Fix the diff in the merge request view when converting a symlink to a regular file.

In this specific case using file_path as a cache key is not enough,
because there are two entries with the same path.
Closes #21610.

2 files changed, 9 insertions, 5 deletions

diff --git a/lib/gitlab/diff/file.rb b/lib/gitlab/diff/file.rb
index e47df508ca2..ce85e5e0123 100644
--- a/lib/gitlab/diff/file.rb
+++ b/lib/gitlab/diff/file.rb
@@ -125,6 +125,10 @@ module Gitlab
 
         repository.blob_at(commit.id, file_path)
       end
+
+      def cache_key
+        "#{file_path}-#{new_file}-#{deleted_file}-#{renamed_file}"
+      end
     end
   end
 end
diff --git a/lib/gitlab/diff/file_collection/merge_request_diff.rb b/lib/gitlab/diff/file_collection/merge_request_diff.rb
index 36348b33943..dc4d47c878b 100644
--- a/lib/gitlab/diff/file_collection/merge_request_diff.rb
+++ b/lib/gitlab/diff/file_collection/merge_request_diff.rb
@@ -35,16 +35,16 @@ module Gitlab
         # for the highlighted ones, so we just skip their execution.
         # If the highlighted diff files lines are not cached we calculate and cache them.
         #
-        # The content of the cache is a Hash where the key correspond to the file_path and the values are Arrays of
+        # The content of the cache is a Hash where the key identifies the file and the values are Arrays of
         # hashes that represent serialized diff lines.
         #
         def cache_highlight!(diff_file)
-          file_path = diff_file.file_path
+          item_key = diff_file.cache_key
 
-          if highlight_cache[file_path]
-            highlight_diff_file_from_cache!(diff_file, highlight_cache[file_path])
+          if highlight_cache[item_key]
+            highlight_diff_file_from_cache!(diff_file, highlight_cache[item_key])
           else
-            highlight_cache[file_path] = diff_file.highlighted_diff_lines.map(&:to_hash)
+            highlight_cache[item_key] = diff_file.highlighted_diff_lines.map(&:to_hash)
           end
         end