Handle post processed content scenario for MR discussions cache

We cache MR discussions after post processing. Before this fix, we
aren't considering the post processing result (e.g. reference state
change) which results to stale cache.

This fix adds a `Note#post_processed_cache_key` that is also added
to `Discussion#cache_key`. This way, when the post processing
changes the note's content for a certain user, we will be able to
show updated content.

app/models/discussion.rb

@@ -163,16 +163,15 @@ class Discussion
   end
 
   def cache_key
-    # Need this so cache will be invalidated when note within a discussion
-    # has been deleted.
-    notes_sha = Digest::SHA1.hexdigest(notes.map(&:id).join(':'))
+    # Need to use the notes' cache key so cache will be invalidated when note
+    # within a discussion has been deleted or has different data after post
+    # processing of content.
+    notes_sha = Digest::SHA1.hexdigest(notes.map(&:post_processed_cache_key).join(':'))
 
     [
       CACHE_VERSION,
-      notes.last.latest_cached_markdown_version,
       id,
       notes_sha,
-      notes.max_by(&:updated_at).updated_at,
       resolved_at
     ].join(':')
   end

app/models/note.rb

@@ -586,6 +586,13 @@ class Note < ApplicationRecord
     review.present? || !author.can_trigger_notifications?
   end
 
+  def post_processed_cache_key
+    cache_key_items = [cache_key]
+    cache_key_items << Digest::SHA1.hexdigest(redacted_note_html) if redacted_note_html.present?
+
+    cache_key_items.join(':')
+  end
+
   private
 
   # Using this method followed by a call to *save* may result in *ActiveRecord::RecordNotUnique* exception

spec/models/diff_discussion_spec.rb

@@ -129,10 +129,10 @@ RSpec.describe DiffDiscussion do
 
   describe '#cache_key' do
     it 'returns the cache key with the position sha' do
-      notes_sha = Digest::SHA1.hexdigest("#{diff_note.id}")
+      notes_sha = Digest::SHA1.hexdigest("#{diff_note.post_processed_cache_key}")
       position_sha = Digest::SHA1.hexdigest(diff_note.position.to_json)
 
-      expect(subject.cache_key).to eq("#{described_class::CACHE_VERSION}:#{diff_note.latest_cached_markdown_version}:#{subject.id}:#{notes_sha}:#{diff_note.updated_at}::#{position_sha}")
+      expect(subject.cache_key).to eq("#{described_class::CACHE_VERSION}:#{subject.id}:#{notes_sha}::#{position_sha}")
     end
   end
 end

spec/models/discussion_spec.rb

@@ -53,10 +53,10 @@ RSpec.describe Discussion do
   end
 
   describe '#cache_key' do
-    let(:notes_sha) { Digest::SHA1.hexdigest("#{first_note.id}:#{second_note.id}:#{third_note.id}") }
+    let(:notes_sha) { Digest::SHA1.hexdigest("#{first_note.post_processed_cache_key}:#{second_note.post_processed_cache_key}:#{third_note.post_processed_cache_key}") }
 
-    it 'returns the cache key with ID and latest updated note updated at' do
-      expect(subject.cache_key).to eq("#{described_class::CACHE_VERSION}:#{third_note.latest_cached_markdown_version}:#{subject.id}:#{notes_sha}:#{third_note.updated_at}:")
+    it 'returns the cache key' do
+      expect(subject.cache_key).to eq("#{described_class::CACHE_VERSION}:#{subject.id}:#{notes_sha}:")
     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("#{described_class::CACHE_VERSION}:#{third_note.latest_cached_markdown_version}:#{subject.id}:#{notes_sha}:#{third_note.updated_at}:#{subject.resolved_at}")
+        expect(subject.cache_key).to eq("#{described_class::CACHE_VERSION}:#{subject.id}:#{notes_sha}:#{subject.resolved_at}")
       end
     end
   end

spec/models/note_spec.rb

@@ -1538,4 +1538,24 @@ RSpec.describe Note do
       expect(attachment).not_to be_exist
     end
   end
+
+  describe '#post_processed_cache_key' do
+    let(:note) { build(:note) }
+
+    it 'returns cache key by default' do
+      expect(note.post_processed_cache_key).to eq(note.cache_key)
+    end
+
+    context 'when note has redacted_note_html' do
+      let(:redacted_note_html) { 'redacted note html' }
+
+      before do
+        note.redacted_note_html = redacted_note_html
+      end
+
+      it 'returns cache key with redacted_note_html sha' do
+        expect(note.post_processed_cache_key).to eq("#{note.cache_key}:#{Digest::SHA1.hexdigest(redacted_note_html)}")
+      end
+    end
+  end
 end

spec/requests/projects/merge_requests_discussions_spec.rb

@@ -54,7 +54,8 @@ RSpec.describe 'merge requests discussions' do
     end
 
     context 'caching', :use_clean_rails_memory_store_caching do
-      let!(:first_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project) }
+      let(:reference) { create(:issue, project: project) }
+      let!(:first_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, note: "reference: #{reference.to_reference}") }
       let!(:second_note) { create(:diff_note_on_merge_request, in_reply_to: first_note, noteable: merge_request, project: project) }
       let!(:award_emoji) { create(:award_emoji, awardable: first_note) }
 
@@ -93,6 +94,16 @@ RSpec.describe 'merge requests discussions' do
         end
       end
 
+      context 'when a note in a discussion got its reference state updated' do
+        before do
+          reference.close!
+        end
+
+        it_behaves_like 'cache miss' do
+          let(:changed_notes) { [first_note, second_note] }
+        end
+      end
+
       context 'when a note in a discussion got resolved' do
         before do
           travel_to(1.minute.from_now) do
@@ -147,17 +158,6 @@ 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 the diff note position changes' do
         before do
           # This replicates a position change wherein timestamps aren't updated