Allow external diffs to be used conditionally

Since external diffs are likely to be a bit slower than in-database
ones, add a mode that makes diffs external after they've been obsoleted
by events. This should strike a balance between performance and disk
space.

A background cron drives the majority of migrations, since diffs become
outdated through user actions.

1 files changed, 23 insertions, 0 deletions

diff --git a/app/services/merge_requests/migrate_external_diffs_service.rb b/app/services/merge_requests/migrate_external_diffs_service.rb
new file mode 100644
index 00000000000..16050244637
--- /dev/null
+++ b/app/services/merge_requests/migrate_external_diffs_service.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module MergeRequests
+  class MigrateExternalDiffsService < ::BaseService
+    MAX_JOBS = 1000.freeze
+
+    attr_reader :diff
+
+    def self.enqueue!
+      ids = MergeRequestDiff.ids_for_external_storage_migration(limit: MAX_JOBS)
+
+      MigrateExternalDiffsWorker.bulk_perform_async(ids.map { |id| [id] })
+    end
+
+    def initialize(merge_request_diff)
+      @diff = merge_request_diff
+    end
+
+    def execute
+      diff.migrate_files_to_external_storage!
+    end
+  end
+end