Add a helper to rename a column using a background migration

This works the same way as change_column_type_using_background_migration, but
for renaming a column. It takes a table, not a relation, to match its concurrent
counterpart.

Also, generalise the cleanup migrations to reduce code duplication.

3 files changed, 70 insertions, 44 deletions

diff --git a/lib/gitlab/background_migration/cleanup_concurrent_rename.rb b/lib/gitlab/background_migration/cleanup_concurrent_rename.rb
new file mode 100644
index 00000000000..d3f366f3480
--- /dev/null
+++ b/lib/gitlab/background_migration/cleanup_concurrent_rename.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # Background migration for cleaning up a concurrent column rename.
+    class CleanupConcurrentRename < CleanupConcurrentSchemaChange
+      RESCHEDULE_DELAY = 10.minutes
+
+      def cleanup_concurrent_schema_change(table, old_column, new_column)
+        cleanup_concurrent_column_rename(table, old_column, new_column)
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/background_migration/cleanup_concurrent_schema_change.rb b/lib/gitlab/background_migration/cleanup_concurrent_schema_change.rb
new file mode 100644
index 00000000000..54f77f184d5
--- /dev/null
+++ b/lib/gitlab/background_migration/cleanup_concurrent_schema_change.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # Base class for cleaning up concurrent schema changes.
+    class CleanupConcurrentSchemaChange
+      include Database::MigrationHelpers
+
+      # table - The name of the table the migration is performed for.
+      # old_column - The name of the old (to drop) column.
+      # new_column - The name of the new column.
+      def perform(table, old_column, new_column)
+        return unless column_exists?(table, new_column)
+
+        rows_to_migrate = define_model_for(table)
+          .where(new_column => nil)
+          .where
+          .not(old_column => nil)
+
+        if rows_to_migrate.any?
+          BackgroundMigrationWorker.perform_in(
+            RESCHEDULE_DELAY,
+            self.class.name,
+            [table, old_column, new_column]
+          )
+        else
+          cleanup_concurrent_schema_change(table, old_column, new_column)
+        end
+      end
+
+      # These methods are necessary so we can re-use the migration helpers in
+      # this class.
+      def connection
+        ActiveRecord::Base.connection
+      end
+
+      def method_missing(name, *args, &block)
+        connection.__send__(name, *args, &block) # rubocop: disable GitlabSecurity/PublicSend
+      end
+
+      def respond_to_missing?(*args)
+        connection.respond_to?(*args) || super
+      end
+
+      def define_model_for(table)
+        Class.new(ActiveRecord::Base) do
+          self.table_name = table
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/background_migration/cleanup_concurrent_type_change.rb b/lib/gitlab/background_migration/cleanup_concurrent_type_change.rb
index de622f657b2..48411095dbb 100644
--- a/lib/gitlab/background_migration/cleanup_concurrent_type_change.rb
+++ b/lib/gitlab/background_migration/cleanup_concurrent_type_change.rb
@@ -2,52 +2,12 @@
 
 module Gitlab
   module BackgroundMigration
-    # Background migration for cleaning up a concurrent column rename.
-    class CleanupConcurrentTypeChange
-      include Database::MigrationHelpers
-
+    # Background migration for cleaning up a concurrent column type changeb.
+    class CleanupConcurrentTypeChange < CleanupConcurrentSchemaChange
       RESCHEDULE_DELAY = 10.minutes
 
-      # table - The name of the table the migration is performed for.
-      # old_column - The name of the old (to drop) column.
-      # new_column - The name of the new column.
-      def perform(table, old_column, new_column)
-        return unless column_exists?(:issues, new_column)
-
-        rows_to_migrate = define_model_for(table)
-          .where(new_column => nil)
-          .where
-          .not(old_column => nil)
-
-        if rows_to_migrate.any?
-          BackgroundMigrationWorker.perform_in(
-            RESCHEDULE_DELAY,
-            'CleanupConcurrentTypeChange',
-            [table, old_column, new_column]
-          )
-        else
-          cleanup_concurrent_column_type_change(table, old_column)
-        end
-      end
-
-      # These methods are necessary so we can re-use the migration helpers in
-      # this class.
-      def connection
-        ActiveRecord::Base.connection
-      end
-
-      def method_missing(name, *args, &block)
-        connection.__send__(name, *args, &block) # rubocop: disable GitlabSecurity/PublicSend
-      end
-
-      def respond_to_missing?(*args)
-        connection.respond_to?(*args) || super
-      end
-
-      def define_model_for(table)
-        Class.new(ActiveRecord::Base) do
-          self.table_name = table
-        end
+      def cleanup_concurrent_schema_change(table, old_column, new_column)
+        cleanup_concurrent_column_type_change(table, old_column)
       end
     end
   end