Do not cache attention requests count

In https://gitlab.com/gitlab-org/gitlab/-/issues/357385, it was
reported that the cached count is inaccurate with the actual results.

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/84140, the
count query has been improved (from ~400ms down to ~14ms).

Since the query is performing better, we don't need to cache the
count.

This is behind `uncached_mr_attention_requests_count` feature flag.
The invalidation calls are not behind the feature flag to keep it
simple. When we decide to remove the feature flag, we can also
remove the calls for `User#invalidate_attention_requested_count`.

app/models/user.rb

@@ -1712,10 +1712,14 @@ class User < ApplicationRecord
   end
 
   def attention_requested_open_merge_requests_count(force: false)
+    if Feature.enabled?(:uncached_mr_attention_requests_count, self, default_enabled: :yaml)
+      MergeRequestsFinder.new(self, attention: self.username, state: 'opened', non_archived: true).execute.count
+    else
       Rails.cache.fetch(attention_request_cache_key, force: force, expires_in: COUNT_CACHE_VALIDITY_PERIOD) do
         MergeRequestsFinder.new(self, attention: self.username, state: 'opened', non_archived: true).execute.count
       end
     end
+  end
 
   def assigned_open_issues_count(force: false)
     Rails.cache.fetch(['users', id, 'assigned_open_issues_count'], force: force, expires_in: COUNT_CACHE_VALIDITY_PERIOD) do

config/feature_flags/development/uncached_mr_attention_requests_count.yml

+---
+name: uncached_mr_attention_requests_count
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/84145
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/357480
+milestone: '14.10'
+type: development
+group: group::code review
+default_enabled: false

spec/models/user_spec.rb

@@ -4900,17 +4900,36 @@ RSpec.describe User do
   end
 
   describe '#attention_requested_open_merge_requests_count' do
-    it 'returns number of open merge requests from non-archived projects' do
-      user    = create(:user)
-      project = create(:project, :public)
-      archived_project = create(:project, :public, :archived)
+    let(:user) { create(:user) }
+    let(:project) { create(:project, :public) }
+    let(:archived_project) { create(:project, :public, :archived) }
 
+    before do
       create(:merge_request, source_project: project, author: user, reviewers: [user])
       create(:merge_request, :closed, source_project: project, author: user, reviewers: [user])
       create(:merge_request, source_project: archived_project, author: user, reviewers: [user])
+    end
 
+    it 'returns number of open merge requests from non-archived projects' do
+      expect(Rails.cache).not_to receive(:fetch)
       expect(user.attention_requested_open_merge_requests_count(force: true)).to eq 1
     end
+
+    context 'when uncached_mr_attention_requests_count is disabled' do
+      before do
+        stub_feature_flags(uncached_mr_attention_requests_count: false)
+      end
+
+      it 'fetches from cache' do
+        expect(Rails.cache).to receive(:fetch).with(
+          user.attention_request_cache_key,
+          force: false,
+          expires_in: described_class::COUNT_CACHE_VALIDITY_PERIOD
+        ).and_call_original
+
+        expect(user.attention_requested_open_merge_requests_count).to eq 1
+      end
+    end
   end
 
   describe '#assigned_open_issues_count' do