Only assign merge params when allowed

When a user updates a merge request coming from a fork, they should
not be able to set `force_remove_source_branch` if they cannot push
code to the source project.

Otherwise developers of the target project could remove the source
branch of the source project by setting this flag through the API.

2 files changed, 26 insertions, 1 deletions

diff --git a/spec/services/merge_requests/build_service_spec.rb b/spec/services/merge_requests/build_service_spec.rb
index 46e86d5b4cb..9b358839c06 100644
--- a/spec/services/merge_requests/build_service_spec.rb
+++ b/spec/services/merge_requests/build_service_spec.rb
@@ -83,8 +83,9 @@ describe MergeRequests::BuildService do
       expect(merge_request.force_remove_source_branch?).to be_truthy
     end
 
-    context 'with force_remove_source_branch parameter' do
+    context 'with force_remove_source_branch parameter when the user is authorized' do
       let(:mr_params) { params.merge(force_remove_source_branch: '1') }
+      let(:source_project) { fork_project(project, user) }
       let(:merge_request) { described_class.new(project, user, mr_params).execute }
 
       it 'assigns force_remove_source_branch' do
diff --git a/spec/services/merge_requests/update_service_spec.rb b/spec/services/merge_requests/update_service_spec.rb
index 8c796475de0..741420d76a7 100644
--- a/spec/services/merge_requests/update_service_spec.rb
+++ b/spec/services/merge_requests/update_service_spec.rb
@@ -646,5 +646,29 @@ describe MergeRequests::UpdateService, :mailer do
         expect(merge_request.allow_collaboration).to be_truthy
       end
     end
+
+    context 'updating `force_remove_source_branch`' do
+      let(:target_project) { create(:project, :repository, :public) }
+      let(:source_project) { fork_project(target_project, nil, repository: true) }
+      let(:user) { target_project.owner }
+      let(:merge_request) do
+        create(:merge_request,
+               source_project: source_project,
+               source_branch: 'fixes',
+               target_project: target_project)
+      end
+
+      it "cannot be done by members of the target project when they don't have access" do
+        expect { update_merge_request(force_remove_source_branch: true) }
+          .not_to change { merge_request.reload.force_remove_source_branch? }.from(nil)
+      end
+
+      it 'can be done by members of the target project if they can push to the source project' do
+        source_project.add_developer(user)
+
+        expect { update_merge_request(force_remove_source_branch: true) }
+          .to change { merge_request.reload.force_remove_source_branch? }.from(nil).to(true)
+      end
+    end
   end
 end