Run project housekeeping for pull mirrors

This should reduce the storage cost of pull mirrors, which are a small,
but noticeable, proportion of the total on GitLab.com

ee/app/services/projects/update_mirror_service.rb

@@ -42,6 +42,11 @@ module Projects
       # Let's skip this if the repository hasn't changed.
       update_lfs_objects if project.repository.checksum != checksum_before
 
+      # Running git fetch in the repository creates loose objects in the same
+      # way running git push *to* the repository does, so ensure we run regular
+      # garbage collection
+      run_housekeeping
+
       success
     rescue Gitlab::Shell::Error, Gitlab::Git::BaseError, UpdateError => e
       error(e.message)
@@ -149,6 +154,15 @@ module Projects
       end
     end
 
+    def run_housekeeping
+      service = Projects::HousekeepingService.new(project)
+
+      service.increment!
+      service.execute if service.needed?
+    rescue Projects::HousekeepingService::LeaseTaken
+      # best-effort
+    end
+
     # In Git is possible to tag blob objects, and those blob objects don't point to a Git commit so those tags
     # have no target.
     def repository_tags_with_target

ee/changelogs/unreleased/214092-gc-when-project-mirroring-fetches.yml

+---
+title: Run project housekeeping for pull mirrors
+merge_request: 46493
+author:
+type: performance

ee/spec/services/projects/update_mirror_service_spec.rb

@@ -30,6 +30,19 @@ RSpec.describe Projects::UpdateMirrorService do
       service.execute
     end
 
+    it 'runs project housekeeping' do
+      stub_fetch_mirror(project)
+
+      expect_next_instance_of(Projects::HousekeepingService) do |svc|
+        expect(svc.project).to eq(project)
+        expect(svc).to receive(:increment!)
+        expect(svc).to receive(:needed?).and_return(true)
+        expect(svc).to receive(:execute)
+      end
+
+      service.execute
+    end
+
     it 'rescues exceptions from Repository#ff_merge' do
       stub_fetch_mirror(project)