Skip LFS updates in mirrors if repository has not changed

The GetAllLFSPointers RPC is expensive and was being called for every
mirror. If the repository has not changed, we can assume that the LFS
pointers also did not change. We can skip calling this RPC if there are
no updates.

Since we don't actually get a return code if branches or tags were
updated, we use the repository checksum as a cheaper mechanism (which
XORs the ref contents) to determine whether we run the LFS scan.

Closes https://gitlab.com/gitlab-org/gitlab/issues/38256

changelogs/unreleased/sh-skip-lfs-updates-mirroring.yml

+---
+title: Skip updating LFS objects in mirror updates if repository has not changed
+merge_request: 21744
+author:
+type: performance

ee/app/services/projects/update_mirror_service.rb

@@ -24,12 +24,17 @@ module Projects
         return error("The mirror user is not allowed to push code to all branches on this project.")
       end
 
+      checksum_before = project.repository.checksum
+
       update_tags do
         project.fetch_mirror(forced: true)
       end
 
       update_branches
-      update_lfs_objects
+
+      # Updating LFS objects is expensive since it requires scanning for blobs with pointers.
+      # Let's skip this if the repository hasn't changed.
+      update_lfs_objects if project.repository.checksum != checksum_before
 
       success
     rescue Gitlab::Shell::Error, Gitlab::Git::BaseError, UpdateError => e

ee/spec/services/projects/update_mirror_service_spec.rb

@@ -325,13 +325,26 @@ describe Projects::UpdateMirrorService do
       end
     end
 
-    context 'updating Lfs objects' do
+    context 'updating LFS objects' do
+      context 'when repository does not change' do
+        before do
+          allow(project).to receive(:lfs_enabled?).and_return(true)
+        end
+
+        it 'does not attempt to update LFS objects' do
+          expect(Projects::LfsPointers::LfsImportService).not_to receive(:new)
+
+          service.execute
+        end
+      end
+
+      context 'when repository changes' do
         before do
           stub_fetch_mirror(project)
         end
 
         context 'when Lfs is disabled in the project' do
-        it 'does not update Lfs objects' do
+          it 'does not update LFS objects' do
             allow(project).to receive(:lfs_enabled?).and_return(false)
             expect(Projects::LfsPointers::LfsObjectDownloadListService).not_to receive(:new)
 
@@ -344,7 +357,7 @@ describe Projects::UpdateMirrorService do
             allow(project).to receive(:lfs_enabled?).and_return(true)
           end
 
-        it 'updates Lfs objects' do
+          it 'updates LFS objects' do
             expect(Projects::LfsPointers::LfsImportService).to receive(:new).and_call_original
             expect_any_instance_of(Projects::LfsPointers::LfsObjectDownloadListService).to receive(:execute).and_return({})
 
@@ -383,6 +396,7 @@ describe Projects::UpdateMirrorService do
           end
         end
       end
+    end
 
     it "fails when the mirror user doesn't have access" do
       stub_fetch_mirror(project)