Migrati
       .where('job_arguments = ?', %w[event_id event_id_convert_to_bigint].to_json)
       .delete_all
   end
+
+  private
+
+  def should_run?
+    Gitlab.dev_or_test_env? || Gitlab.com?
+  end
 end

lib/gitlab/database/migrations/background_migration_helpers.rb

@@ -190,11 +190,7 @@ module Gitlab
           migration_status = batch_max_value.nil? ? :finished : :active
           batch_max_value ||= batch_min_value
 
-          # We keep track of the estimated number of tuples to reason later
-          # about the overall progress of a migration.
-          total_tuple_count = Gitlab::Database::PgClass.for_table(batch_table_name)&.cardinality_estimate
-
-          Gitlab::Database::BackgroundMigration::BatchedMigration.create!(
+          migration = Gitlab::Database::BackgroundMigration::BatchedMigration.create!(
             job_class_name: job_class_name,
             table_name: batch_table_name,
             column_name: batch_column_name,
@@ -205,8 +201,18 @@ module Gitlab
             batch_size: batch_size,
             sub_batch_size: sub_batch_size,
             job_arguments: job_arguments,
-            status: migration_status,
-            total_tuple_count: total_tuple_count)
+            status: migration_status)
+
+          # This guard is necessary since #total_tuple_count was only introduced schema-wise,
+          # after this migration helper had been used for the first time.
+          return migration unless migration.respond_to?(:total_tuple_count)
+
+          # We keep track of the estimated number of tuples to reason later
+          # about the overall progress of a migration.
+          migration.total_tuple_count = Gitlab::Database::PgClass.for_table(batch_table_name)&.cardinality_estimate
+          migration.save!
+
+          migration
         end
 
         def perform_background_migration_inline?