Include position for cache key of DiffDiscussion

In https://gitlab.com/gitlab-org/gitlab/-/issues/332967, we
introduced caching on discussions endpoint.

Upon testing it further while rolling it out, found a bug wherein
the diff note that should still be displayed won't display anymore
because position changed. The position stored in cache no longer
applies as it's outdated.

With this fix, we include the position of the first note in diff
discussion (as sha) in the cache_key. This way, when the position
changes, the cache key will be updated and will result to a cache
miss and will return updated data.

app/models/diff_discussion.rb

@@ -42,6 +42,13 @@ class DiffDiscussion < Discussion
     )
   end
 
+  def cache_key
+    [
+      super,
+      Digest::SHA1.hexdigest(position.to_json)
+    ].join(':')
+  end
+
   private
 
   def get_params

spec/models/diff_discussion_spec.rb

@@ -126,4 +126,13 @@ RSpec.describe DiffDiscussion do
       end
     end
   end
+
+  describe '#cache_key' do
+    it 'returns the cache key with the position sha' do
+      notes_sha = Digest::SHA1.hexdigest("#{diff_note.id}")
+      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}")
+    end
+  end
 end

spec/requests/projects/merge_requests_discussions_spec.rb

@@ -158,6 +158,29 @@ RSpec.describe 'merge requests discussions' do
         end
       end
 
+      context 'when the diff note position changes' do
+        before do
+          # This replicates a position change wherein timestamps aren't updated
+          # which is why `Gitlab::Timeless.timeless` is utilized. This is the
+          # same approach being used in Discussions::UpdateDiffPositionService
+          # which is responsible for updating the positions of diff discussions
+          # when MR updates.
+          first_note.position = Gitlab::Diff::Position.new(
+            old_path: first_note.position.old_path,
+            new_path: first_note.position.new_path,
+            old_line: first_note.position.old_line,
+            new_line: first_note.position.new_line + 1,
+            diff_refs: first_note.position.diff_refs
+          )
+
+          Gitlab::Timeless.timeless(first_note, &:save)
+        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)