Reduce excessive GC on pull mirrors

In https://gitlab.com/gitlab-org/gitaly/issues/1686 and
https://gitlab.com/gitlab-com/gl-infra/scalability/issues/20, we saw a
lot of I/O consumed by Git garbage collection running on pull mirrors.

It looks like the problem happened because
`Projects::AfterImportService` always ran after completion of a project
import sync, which caused `HousekeepingService` to run. Since the
`pushes_since_gc` counter never changed, `HousekeepingService` would
always run GC again.

This commit increments the `pushes_since_gc` counter so that GCs are
only run every 200 syncs. Ideally we'd only run `HousekeepingService`
only if the repository actually changed, but right now we don't have
an easy way of detecting that.

Note that this problem does NOT happen if the mirror is a imported
directly via a URL. It only happens if the `import_type` is
`Gitlab::ImportSources.importer_names`.

Closes https://gitlab.com/gitlab-org/gitaly/issues/1686

app/services/projects/after_import_service.rb

@@ -9,9 +9,16 @@ module Projects
     end
 
     def execute
-      Projects::HousekeepingService.new(@project).execute do
+      service = Projects::HousekeepingService.new(@project)
+
+      service.execute do
         repository.delete_all_refs_except(RESERVED_REF_PREFIXES)
       end
+
+      # Right now we don't actually have a way to know if a project
+      # import actually changed, so we increment the counter to avoid
+      # causing GC to run every time.
+      service.increment!
     rescue Projects::HousekeepingService::LeaseTaken => e
       Rails.logger.info( # rubocop:disable Gitlab/RailsLogger
         "Could not perform housekeeping for project #{@project.full_path} (#{@project.id}): #{e}")

ee/changelogs/unreleased/sh-fix-excessive-project-mirror-gc.yml

+---
+title: Reduce excessive GC on pull mirrors
+merge_request: 17931
+author:
+type: performance

spec/services/projects/after_import_service_spec.rb

@@ -19,6 +19,8 @@ describe Projects::AfterImportService do
 
       allow(housekeeping_service)
         .to receive(:execute).and_yield
+
+      expect(housekeeping_service).to receive(:increment!)
     end
 
     it 'performs housekeeping' do