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.

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

changelogs/unreleased/osw-use-cached-highlighted-content-for-discussions.yml

+---
+title: Use persisted diff data instead fetching Git on discussions
+merge_request:
+author:
+type: performance

lib/gitlab/diff/file_collection/base.rb

@@ -36,6 +36,8 @@ module Gitlab
         private
 
         def decorate_diff!(diff)
+          return diff if diff.is_a?(File)
+
           Gitlab::Diff::File.new(diff, repository: project.repository, diff_refs: diff_refs, fallback_diff_refs: fallback_diff_refs)
         end
       end

spec/models/diff_note_spec.rb

@@ -85,7 +85,8 @@ describe DiffNote do
   end
 
   describe "#diff_file" do
-    it "returns the correct diff file" do
+    context 'when the discussion was created in the diff' do
+      it 'returns correct diff file' do
         diff_file = subject.diff_file
 
         expect(diff_file.old_path).to eq(position.old_path)
@@ -94,6 +95,28 @@ describe DiffNote do
       end
     end
 
+    context 'when discussion is outdated or not created in the diff' do
+      let(:diff_refs) { project.commit(sample_commit.id).diff_refs }
+      let(:position) do
+        Gitlab::Diff::Position.new(
+          old_path: "files/ruby/popen.rb",
+          new_path: "files/ruby/popen.rb",
+          old_line: nil,
+          new_line: 14,
+          diff_refs: diff_refs
+        )
+      end
+
+      it 'returns the correct diff file' do
+        diff_file = subject.diff_file
+
+        expect(diff_file.old_path).to eq(position.old_path)
+        expect(diff_file.new_path).to eq(position.new_path)
+        expect(diff_file.diff_refs).to eq(position.diff_refs)
+      end
+    end
+  end
+
   describe "#diff_line" do
     it "returns the correct diff line" do
       diff_line = subject.diff_line