Allow maintainers to push to a single branch

lib/gitlab/user_access.rb

@@ -68,8 +68,12 @@ module Gitlab
         return true if project.user_can_push_to_empty_repo?(user)
 
         protected_branch_accessible_to?(ref, action: :push)
+      elsif user.can?(:push_code, project)
+        true
+      elsif user.can?(:push_single_branch, project)
+        project.branches_allowing_maintainer_access_to_user(user).include?(ref)
       else
-        user.can?(:push_code, project)
+        false
       end
     end
 

spec/lib/gitlab/user_access_spec.rb

 require 'spec_helper'
 
 describe Gitlab::UserAccess do
+  include ProjectForksHelper
+
   let(:access) { described_class.new(user, project: project) }
   let(:project) { create(:project, :repository) }
   let(:user) { create(:user) }
@@ -118,6 +120,39 @@ describe Gitlab::UserAccess do
       end
     end
 
+    describe 'allowing pushes to maintainers of forked projects' do
+      let(:canonical_project) { create(:project, :public, :repository) }
+      let(:project) { fork_project(canonical_project, create(:user), repository: true) }
+
+      before do
+        create(
+          :merge_request,
+          target_project: canonical_project,
+          source_project: project,
+          source_branch: 'awesome-feature',
+          allow_maintainer_to_push: true
+        )
+      end
+
+      it 'allows users that have push access to the canonical project to push to the MR branch' do
+        canonical_project.add_developer(user)
+
+        expect(access.can_push_to_branch?('awesome-feature')).to be_truthy
+      end
+
+      it 'does not allow the user to push to other branches' do
+        canonical_project.add_developer(user)
+
+        expect(access.can_push_to_branch?('master')).to be_falsey
+      end
+
+      it 'does not allow the user to push if he does not have push access to the canonical project' do
+        canonical_project.add_guest(user)
+
+        expect(access.can_push_to_branch?('awesome-feature')).to be_falsey
+      end
+    end
+
     describe 'merge to protected branch if allowed for developers' do
       before do
         @branch = create :protected_branch, :developers_can_merge, project: project