Use persisted diff data instead fetching Git on discussions

Today, when fetching diffs of a note, we always go to Gitaly in order to diff between commits and return the diff of each discussion note. With this change we avoid doing that for notes on the "current version" of the MR.

1 files changed, 14 insertions, 1 deletions

diff --git a/app/models/diff_note.rb b/app/models/diff_note.rb
index 15122cbc693..616a626419b 100644
--- a/app/models/diff_note.rb
+++ b/app/models/diff_note.rb
@@ -54,7 +54,20 @@ class DiffNote < Note
   end
 
   def diff_file
-    @diff_file ||= self.original_position.diff_file(self.project.repository)
+    @diff_file ||=
+      begin
+        if created_at_diff?(noteable.diff_refs)
+          # We're able to use the already persisted diffs (Postgres) if we're
+          # presenting a "current version" of the MR discussion diff.
+          # So no need to make an extra Gitaly diff request for it.
+          # As an extra benefit, the returned `diff_file` already
+          # has `highlighted_diff_lines` data set from Redis on
+          # `Diff::FileCollection::MergeRequestDiff`.
+          noteable.diffs(paths: original_position.paths, expanded: true).diff_files.first
+        else
+          original_position.diff_file(self.project.repository)
+        end
+      end
   end
 
   def diff_line