Invalidate reviews counter cache when MR gets created

Since the "Review requests for you" counter is cached and will
only invalidate after 20 minutes, we need to invalidate the cache
when MR gets created. Doing that, we will be able to show accurate
count on the dropdown when it happens.

app/services/issuable_base_service.rb

@@ -158,7 +158,9 @@ class IssuableBaseService < BaseService
 
       after_create(issuable)
       execute_hooks(issuable)
-      invalidate_cache_counts(issuable, users: issuable.assignees)
+
+      users_to_invalidate = issuable.allows_reviewers? ? issuable.assignees | issuable.reviewers : issuable.assignees
+      invalidate_cache_counts(issuable, users: users_to_invalidate)
       issuable.update_project_counter_caches
     end
 

changelogs/unreleased/296842-invalidate-reviewer-counter-on-create.yml

+---
+title: Invalidate reviews counter cache when MR gets created
+merge_request: 51316
+author:
+type: fixed

spec/services/merge_requests/create_service_spec.rb

@@ -155,6 +155,12 @@ RSpec.describe MergeRequests::CreateService, :clean_gitlab_redis_shared_state do
 
           expect(Todo.where(attributes).count).to eq 1
         end
+
+        it 'invalidates counter cache for reviewers', :use_clean_rails_memory_store_caching do
+          expect { merge_request }
+            .to change { user2.review_requested_open_merge_requests_count }
+            .by(1)
+        end
       end
 
       context 'when head pipelines already exist for merge request source branch', :sidekiq_inline do