Improve cache sharing between cached markdown fields and mentionables

This change makes an item of implicit coupling between the Mentionable
and CachedMarkdownField concerns. Mentionable needs to render the
fields it depends on to extract references; when those fields are also
cached markdown fields, it currently uses that cache to skip the render
step.

For the Commit model, we couldn't share the cache because Mentionable
was looking at safe_message, while CachedMarkdownField was looking at
`title`, `full_title`, and `description`. However, the combination of
the latter two is equivalent to `safe_message`, so we can use them
instead.

This opens up a couple of cans of worms - in particular, we now call
Commit#store_mentions!, which we've never done before. However, it all
seems to work out in the end.

Changelog: performance

app/models/commit.rb

@@ -15,8 +15,6 @@ class Commit
   include ActsAsPaginatedDiff
   include CacheMarkdownField
 
-  attr_mentionable :safe_message, pipeline: :single_line
-
   participant :author
   participant :committer
   participant :notes_with_associations
@@ -39,6 +37,10 @@ class Commit
   cache_markdown_field :full_title, pipeline: :single_line, limit: 1.kilobyte
   cache_markdown_field :description, pipeline: :commit_description, limit: 1.megabyte
 
+  # Share the cache used by the markdown fields
+  attr_mentionable :full_title, pipeline: :single_line, limit: 1.kilobyte
+  attr_mentionable :description, pipeline: :commit_description, limit: 1.megabyte
+
   class << self
     def decorate(commits, container)
       commits.map do |commit|

app/models/concerns/cache_markdown_field.rb

@@ -157,6 +157,9 @@ module CacheMarkdownField
   end
 
   def store_mentions!
+    # We can only store mentions if the mentionable is a database object
+    return unless self.is_a?(ApplicationRecord)
+
     refs = all_references(self.author)
 
     references = {}

ee/spec/support/shared_examples/models/mentionable_shared_examples.rb

@@ -33,7 +33,7 @@ RSpec.shared_examples 'a mentionable with EE-specific mentions' do
     expect(refs).to include(mentioned_epic)
   end
 
-  it 'creates cross-reference notes' do
+  it 'creates cross-reference notes', :clean_gitlab_redis_cache do
     mentioned_objects = [mentioned_epic]
 
     mentioned_objects.each do |referenced|

spec/models/commit_spec.rb

@@ -471,16 +471,25 @@ eos
   end
 
   it_behaves_like 'a mentionable' do
-    subject { create(:project, :repository).commit }
+    subject(:commit) { create(:project, :repository).commit }
 
     let(:author) { create(:user, email: subject.author_email) }
     let(:backref_text) { "commit #{subject.id}" }
     let(:set_mentionable_text) do
-      ->(txt) { allow(subject).to receive(:safe_message).and_return(txt) }
+      ->(txt) { allow(commit).to receive(:safe_message).and_return(txt) }
     end
 
     # Include the subject in the repository stub.
-    let(:extra_commits) { [subject] }
+    let(:extra_commits) { [commit] }
+
+    it 'uses the CachedMarkdownField cache instead of the Mentionable cache', :use_clean_rails_redis_caching do
+      expect(commit.title_html).not_to be_present
+
+      commit.all_references(project.owner).all
+
+      expect(commit.title_html).to be_present
+      expect(Rails.cache.read("banzai/commit:#{commit.id}/safe_message/single_line")).to be_nil
+    end
   end
 
   describe '#hook_attrs' do

spec/support/shared_examples/models/mentionable_shared_examples.rb

@@ -66,7 +66,7 @@ RSpec.shared_examples 'a mentionable' do
     expect(subject.gfm_reference).to eq(backref_text)
   end
 
-  it "extracts references from its reference property" do
+  it "extracts references from its reference property", :clean_gitlab_redis_cache do
     # De-duplicate and omit itself
     refs = subject.referenced_mentionables
     expect(refs.size).to eq(6)
@@ -98,7 +98,7 @@ RSpec.shared_examples 'a mentionable' do
     end
   end
 
-  it 'creates cross-reference notes' do
+  it 'creates cross-reference notes', :clean_gitlab_redis_cache do
     mentioned_objects = [mentioned_issue, mentioned_mr, mentioned_commit,
                          ext_issue, ext_mr, ext_commit]
 

spec/workers/process_commit_worker_spec.rb

@@ -138,7 +138,7 @@ RSpec.describe ProcessCommitWorker do
     end
   end
 
-  describe '#update_issue_metrics' do
+  describe '#update_issue_metrics', :clean_gitlab_redis_cache do
     context 'when commit has issue reference' do
       subject(:update_metrics_and_reload) do
         -> {