Remove tempfile from external diff creation

Since we already hold the diff in RAM, and this is a limited diff,
going to a temporary file is unnecessary and may introduce unexpected
latency in general.

app/models/merge_request_diff.rb

@@ -487,8 +487,6 @@ class MergeRequestDiff < ApplicationRecord
     self.stored_externally = true
 
     rows
-  ensure
-    tempfile&.unlink
   end
 
   def create_merge_request_diff_files(rows)
@@ -499,17 +497,19 @@ class MergeRequestDiff < ApplicationRecord
   end
 
   def build_external_diff_tempfile(rows)
-    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
+    pos = 0
 
-        file.write(data)
-      end
+    segments = rows.map do |row|
+      segment = row.delete(:diff)
+
+      row[:external_diff_offset] = pos
+      row[:external_diff_size] = segment.bytesize
+      pos += segment.bytesize
 
-      file
+      segment
     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
-  def original_filename
-    ""
+  attr_reader :original_filename
+
+  def initialize(data, original_filename = "")
+    super(data)
+
+    @original_filename = original_filename
   end
 end