Add cache version and cached markdown version on cache key

This is needed so we can invalidate the cache by bumping the
CACHE_VERSION and also gets invalidated when the cached markdown
version gets bump.

app/models/discussion.rb

@@ -7,6 +7,9 @@ class Discussion
   include GlobalID::Identification
   include ResolvableDiscussion
 
+  # Bump this if we need to refresh the cached versions of discussions
+  CACHE_VERSION = 1
+
   attr_reader :notes, :context_noteable
 
   delegate  :created_at,
@@ -165,6 +168,8 @@ class Discussion
     notes_sha = Digest::SHA1.hexdigest(notes.map(&:id).join(':'))
 
     [
+      CACHE_VERSION,
+      notes.last.latest_cached_markdown_version,
       id,
       notes_sha,
       notes.max_by(&:updated_at).updated_at,

spec/models/discussion_spec.rb

@@ -56,7 +56,7 @@ RSpec.describe Discussion do
     let(:notes_sha) { Digest::SHA1.hexdigest("#{first_note.id}:#{second_note.id}:#{third_note.id}") }
 
     it 'returns the cache key with ID and latest updated note updated at' do
-      expect(subject.cache_key).to eq("#{subject.id}:#{notes_sha}:#{third_note.updated_at}:")
+      expect(subject.cache_key).to eq("#{described_class::CACHE_VERSION}:#{third_note.latest_cached_markdown_version}:#{subject.id}:#{notes_sha}:#{third_note.updated_at}:")
     end
 
     context 'when discussion is resolved' do
@@ -65,7 +65,7 @@ RSpec.describe Discussion do
       end
 
       it 'returns the cache key with resolved at' do
-        expect(subject.cache_key).to eq("#{subject.id}:#{notes_sha}:#{third_note.updated_at}:#{subject.resolved_at}")
+        expect(subject.cache_key).to eq("#{described_class::CACHE_VERSION}:#{third_note.latest_cached_markdown_version}:#{subject.id}:#{notes_sha}:#{third_note.updated_at}:#{subject.resolved_at}")
       end
     end
   end

spec/requests/projects/merge_requests_discussions_spec.rb

@@ -147,6 +147,17 @@ RSpec.describe 'merge requests discussions' do
         end
       end
 
+      context 'when cached markdown version gets bump' do
+        before do
+          settings = Gitlab::CurrentSettings.current_application_settings
+          settings.update!(local_markdown_version: settings.local_markdown_version + 1)
+        end
+
+        it_behaves_like 'cache miss' do
+          let(:changed_notes) { [first_note, second_note] }
+        end
+      end
+
       context 'when merge_request_discussion_cache is disabled' do
         before do
           stub_feature_flags(merge_request_discussion_cache: false)