Allow collaboration with forks through the API

These APIs are used by the web IDE.

They need to be on par with git & web access, allowing edits from
maintainers to forks with merge requests that allow access.

lib/api/commits.rb

@@ -6,6 +6,18 @@ module API
 
     before { authorize! :download_code, user_project }
 
+    helpers do
+      def user_access
+        @user_access ||= Gitlab::UserAccess.new(current_user, project: user_project)
+      end
+
+      def authorize_push_to_branch!(branch)
+        unless user_access.can_push_to_branch?(branch)
+          forbidden!("You are not allowed to push into this branch")
+        end
+      end
+    end
+
     params do
       requires :id, type: String, desc: 'The ID of a project'
     end
@@ -67,7 +79,7 @@ module API
         optional :author_name, type: String, desc: 'Author name for commit'
       end
       post ':id/repository/commits' do
-        authorize! :push_code, user_project
+        authorize_push_to_branch!(params[:branch])
 
         attrs = declared_params
         attrs[:branch_name] = attrs.delete(:branch)
@@ -142,7 +154,7 @@ module API
         requires :branch, type: String, desc: 'The name of the branch'
       end
       post ':id/repository/commits/:sha/cherry_pick', requirements: API::COMMIT_ENDPOINT_REQUIREMENTS do
-        authorize! :push_code, user_project
+        authorize_push_to_branch!(params[:branch])
 
         commit = user_project.commit(params[:sha])
         not_found!('Commit') unless commit

spec/requests/api/commits_spec.rb

@@ -514,6 +514,38 @@ describe API::Commits do
         expect(response).to have_gitlab_http_status(400)
       end
     end
+
+    context 'when committing into a fork as a maintainer' do
+      include_context 'merge request allowing collaboration'
+
+      let(:project_id) { forked_project.id }
+
+      def push_params(branch_name)
+        {
+          branch: branch_name,
+          commit_message: 'Hello world',
+          actions: [
+            {
+              action: 'create',
+              file_path: 'foo/bar/baz.txt',
+              content: 'puts 8'
+            }
+          ]
+        }
+      end
+
+      it 'allows pushing to the source branch of the merge request' do
+        post api(url, user), push_params('feature')
+
+        expect(response).to have_gitlab_http_status(:created)
+      end
+
+      it 'denies pushing to another branch' do
+        post api(url, user), push_params('other-branch')
+
+        expect(response).to have_gitlab_http_status(:forbidden)
+      end
+    end
   end
 
   describe 'GET /projects/:id/repository/commits/:sha/refs' do
@@ -1065,9 +1097,27 @@ describe API::Commits do
         it 'returns 400 if you are not allowed to push to the target branch' do
           post api(route, current_user), branch: 'feature'
 
-          expect(response).to have_gitlab_http_status(400)
-          expect(json_response['message']).to eq('You are not allowed to push into this branch')
+          expect(response).to have_gitlab_http_status(:forbidden)
+          expect(json_response['message']).to match(/You are not allowed to push into this branch/)
+        end
+      end
     end
+
+    context 'when cherry picking to a fork as a maintainer' do
+      include_context 'merge request allowing collaboration'
+
+      let(:project_id) { forked_project.id }
+
+      it 'allows access from a maintainer that to the source branch' do
+        post api(route, user), branch: 'feature'
+
+        expect(response).to have_gitlab_http_status(:created)
+      end
+
+      it 'denies cherry picking to another branch' do
+        post api(route, user), branch: 'master'
+
+        expect(response).to have_gitlab_http_status(:forbidden)
       end
     end
   end

spec/support/shared_contexts/merge_requests_allowing_collaboration.rb

+shared_context 'merge request allowing collaboration' do
+  include ProjectForksHelper
+
+  let(:canonical) { create(:project, :public, :repository) }
+  let(:forked_project) { fork_project(canonical, nil, repository: true) }
+
+  before do
+    canonical.add_maintainer(user)
+    create(:merge_request,
+           target_project: canonical,
+           source_project: forked_project,
+           source_branch: 'feature',
+           allow_collaboration: true)
+  end
+end