Cache diff highlight in discussions

This commit handles note diffs caching, which considerably improves
the performance on merge requests with lots of comments.
Important to note that the caching approach taken here is different
from `Gitlab::Diff::HighlightCache`. We do not reset the whole cache
when a new push is sent or anything else. That's because discussions
diffs are persisted and do not change.

1 files changed, 19 insertions, 1 deletions

diff --git a/app/models/concerns/discussion_on_diff.rb b/app/models/concerns/discussion_on_diff.rb
index 86b61248534..e4e5928f5cf 100644
--- a/app/models/concerns/discussion_on_diff.rb
+++ b/app/models/concerns/discussion_on_diff.rb
@@ -9,7 +9,7 @@ module DiscussionOnDiff
   included do
     delegate  :line_code,
               :original_line_code,
-              :diff_file,
+              :note_diff_file,
               :diff_line,
               :active?,
               :created_at_diff?,
@@ -60,6 +60,13 @@ module DiscussionOnDiff
     prev_lines
   end
 
+  def diff_file
+    strong_memoize(:diff_file) do
+      # Falling back here is important as `note_diff_files` are created async.
+      fetch_preloaded_diff_file || first_note.diff_file
+    end
+  end
+
   def line_code_in_diffs(diff_refs)
     if active?(diff_refs)
       line_code
@@ -67,4 +74,15 @@ module DiscussionOnDiff
       original_line_code
     end
   end
+
+  private
+
+  def fetch_preloaded_diff_file
+    fetch_preloaded_diff =
+      context_noteable &&
+      context_noteable.preloads_discussion_diff_highlighting? &&
+      note_diff_file
+
+    context_noteable.discussions_diffs.find_by_id(note_diff_file.id) if fetch_preloaded_diff
+  end
 end