Fix Gitlab::Popen.popen thread-safety issue

Fixes #21842

CHANGELOG

@@ -131,6 +131,7 @@ v 8.12.0 (unreleased)
   - Remove duplication between project builds and admin builds view !5680 (Katarzyna Kobierska Ula Budziszewska)
   - Deleting source project with existing fork link will close all related merge requests !6177 (Katarzyna Kobierska Ula Budziszeska)
   - Return 204 instead of 404 for /ci/api/v1/builds/register.json if no builds are scheduled for a runner !6225
+  - Fix Gitlab::Popen.popen thread-safety issue
 
 v 8.11.6 (unreleased)
   - Fix an error where we were unable to create a CommitStatus for running state

lib/gitlab/popen.rb

@@ -18,18 +18,18 @@ module Gitlab
         FileUtils.mkdir_p(path)
       end
 
-      @cmd_output = ""
-      @cmd_status = 0
+      cmd_output = ""
+      cmd_status = 0
       Open3.popen3(vars, *cmd, options) do |stdin, stdout, stderr, wait_thr|
         yield(stdin) if block_given?
         stdin.close
 
-        @cmd_output << stdout.read
-        @cmd_output << stderr.read
-        @cmd_status = wait_thr.value.exitstatus
+        cmd_output << stdout.read
+        cmd_output << stderr.read
+        cmd_status = wait_thr.value.exitstatus
       end
 
-      [@cmd_output, @cmd_status]
+      [cmd_output, cmd_status]
     end
   end
 end