Merge branch 'sh-fix-error-500-updating-wikis' into 'master'

Fix Error 500s due to encoding issues when Wiki hooks fire

Closes #50590

See merge request gitlab-org/gitlab-ce!21414

changelogs/unreleased/sh-fix-error-500-updating-wikis.yml

+---
+title: Fix Error 500s due to encoding issues when Wiki hooks fire
+merge_request: 21414
+author:
+type: fixed

lib/gitlab/encoding_helper.rb

@@ -75,7 +75,7 @@ module Gitlab
     end
 
     def binary_stringio(str)
-      StringIO.new(str || '').tap { |io| io.set_encoding(Encoding::ASCII_8BIT) }
+      StringIO.new(str.freeze || '').tap { |io| io.set_encoding(Encoding::ASCII_8BIT) }
     end
 
     private

spec/lib/gitlab/encoding_helper_spec.rb

+# coding: utf-8
 require "spec_helper"
 
 describe Gitlab::EncodingHelper do
@@ -187,4 +188,15 @@ describe Gitlab::EncodingHelper do
       end
     end
   end
+
+  describe '#binary_stringio' do
+    it 'does not mutate the original string encoding' do
+      test = 'my-test'
+
+      io_stream = ext_class.binary_stringio(test)
+
+      expect(io_stream.external_encoding.name).to eq('ASCII-8BIT')
+      expect(test.encoding.name).to eq('UTF-8')
+    end
+  end
 end