Fix MR creation on fork of deleted project

The `selected_target_project` helper method is used for actions where
`target_project_id` is a top-level parameter, not nested under the
`merge_request` hash.

Even though that logic was wrong, if the target project wasn't a fork,
or was a fork of an active project, this would work through
coincidence (although it might check the wrong value for
`approvals_before_merge`).

If a project was a fork of a deleted project, however, then
`selected_target_project` would be `nil`, causing a 500 error. Ensure
that there is always a target project to compare to.

CHANGELOG-EE

@@ -4,6 +4,7 @@ v 8.10.0 (unreleased)
 
 v 8.9.3 (unreleased)
   - Fix encrypted data backwards compatibility after upgrading attr_encrypted gem
+  - Fix creating MRs on forks of deleted projects
 
 v 8.9.1
   - Improve Geo documentation. !431

app/controllers/projects/merge_requests_controller.rb

@@ -410,7 +410,10 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   end
 
   def clamp_approvals_before_merge(mr_params)
-    if mr_params[:approvals_before_merge].to_i <= selected_target_project.approvals_before_merge
+    target_project = @project.forked_from_project if @project.id.to_s != mr_params[:target_project_id]
+    target_project ||= @project
+
+    if mr_params[:approvals_before_merge].to_i <= target_project.approvals_before_merge
       mr_params.delete(:approvals_before_merge)
     end
 

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -92,6 +92,25 @@ describe Projects::MergeRequestsController do
           expect(response).to redirect_to(namespace_project_merge_request_path(id: created_merge_request.iid, project_id: project.to_param))
         end
       end
+
+      context 'when the target project is a fork of a deleted project' do
+        before do
+          original_project = create(:empty_project)
+          project.update_attributes(forked_from_project: original_project, approvals_before_merge: 4)
+          original_project.update_attributes(pending_delete: true)
+
+          create_merge_request(approvals_before_merge: 3)
+        end
+
+        it 'uses the default from the target project' do
+          expect(created_merge_request.approvals_before_merge).to eq(nil)
+        end
+
+        it 'creates the merge request' do
+          expect(created_merge_request).to be_valid
+          expect(response).to redirect_to(namespace_project_merge_request_path(id: created_merge_request.iid, project_id: project.to_param))
+        end
+      end
     end
 
     context 'when the merge request is invalid' do