Check if mapping is empty before caching

- check if mapping is empty before calling `write_multiple`
- add spec

changelogs/unreleased/28211-check-if-mapping-is-empty-before-caching.yml

+---
+title: Check if mapping is empty before caching in File Collections
+merge_request: 18290
+author: briankabiro
+type: performance

lib/gitlab/discussions_diff/file_collection.rb

@@ -27,12 +27,14 @@ module Gitlab
       # - The cache content is not updated (there's no need to do so)
       def load_highlight
         ids = highlightable_collection_ids
+        return if ids.empty?
+
         cached_content = read_cache(ids)
 
         uncached_ids = ids.select.each_with_index { |_, i| cached_content[i].nil? }
         mapping = highlighted_lines_by_ids(uncached_ids)
 
-        HighlightCache.write_multiple(mapping)
+        HighlightCache.write_multiple(mapping) if mapping.any?
 
         diffs = diff_files_indexed_by_id.values_at(*ids)
 

spec/lib/gitlab/discussions_diff/file_collection_spec.rb

@@ -40,6 +40,14 @@ describe Gitlab::DiscussionsDiff::FileCollection do
       subject.load_highlight
     end
 
+    it 'does not write cache for empty mapping' do
+      allow(subject).to receive(:highlighted_lines_by_ids).and_return([])
+
+      expect(Gitlab::DiscussionsDiff::HighlightCache).not_to receive(:write_multiple)
+
+      subject.load_highlight
+    end
+
     it 'does not write cache for resolved notes' do
       diff_note_a.update_column(:resolved_at, Time.now)