Revert "Merge branch '216458-remove-tempfile-from-external-diff-creation' into 'master'"

This reverts merge request !35750

app/models/merge_request_diff.rb

@@ -493,6 +493,8 @@ class MergeRequestDiff < ApplicationRecord
     self.stored_externally = true
 
     rows
+  ensure
+    tempfile&.unlink
   end
 
   def create_merge_request_diff_files(rows)
@@ -503,19 +505,17 @@ class MergeRequestDiff < ApplicationRecord
   end
 
   def build_external_diff_tempfile(rows)
-    pos = 0
+    Tempfile.open(external_diff.filename) do |file|
+      rows.each do |row|
+        data = row.delete(:diff)
+        row[:external_diff_offset] = file.pos
+        row[:external_diff_size] = data.bytesize
 
-    segments = rows.map do |row|
-      segment = row.delete(:diff)
-
-      row[:external_diff_offset] = pos
-      row[:external_diff_size] = segment.bytesize
-      pos += segment.bytesize
+        file.write(data)
+      end
 
-      segment
+      file
     end
-
-    CarrierWaveStringFile.new(segments.join(''), external_diff.filename)
   end
 
   def build_merge_request_diff_files(diffs)

changelogs/unreleased/216458-remove-tempfile-from-external-diff-creation.yml

----
-title: Remove tempfile from external diff creation
-merge_request: 35750
-author:
-type: performance

lib/carrier_wave_string_file.rb

 # frozen_string_literal: true
 
 class CarrierWaveStringFile < StringIO
-  attr_reader :original_filename
-
-  def initialize(data, original_filename = "")
-    super(data)
-
-    @original_filename = original_filename
+  def original_filename
+    ""
   end
 end