Fix diverged from upstream detection

In case pull_mirror_branch_prefix is set, upstream branch names do not
have the prefix

ee/app/models/ee/repository.rb

@@ -15,6 +15,7 @@ module EE
       include Elastic::RepositoriesSearch
 
       delegate :checksum, :find_remote_root_ref, to: :raw_repository
+      delegate :pull_mirror_branch_prefix, to: :project
     end
 
     # Transiently sets a configuration variable
@@ -33,6 +34,19 @@ module EE
       expire_content_cache
     end
 
+    def upstream_branch_name(branch_name)
+      return branch_name unless ::Feature.enabled?(:pull_mirror_branch_prefix, project)
+      return branch_name unless pull_mirror_branch_prefix
+
+      # when pull_mirror_branch_prefix is set, a branch not starting with it
+      # is a local branch that doesn't tracking upstream
+      if branch_name.start_with?(pull_mirror_branch_prefix)
+        branch_name.delete_prefix(pull_mirror_branch_prefix)
+      else
+        nil
+      end
+    end
+
     def fetch_upstream(url, forced: false)
       add_remote(MIRROR_REMOTE, url)
       fetch_remote(MIRROR_REMOTE, ssh_auth: project&.import_data, forced: forced)
@@ -43,8 +57,11 @@ module EE
     end
 
     def diverged_from_upstream?(branch_name)
+      upstream_branch = upstream_branch_name(branch_name)
+      return false unless upstream_branch
+
       branch_commit = commit("refs/heads/#{branch_name}")
-      upstream_commit = commit("refs/remotes/#{MIRROR_REMOTE}/#{branch_name}")
+      upstream_commit = commit("refs/remotes/#{MIRROR_REMOTE}/#{upstream_branch}")
 
       if branch_commit && upstream_commit
         !raw_repository.ancestor?(branch_commit.id, upstream_commit.id)
@@ -65,8 +82,11 @@ module EE
     end
 
     def up_to_date_with_upstream?(branch_name)
+      upstream_branch = upstream_branch_name(branch_name)
+      return false unless upstream_branch
+
       branch_commit = commit("refs/heads/#{branch_name}")
-      upstream_commit = commit("refs/remotes/#{MIRROR_REMOTE}/#{branch_name}")
+      upstream_commit = commit("refs/remotes/#{MIRROR_REMOTE}/#{upstream_branch}")
 
       if branch_commit && upstream_commit
         ancestor?(branch_commit.id, upstream_commit.id)

ee/spec/models/repository_spec.rb

@@ -20,6 +20,14 @@ describe Repository do
     rugged.references.create("refs/remotes/#{remote_name}/#{branch_name}", target.id)
   end
 
+  describe 'delegated methods' do
+    subject { repository }
+
+    it { is_expected.to delegate_method(:checksum).to(:raw_repository) }
+    it { is_expected.to delegate_method(:find_remote_root_ref).to(:raw_repository) }
+    it { is_expected.to delegate_method(:pull_mirror_branch_prefix).to(:project) }
+  end
+
   describe '#after_sync' do
     it 'expires repository cache' do
       expect(repository).to receive(:expire_all_method_caches)
@@ -208,4 +216,61 @@ describe Repository do
       expect(project.repository.insights_config_for(project.repository.root_ref)).to eq("monthlyBugsCreated:\n  title: My chart")
     end
   end
+
+  describe '#upstream_branch_name' do
+    let(:pull_mirror_branch_prefix) { 'upstream/' }
+    let(:branch_name) { 'upstream/master' }
+
+    subject { repository.upstream_branch_name(branch_name) }
+
+    before do
+      project.update(pull_mirror_branch_prefix: pull_mirror_branch_prefix)
+    end
+
+    it { is_expected.to eq('master') }
+
+    context 'when the branch is local (not mirrored)' do
+      let(:branch_name) { 'a-local-branch' }
+
+      it { is_expected.to be_nil }
+    end
+
+    context 'when pull_mirror_branch_prefix is nil' do
+      let(:pull_mirror_branch_prefix) { nil }
+
+      it { is_expected.to eq(branch_name) }
+    end
+
+    context 'when pull_mirror_branch_prefix is empty' do
+      let(:pull_mirror_branch_prefix) { '' }
+
+      it { is_expected.to eq(branch_name) }
+    end
+
+    context 'when pull_mirror_branch_prefix feature flag is disabled' do
+      before do
+        stub_feature_flags(pull_mirror_branch_prefix: false)
+      end
+
+      it { is_expected.to eq(branch_name) }
+
+      context 'when the branch is local (not mirrored)' do
+        let(:branch_name) { 'a-local-branch' }
+
+        it { is_expected.to eq(branch_name) }
+      end
+
+      context 'when pull_mirror_branch_prefix is nil' do
+        let(:pull_mirror_branch_prefix) { nil }
+
+        it { is_expected.to eq(branch_name) }
+      end
+
+      context 'when pull_mirror_branch_prefix is empty' do
+        let(:pull_mirror_branch_prefix) { '' }
+
+        it { is_expected.to eq(branch_name) }
+      end
+    end
+  end
 end