Persist markdown cache when accessing mentions

This fixes a performance regression when mentions are being read
repeatedly from the API and we have to re-render all the notes to
parse the HTML

app/models/concerns/cache_markdown_field.rb

@@ -102,7 +102,7 @@ module CacheMarkdownField
   def updated_cached_html_for(markdown_field)
     return unless cached_markdown_fields.markdown_fields.include?(markdown_field)
 
-    refresh_markdown_cache if attribute_invalidated?(cached_markdown_fields.html_field(markdown_field))
+    refresh_markdown_cache! if attribute_invalidated?(cached_markdown_fields.html_field(markdown_field))
 
     cached_html_for(markdown_field)
   end

changelogs/unreleased/218745-single-issue-api-performance-regression-degraded-by-8000ms-after-u.yml

+---
+title: Fix issue and MR API performance regression when Markdown cache is stale
+merge_request: 33235
+author:
+type: fixed

spec/models/concerns/cache_markdown_field_spec.rb

@@ -209,8 +209,8 @@ describe CacheMarkdownField, :clean_gitlab_redis_cache do
           thing.cached_markdown_version += 1
         end
 
-        it 'calls #refresh_markdown_cache' do
-          expect(thing).to receive(:refresh_markdown_cache)
+        it 'calls #refresh_markdown_cache!' do
+          expect(thing).to receive(:refresh_markdown_cache!)
 
           expect(thing.updated_cached_html_for(:description)).to eq(html)
         end
@@ -227,8 +227,8 @@ describe CacheMarkdownField, :clean_gitlab_redis_cache do
           thing.try(:save)
         end
 
-        it 'does not call #refresh_markdown_cache' do
-          expect(thing).not_to receive(:refresh_markdown_cache)
+        it 'does not call #refresh_markdown_cache!' do
+          expect(thing).not_to receive(:refresh_markdown_cache!)
 
           expect(thing.updated_cached_html_for(:description)).to eq(html)
         end

spec/support/shared_examples/models/mentionable_shared_examples.rb

@@ -80,13 +80,11 @@ RSpec.shared_examples 'a mentionable' do
 
   context 'when there are cached markdown fields' do
     before do
-      if subject.is_a?(CacheMarkdownField)
-        subject.refresh_markdown_cache
-      end
+      skip unless subject.is_a?(CacheMarkdownField)
     end
 
     it 'sends in cached markdown fields when appropriate' do
-      if subject.is_a?(CacheMarkdownField) && subject.extractors[author].blank?
+      subject.extractors[author] = nil
       expect_next_instance_of(Gitlab::ReferenceExtractor) do |ext|
         attrs = subject.class.mentionable_attrs.collect(&:first) & subject.cached_markdown_fields.markdown_fields
         attrs.each do |field|
@@ -94,13 +92,11 @@ RSpec.shared_examples 'a mentionable' do
         end
       end
 
-        expect(subject).not_to receive(:refresh_markdown_cache)
       expect(subject).to receive(:cached_markdown_fields).at_least(:once).and_call_original
 
       subject.all_references(author)
     end
   end
-  end
 
   it 'creates cross-reference notes' do
     mentioned_objects = [mentioned_issue, mentioned_mr, mentioned_commit,
@@ -126,28 +122,42 @@ RSpec.shared_examples 'an editable mentionable' do
 
   context 'when there are cached markdown fields' do
     before do
-      if subject.is_a?(CacheMarkdownField)
-        subject.refresh_markdown_cache
-      end
-    end
+      skip unless subject.is_a?(CacheMarkdownField)
 
-    it 'refreshes markdown cache if necessary' do
       subject.save!
+    end
 
+    it 'refreshes markdown cache if necessary' do
       set_mentionable_text.call('This is a text')
 
-      if subject.is_a?(CacheMarkdownField) && subject.extractors[author].blank?
+      subject.extractors[author] = nil
       expect_next_instance_of(Gitlab::ReferenceExtractor) do |ext|
         subject.cached_markdown_fields.markdown_fields.each do |field|
           expect(ext).to receive(:analyze).with(subject.send(field), hash_including(rendered: anything))
         end
       end
 
-        expect(subject).to receive(:refresh_markdown_cache)
+      expect(subject).to receive(:refresh_markdown_cache).and_call_original
       expect(subject).to receive(:cached_markdown_fields).at_least(:once).and_call_original
 
       subject.all_references(author)
     end
+
+    context 'when the markdown cache is stale' do
+      before do
+        expect(subject).to receive(:latest_cached_markdown_version).at_least(:once) do
+          (Gitlab::MarkdownCache::CACHE_COMMONMARK_VERSION + 1) << 16
+        end
+      end
+
+      it 'persists the refreshed cache so that it does not have to be refreshed every time' do
+        expect(subject).to receive(:refresh_markdown_cache).once.and_call_original
+
+        subject.all_references(author)
+
+        subject.reload
+        subject.all_references(author)
+      end
     end
   end