Merge branch '210500-missing-merge-conflicts-email-notification' into 'master'

Fix missing unmergeable email notification for async mergeability check

See merge request gitlab-org/gitlab!27870

app/models/merge_request.rb

@@ -189,7 +189,7 @@ class MergeRequest < ApplicationRecord
     end
 
     # rubocop: disable CodeReuse/ServiceClass
-    after_transition unchecked: :cannot_be_merged do |merge_request, transition|
+    after_transition [:unchecked, :checking] => :cannot_be_merged do |merge_request, transition|
       if merge_request.notify_conflict?
         NotificationService.new.merge_request_unmergeable(merge_request)
         TodoService.new.merge_request_became_unmergeable(merge_request)

spec/models/merge_request_spec.rb

@@ -3223,6 +3223,14 @@ describe MergeRequest do
             subject.mark_as_unmergeable
           end
 
+          it 'notifies conflict, with enabled async mergability check' do
+            expect(notification_service).to receive(:merge_request_unmergeable).with(subject).once
+            expect(todo_service).to receive(:merge_request_became_unmergeable).with(subject).once
+
+            subject.mark_as_checking
+            subject.mark_as_unmergeable
+          end
+
           it 'does not notify whenever merge request is newly unmergeable due to other reasons' do
             allow(subject.project.repository).to receive(:can_be_merged?).and_return(true)