Run housekeeping after moving a repository between shards

The current inter-shard move is implemented as a `git fetch` from the
original shard to an empty repository on the new shard. The repository
on the new shard lacks bitmaps, and may be less well packed, than the
repository on the old shard. This has a measurable performance impact.

We may be changing from "FetchInternalRemote" to "ReplicateRepository"
in the future, but until then, forcing housekeeping to run is a good
performance optimisation - we trade off some short-term I/O load on the
new shard for better performance across a wide range of RPCs, including
an order-of-magnitude improvement in `IsAncestor`, which is the find
that motivated this change.

changelogs/unreleased/gitaly-2108-repos-gc-after-move.yml

+---
+title: Run housekeeping after moving a repository between shards
+merge_request: 20863
+author:
+type: performance

ee/app/services/projects/update_repository_storage_service.rb

@@ -33,6 +33,8 @@ module Projects
         project.update(repository_storage: new_repository_storage_key, repository_read_only: false)
         project.leave_pool_repository
         project.track_project_repository
+
+        enqueue_housekeeping
       else
         project.update(repository_read_only: false)
       end
@@ -89,6 +91,18 @@ module Projects
       "#{path}+#{project.id}+moved+#{Time.now.to_i}"
     end
 
+    # The underlying FetchInternalRemote call uses a `git fetch` to move data
+    # to the new repository, which leaves it in a less-well-packed state and
+    # lacking bitmaps. Housekeeping will boost performance significantly.
+    def enqueue_housekeeping
+      return unless Gitlab::CurrentSettings.housekeeping_enabled?
+      return unless Feature.enabled?(:repack_after_shard_migration, project)
+
+      Projects::HousekeepingService.new(project, :full_repack).execute
+    rescue Projects::HousekeepingService::LeaseTaken
+      # No action required
+    end
+
     def wait_for_pushes(type)
       reference_counter = project.reference_counter(type: type)
 

ee/spec/services/projects/update_repository_storage_service_spec.rb

@@ -73,7 +73,19 @@ describe Projects::UpdateRepositoryStorageService do
           .and_return(repository_double)
       end
 
-      context 'when the move succeeds' do
+      context 'when the move succeeds', :clean_gitlab_redis_shared_state do
+        before do
+          allow(project_repository_double)
+            .to receive(:fetch_repository_as_mirror)
+            .with(project.repository.raw)
+            .and_return(true)
+
+          allow(repository_double)
+            .to receive(:fetch_repository_as_mirror)
+            .with(repository.raw)
+            .and_return(true)
+        end
+
         it "moves the project and its #{repository_type} repository to the new storage and unmarks the repository as read only" do
           old_project_repository_path = Gitlab::GitalyClient::StorageSettings.allow_disk_access do
             project.repository.path_to_repo
@@ -81,12 +93,6 @@ describe Projects::UpdateRepositoryStorageService do
 
           old_repository_path = repository.full_path
 
-          allow(project_repository_double).to receive(:fetch_repository_as_mirror)
-            .with(project.repository.raw).and_return(true)
-
-          allow(repository_double).to receive(:fetch_repository_as_mirror)
-            .with(repository.raw).and_return(true)
-
           subject.execute('test_second_storage')
 
           expect(project).not_to be_repository_read_only
@@ -94,6 +100,35 @@ describe Projects::UpdateRepositoryStorageService do
           expect(gitlab_shell.repository_exists?('default', old_project_repository_path)).to be(false)
           expect(gitlab_shell.repository_exists?('default', old_repository_path)).to be(false)
         end
+
+        context ':repack_after_shard_migration feature flag disabled' do
+          before do
+            stub_feature_flags(repack_after_shard_migration: false)
+          end
+
+          it 'does not enqueue a GC run' do
+            expect { subject.execute('test_second_storage') }
+              .not_to change(GitGarbageCollectWorker.jobs, :count)
+          end
+        end
+
+        context ':repack_after_shard_migration feature flag enabled' do
+          before do
+            stub_feature_flags(repack_after_shard_migration: true)
+          end
+
+          it 'does not enqueue a GC run if housekeeping is disabled' do
+            stub_application_setting(housekeeping_enabled: false)
+
+            expect { subject.execute('test_second_storage') }
+              .not_to change(GitGarbageCollectWorker.jobs, :count)
+          end
+
+          it 'enqueues a GC run' do
+            expect { subject.execute('test_second_storage') }
+              .to change(GitGarbageCollectWorker.jobs, :count).by(1)
+          end
+        end
       end
 
       context 'when the project is already on the target storage' do