Prevent simultaneous use of /rebase and /merge actions

Since /rebase and /merge actions are not atomic, we may run into
confusing race conditions if these are applied together. For now, we
just ignore the /merge and proceed with the rebase.

app/services/merge_requests/update_service.rb

@@ -126,6 +126,12 @@ module MergeRequests
     override :handle_quick_actions
     def handle_quick_actions(merge_request)
       super
+
+      # Ignore /merge if /rebase is used to avoid an unexpected race
+      params.delete(:merge) if params[:rebase] && params[:merge]
+      # Rebase is handled in MergeRequestActions to provide user feedback
+      params.delete(:rebase)
+
       merge_from_quick_action(merge_request) if params[:merge]
     end
 

lib/gitlab/quick_actions/merge_request_actions.rb

@@ -56,6 +56,9 @@ module Gitlab
           access_check.can_push_to_branch?(merge_request.source_branch)
         end
         command :rebase do
+          # This will be used to avoid simultaneous /merge and /rebase actions
+          @updates[:rebase] = true
+
           result = MergeRequests::RebaseService.new(quick_action_target.project, current_user)
                       .execute(quick_action_target)
 

spec/support/shared_examples/quick_actions/merge_request/rebase_quick_action_shared_examples.rb

@@ -14,6 +14,14 @@ RSpec.shared_examples 'rebase quick action' do
       expect(page).not_to have_content('commit behind the target branch')
       expect(merge_request.reload).not_to be_merged
     end
+
+    it 'ignores /merge if /rebase is specified', :sidekiq_inline do
+      fill_in('Description', with: "/merge\n/rebase")
+      click_button('Save changes')
+
+      expect(page).not_to have_content('commit behind the target branch')
+      expect(merge_request.reload).not_to be_merged
+    end
   end
 
   context 'when creating a new note' do