Synchronize the default branch when updating a remote mirror

app/models/project.rb

@@ -2063,6 +2063,12 @@ class Project < ActiveRecord::Base
     auto_cancel_pending_pipelines == 'enabled'
   end
 
+  # 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
+  end
+
   private
 
   def rename_or_migrate_repository!

app/models/repository.rb

@@ -24,6 +24,7 @@ class Repository
 
   delegate :ref_name_for_sha, to: :raw_repository
   delegate :bundle_to_disk, to: :raw_repository
+  delegate :find_remote_root_ref, to: :raw_repository
 
   CreateTreeError = Class.new(StandardError)
 

app/services/projects/update_remote_mirror_service.rb

@@ -5,13 +5,14 @@ module Projects
     attr_reader :errors
 
     def execute(remote_mirror)
-      @errors = []
-
       return success unless remote_mirror.enabled?
 
+      errors = []
+
       begin
         remote_mirror.ensure_remote!
         repository.fetch_remote(remote_mirror.remote_name, no_tags: true)
+        project.update_root_ref(remote_mirror.remote_name)
 
         opts = {}
         if remote_mirror.only_protected_branches?

spec/models/project_spec.rb

@@ -3983,6 +3983,40 @@ describe Project do
     end
   end
 
+  describe '#update_root_ref' do
+    let(:project) { create(:project, :repository) }
+
+    it 'updates the default branch when HEAD has changed' do
+      stub_find_remote_root_ref(project, ref: 'feature')
+
+      expect { project.update_root_ref('origin') }
+        .to change { project.default_branch }
+        .from('master')
+        .to('feature')
+    end
+
+    it 'does not update the default branch 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 }
+    end
+
+    it 'does not update the default branch when HEAD does not exist' do
+      stub_find_remote_root_ref(project, ref: 'foo')
+
+      expect { project.update_root_ref('origin') }
+        .not_to change { project.default_branch }
+    end
+
+    def stub_find_remote_root_ref(project, ref:)
+      allow(project.repository)
+        .to receive(:find_remote_root_ref)
+        .with('origin')
+        .and_return(ref)
+    end
+  end
+
   def rugged_config
     Gitlab::GitalyClient::StorageSettings.allow_disk_access do
       project.repository.rugged.config

spec/services/projects/update_remote_mirror_service_spec.rb

 require 'spec_helper'
 
 describe Projects::UpdateRemoteMirrorService do
-  set(:project) { create(:project, :repository) }
-  let(:owner) { project.owner }
+  let(:project) { create(:project, :repository) }
   let(:remote_project) { create(:forked_project_with_submodules) }
-  let(:repository) { project.repository }
-  let(:raw_repository) { repository.raw }
   let(:remote_mirror) { project.remote_mirrors.create!(url: remote_project.http_url_to_repo, enabled: true, only_protected_branches: false) }
+  let(:remote_name) { remote_mirror.remote_name }
 
-  subject { described_class.new(project, project.creator) }
+  subject(:service) { described_class.new(project, project.creator) }
 
   describe "#execute" do
     before do
-      repository.add_branch(owner, 'existing-branch', 'master')
+      project.repository.add_branch(project.owner, 'existing-branch', 'master')
 
       allow(remote_mirror).to receive(:update_repository).and_return(true)
     end
 
+    it "ensures the remote exists" do
+      stub_fetch_remote(project, remote_name: remote_name)
+      stub_find_remote_root_ref(project, remote_name: remote_name)
+
+      expect(remote_mirror).to receive(:ensure_remote!)
+
+      service.execute(remote_mirror)
+    end
+
     it "fetches the remote repository" do
-      expect(remote_mirror).to receive(:ensure_remote!).and_call_original
-      expect(repository).to receive(:fetch_remote).with(remote_mirror.remote_name, no_tags: true) do
-        sync_remote(repository, remote_mirror.remote_name, local_branch_names)
-      end
+      stub_find_remote_root_ref(project, remote_name: remote_name)
+
+      expect(project.repository)
+        .to receive(:fetch_remote)
+        .with(remote_mirror.remote_name, no_tags: true)
+
+      service.execute(remote_mirror)
+    end
+
+    it "updates the default branch when HEAD has changed" do
+      stub_fetch_remote(project, remote_name: remote_name)
+      stub_find_remote_root_ref(project, remote_name: remote_name, ref: "existing-branch")
 
-      subject.execute(remote_mirror)
+      expect { service.execute(remote_mirror) }
+        .to change { project.default_branch }
+        .from("master")
+        .to("existing-branch")
     end
 
