Merge branch '21211-comment-on-diff-partially-broken-after-updating-to-8-11' into 'master'

Fix diff comments on legacy MRs

## What does this MR do?

Allow diff commenting on MRs without complete diff refs. (Commenting would work before, but the JSON response would fail, so you'd only see your comment after a refresh.)

## Are there points in the code the reviewer needs to double check?

Is this really all I needed to do? I feel like there must be more to it 😊

## Why was this MR needed?

It's a regression!

## What are the relevant issue numbers?

Closes #21211.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6029

CHANGELOG

@@ -49,6 +49,9 @@ v 8.11.4 (unreleased)
 v 8.11.4 (unreleased)
   - Fix resolving conflicts on forks
 
+v 8.11.4 (unreleased)
+  - Fix diff commenting on merge requests created prior to 8.10
+
 v 8.11.3 (unreleased)
   - Do not enforce using hash with hidden key in CI configuration. !6079
   - Allow system info page to handle case where info is unavailable

app/models/concerns/note_on_diff.rb

@@ -28,4 +28,8 @@ module NoteOnDiff
   def can_be_award_emoji?
     false
   end
+
+  def to_discussion
+    Discussion.new([self])
+  end
 end

app/models/diff_note.rb

@@ -107,10 +107,6 @@ class DiffNote < Note
     self.noteable.find_diff_discussion(self.discussion_id)
   end
 
-  def to_discussion
-    Discussion.new([self])
-  end
-
   private
 
   def supported?

spec/features/merge_requests/diff_notes_spec.rb

@@ -147,6 +147,37 @@ feature 'Diff notes', js: true, feature: true do
       end
     end
 
+    context 'when the MR only supports legacy diff notes' do
+      before do
+        @merge_request.merge_request_diff.update_attributes(start_commit_sha: nil)
+        visit diffs_namespace_project_merge_request_path(@project.namespace, @project, @merge_request, view: 'inline')
+      end
+
+      context 'with a new line' do
+        it 'should allow commenting' do
+          should_allow_commenting(find('[id="2f6fcd96b88b36ce98c38da085c795a27d92a3dd_10_9"]'))
+        end
+      end
+
+      context 'with an old line' do
+        it 'should allow commenting' do
+          should_allow_commenting(find('[id="6eb14e00385d2fb284765eb1cd8d420d33d63fc9_22_22"]'))
+        end
+      end
+
+      context 'with an unchanged line' do
+        it 'should allow commenting' do
+          should_allow_commenting(find('[id="2f6fcd96b88b36ce98c38da085c795a27d92a3dd_7_7"]'))
+        end
+      end
+
+      context 'with a match line' do
+        it 'should not allow commenting' do
+          should_not_allow_commenting(find('.match', match: :first))
+        end
+      end
+    end
+
     def should_allow_commenting(line_holder, diff_side = nil)
       line = get_line_components(line_holder, diff_side)
       line[:content].hover