1 files changed, 64 insertions, 0 deletions
diff --git a/lib/gitlab/background_migration/copy_column_using_background_migration_job.rb b/lib/gitlab/background_migration/copy_column_using_background_migration_job.rb
new file mode 100644
index 00000000000..16c0de39a3b
--- /dev/null
+++ b/lib/gitlab/background_migration/copy_column_using_background_migration_job.rb
@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    # Background migration that extends CopyColumn to update the value of a
+    # column using the value of another column in the same table.
+    #
+    # - The {start_id, end_id} arguments are at the start so that it can be used
+    #   with `queue_background_migration_jobs_by_range_at_intervals`
+    # - Provides support for background job tracking through the use of
+    #   Gitlab::Database::BackgroundMigrationJob
+    # - Uses sub-batching so that we can keep each update's execution time at
+    #   low 100s ms, while being able to update more records per 2 minutes
+    #   that we allow background migration jobs to be scheduled one after the other
+    # - We skip the NULL checks as they may result in not using an index scan
+    # - The table that is migrated does _not_ need `id` as the primary key
+    #   We use the provided primary_key column to perform the update.
+    class CopyColumnUsingBackgroundMigrationJob
+      include Gitlab::Database::DynamicModelHelpers
+
+      PAUSE_SECONDS = 0.1
+
+      # start_id - The start ID of the range of rows to update.
+      # end_id - The end ID of the range of rows to update.
+      # table - The name of the table that contains the columns.
+      # primary_key - The primary key column of the table.
+      # copy_from - The column containing the data to copy.
+      # copy_to - The column to copy the data to.
+      # sub_batch_size - We don't want updates to take more than ~100ms
+      #                  This allows us to run multiple smaller batches during
+      #                  the minimum 2.minute interval that we can schedule jobs
+      def perform(start_id, end_id, table, primary_key, copy_from, copy_to, sub_batch_size)
+        quoted_copy_from = connection.quote_column_name(copy_from)
+        quoted_copy_to = connection.quote_column_name(copy_to)
+
+        parent_batch_relation = relation_scoped_to_range(table, primary_key, start_id, end_id)
+
+        parent_batch_relation.each_batch(column: primary_key, of: sub_batch_size) do |sub_batch|
+          sub_batch.update_all("#{quoted_copy_to}=#{quoted_copy_from}")
+
+          sleep(PAUSE_SECONDS)
+        end
+
+        # We have to add all arguments when marking a job as succeeded as they
+        #  are all used to track the job by `queue_background_migration_jobs_by_range_at_intervals`
+        mark_job_as_succeeded(start_id, end_id, table, primary_key, copy_from, copy_to, sub_batch_size)
+      end
+
+      private
+
+      def connection
+        ActiveRecord::Base.connection
+      end
+
+      def mark_job_as_succeeded(*arguments)
+        Gitlab::Database::BackgroundMigrationJob.mark_all_as_succeeded(self.class.name, arguments)
+      end
+
+      def relation_scoped_to_range(source_table, source_key_column, start_id, stop_id)
+        define_batchable_model(source_table).where(source_key_column => start_id..stop_id)
+      end
+    end
+  end
+end