-    it "succeeds" do
-      allow(repository).to receive(:fetch_remote) { sync_remote(repository, remote_mirror.remote_name, local_branch_names) }
+    it "does not update the default branch when HEAD does not change" do
+      stub_fetch_remote(project, remote_name: remote_name)
+      stub_find_remote_root_ref(project, remote_name: remote_name, ref: "master")
+
+      expect { service.execute(remote_mirror) }.not_to change { project.default_branch }
+    end
 
-      result = subject.execute(remote_mirror)
+    it "returns success when updated succeeds" do
+      stub_fetch_remote(project, remote_name: remote_name)
+      stub_find_remote_root_ref(project, remote_name: remote_name)
+
+      result = service.execute(remote_mirror)
 
       expect(result[:status]).to eq(:success)
     end
 
     context 'when syncing all branches' do
       it "push all the branches the first time" do
-        allow(repository).to receive(:fetch_remote)
+        stub_fetch_remote(project, remote_name: remote_name)
+        stub_find_remote_root_ref(project, remote_name: remote_name)
 
         expect(remote_mirror).to receive(:update_repository).with({})
 
-        subject.execute(remote_mirror)
+        service.execute(remote_mirror)
       end
     end
 
     context 'when only syncing protected branches' do
-      let(:unprotected_branch_name) { 'existing-branch' }
-      let(:protected_branch_name) do
-        project.repository.branch_names.find { |n| n != unprotected_branch_name }
-      end
-      let!(:protected_branch) do
-        create(:protected_branch, project: project, name: protected_branch_name)
-      end
-
-      before do
-        project.reload
+      it "sync updated protected branches" do
+        stub_fetch_remote(project, remote_name: remote_name)
+        stub_find_remote_root_ref(project, remote_name: remote_name)
+        protected_branch = create_protected_branch(project)
         remote_mirror.only_protected_branches = true
-      end
 
-      it "sync updated protected branches" do
-        allow(repository).to receive(:fetch_remote)
-        expect(remote_mirror).to receive(:update_repository).with(only_branches_matching: [protected_branch_name])
+        expect(remote_mirror)
+          .to receive(:update_repository)
+          .with(only_branches_matching: [protected_branch.name])
 
-        subject.execute(remote_mirror)
+        service.execute(remote_mirror)
       end
-    end
-  end
 
-  def sync_remote(repository, remote_name, local_branch_names)
-    local_branch_names.each do |branch|
-      commit = repository.commit(branch)
-      repository.write_ref("refs/remotes/#{remote_name}/#{branch}", commit.id) if commit
+      def create_protected_branch(project)
+        branch_name = project.repository.branch_names.find { |n| n != 'existing-branch' }
+        create(:protected_branch, project: project, name: branch_name)
+      end
     end
   end
 
-  def update_remote_branch(repository, remote_name, branch)
-    masterrev = repository.commit('master').id
-
-    repository.write_ref("refs/remotes/#{remote_name}/#{branch}", masterrev, force: true)
-    repository.expire_branches_cache
+  def stub_find_remote_root_ref(project, ref: 'master', remote_name:)
+    allow(project.repository)
+      .to receive(:find_remote_root_ref)
+      .with(remote_name)
+      .and_return(ref)
   end
 
-  def update_branch(repository, branch)
-    masterrev = repository.commit('master').id
-
-    repository.write_ref("refs/heads/#{branch}", masterrev, force: true)
-    repository.expire_branches_cache
+  def stub_fetch_remote(project, remote_name:)
+    allow(project.repository)
+      .to receive(:fetch_remote)
+      .with(remote_name, no_tags: true) { fetch_remote(project.repository, remote_name) }
   end
 
-  def generate_tags(repository, *tag_names)
-    tag_names.each_with_object([]) do |name, tags|
-      tag = repository.find_tag(name)
-      target = tag.try(:target)
-      target_commit = tag.try(:dereferenced_target)
-      tags << Gitlab::Git::Tag.new(repository.raw_repository, {
-        name: name,
-        target: target,
-        target_commit: target_commit
-      })
+  def fetch_remote(repository, remote_name)
+    local_branch_names(repository).each do |branch|
+      commit = repository.commit(branch)
+      repository.write_ref("refs/remotes/#{remote_name}/#{branch}", commit.id) if commit
     end
   end
 
-  def local_branch_names
+  def local_branch_names(repository)
     branch_names = repository.branches.map(&:name)
     # we want the protected branch to be pushed first
     branch_names.unshift(branch_names.delete('master'))