Empty repository mirror no longer creates master branch with README automatically

app/services/create_branch_service.rb

 class CreateBranchService < BaseService
-  def execute(branch_name, ref)
-    create_master_branch if project.empty_repo?
+  def execute(branch_name, ref, create_master_if_empty: true)
+    create_master_branch if create_master_if_empty && project.empty_repo?
 
     result = ValidateNewBranchService.new(project, current_user)
       .execute(branch_name)

app/services/projects/update_mirror_service.rb

@@ -36,7 +36,7 @@ module Projects
         local_branch = local_branches[name]
 
         if local_branch.nil?
-          result = CreateBranchService.new(project, current_user).execute(name, upstream_branch.dereferenced_target.sha)
+          result = CreateBranchService.new(project, current_user).execute(name, upstream_branch.dereferenced_target.sha, create_master_if_empty: false)
           if result[:status] == :error
             errors << result[:message]
           end

changelogs/unreleased-ee/2425-fix-failure-for-mirrors-with-empty-repository.yml

+---
+title: Empty repository mirror no longer creates master branch with README automatically.
+merge_request: 2276
+author:

spec/services/projects/update_mirror_service_spec.rb

@@ -63,6 +63,31 @@ describe Projects::UpdateMirrorService do
         expect(project.repository.find_branch('markdown').dereferenced_target)
           .not_to eq(project.repository.find_branch('master').dereferenced_target)
       end
+
+      describe 'when project is empty' do
+        let(:project) { create(:project_empty_repo, :mirror, import_url: Project::UNKNOWN_IMPORT_URL) }
+
+        it 'does not add a default master branch' do
+          repository = project.repository
+
+          allow(project).to receive(:fetch_mirror) { create_file(repository) }
+          expect(CreateBranchService).not_to receive(:create_master_branch)
+
+          described_class.new(project, project.owner).execute
+
+          expect(repository.branch_names).not_to include('master')
+        end
+      end
+
+      def create_file(repository)
+        repository.create_file(
+          project.owner,
+          '/newfile.txt',
+          'hello',
+          message: 'Add newfile.txt',
+          branch_name: 'newbranch'
+        )
+      end
     end
 
     describe "when the mirror user doesn't have access" do