Check (without locking) if the request merge SHA matches the MR SHA

This check is done in the merge service with locking - but we now
preemptively return a more approriate error.

Issue: gitlab.com/gitlab-org/gitlab/-/issues/355567
MR: gitlab.com/gitlab-org/gitlab/-/merge_requests/82775

lib/gitlab/quick_actions/merge_request_actions.rb

@@ -25,6 +25,8 @@ module Gitlab
         execution_message do
           if params[:merge_request_diff_head_sha].blank?
             _("Merge request diff sha parameter is required for the merge quick action.")
+          elsif params[:merge_request_diff_head_sha] != quick_action_target.diff_head_sha
+            _("Branch has been updated since the merge was requested.")
           elsif preferred_strategy = preferred_auto_merge_strategy(quick_action_target)
             _("Scheduled to merge this merge request (%{strategy}).") % { strategy: preferred_strategy.humanize }
           else
@@ -39,6 +41,8 @@ module Gitlab
         command :merge do
           next unless params[:merge_request_diff_head_sha].present?
 
+          next unless params[:merge_request_diff_head_sha] == quick_action_target.diff_head_sha
+
           @updates[:merge] = params[:merge_request_diff_head_sha]
         end
 

locale/gitlab.pot

@@ -5996,6 +5996,9 @@ msgstr ""
 msgid "Branch changed"
 msgstr ""
 
+msgid "Branch has been updated since the merge was requested."
+msgstr ""
+
 msgid "Branch is already taken"
 msgstr ""
 

spec/services/quick_actions/interpret_service_spec.rb

@@ -759,6 +759,7 @@ RSpec.describe QuickActions::InterpretService do
 
     context 'merge command' do
       let(:service) { described_class.new(project, developer, { merge_request_diff_head_sha: merge_request.diff_head_sha }) }
+      let(:merge_request) { create(:merge_request, source_project: repository_project) }
 
       it_behaves_like 'merge immediately command' do
         let(:content) { '/merge' }
@@ -789,7 +790,7 @@ RSpec.describe QuickActions::InterpretService do
       context 'can not be merged when sha does not match' do
         let(:service) { described_class.new(project, developer, { merge_request_diff_head_sha: 'othersha' }) }
 
-        it_behaves_like 'failed command', 'Could not apply merge command.' do
+        it_behaves_like 'failed command', 'Branch has been updated since the merge was requested.' do
           let(:content) { "/merge" }
           let(:issuable) { merge_request }
         end