Fix stale MR discussion cache when author role changes

Currently, when MR discussion is cached and the author's role has
changed (e.g. Developer to Guest), the role pill and some actions
will show as enabled (e.g. being able to resolve a discussion
but it'll not actually work).

This fix adds the max access of author to the cache key so when it
changes, the cache will be invalidated too.

app/models/note.rb

@@ -583,6 +583,7 @@ class Note < ApplicationRecord
 
   def post_processed_cache_key
     cache_key_items = [cache_key, author&.cache_key]
+    cache_key_items << project.team.human_max_access(author&.id) if author.present?
     cache_key_items << Digest::SHA1.hexdigest(redacted_note_html) if redacted_note_html.present?
 
     cache_key_items.join(':')

spec/models/note_spec.rb

@@ -1573,7 +1573,7 @@ RSpec.describe Note do
     let(:note) { build(:note) }
 
     it 'returns cache key and author cache key by default' do
-      expect(note.post_processed_cache_key).to eq("#{note.cache_key}:#{note.author.cache_key}")
+      expect(note.post_processed_cache_key).to eq("#{note.cache_key}:#{note.author.cache_key}:#{note.project.team.human_max_access(note.author_id)}")
     end
 
     context 'when note has no author' do
@@ -1592,7 +1592,7 @@ RSpec.describe Note do
       end
 
       it 'returns cache key with redacted_note_html sha' do
-        expect(note.post_processed_cache_key).to eq("#{note.cache_key}:#{note.author.cache_key}:#{Digest::SHA1.hexdigest(redacted_note_html)}")
+        expect(note.post_processed_cache_key).to eq("#{note.cache_key}:#{note.author.cache_key}:#{note.project.team.human_max_access(note.author_id)}:#{Digest::SHA1.hexdigest(redacted_note_html)}")
       end
     end
   end

spec/requests/projects/merge_requests_discussions_spec.rb

@@ -59,6 +59,7 @@ RSpec.describe 'merge requests discussions' do
       let!(:first_note) { create(:diff_note_on_merge_request, author: author, 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) }
+      let!(:author_membership) { project.add_maintainer(author) }
 
       before do
         # Make a request to cache the discussions
@@ -229,6 +230,16 @@ RSpec.describe 'merge requests discussions' do
         end
       end
 
+      context 'when author role changes' do
+        before do
+          Members::UpdateService.new(user, access_level: Gitlab::Access::GUEST).execute(author_membership)
+        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)