Geo: Always update the default branch on the secondary

Previously, we would only update the default branch on a Geo secondary
if the default branch didn't match the primary default branch. However,
the Gitaly FindDefaultBranch RPC may return an arbitrary branch if HEAD
points to an invalid branch, so we can't trust that result to be
correct.

To ensure that the default branch is always correct, we now write it out
on the Geo secondary whenever we do a sync. This incurs an additional
filesystem write, but that should be relatively minor.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9084

ee/app/models/ee/project.rb

@@ -476,7 +476,7 @@ module EE
     # Update the default branch querying the remote to determine its HEAD
     def update_root_ref(remote_name)
       root_ref = repository.find_remote_root_ref(remote_name)
-      change_head(root_ref) if root_ref.present? && root_ref != default_branch
+      change_head(root_ref) if root_ref.present?
     end
 
     def feature_flags_client_token

ee/changelogs/unreleased/sh-fix-issue-9084.yml

+---
+title: 'Geo: Always update the default branch on the secondary'
+merge_request: 9064
+author:
+type: fixed

ee/spec/models/project_spec.rb

@@ -1520,11 +1520,16 @@ describe Project do
         .to('feature')
     end
 
-    it 'does not update the default branch when HEAD does not change' do
+    it 'always updates the default branch even when HEAD does not change' do
       stub_find_remote_root_ref(project, ref: 'master')
 
-      expect { project.update_root_ref('origin') }
-        .not_to change { project.default_branch }
+      expect(project).to receive(:change_head).with('master').and_call_original
+
+      project.update_root_ref('origin')
+
+      # For good measure, expunge the root ref cache and reload.
+      project.repository.expire_all_method_caches
+      expect(project.reload.default_branch).to eq('master')
     end
 
     it 'does not update the default branch when HEAD does not exist' do

ee/spec/services/geo/repository_sync_service_spec.rb

@@ -253,8 +253,8 @@ describe Geo::RepositorySyncService do
                 .and_return(project.default_branch)
             end
 
-            it 'does not sync gitattributes to info/attributes' do
-              expect(repository).not_to receive(:copy_gitattributes)
+            it 'syncs gitattributes to info/attributes' do
+              expect(repository).to receive(:copy_gitattributes)
 
               subject.execute
             end
@@ -265,7 +265,7 @@ describe Geo::RepositorySyncService do
                 .twice
                 .and_call_original
 
-              expect(project).not_to receive(:change_head)
+              expect(project).to receive(:change_head).with('master').once
 
               subject.execute
             end