Merge branch 'fix/gitlab-popen-thread-safety' into 'master'

Fix Gitlab::Popen.popen thread-safety issue

## What does this MR do?
It changes the instance variables of `Gitlab::Popen.popen` to a local ones.

## Are there points in the code the reviewer needs to double check?
N/A

## Why was this MR needed?
It fixes a bug!

## Screenshots (if relevant)
N/A

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- Tests
  - [ ] ~~Added for this feature/bug~~
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
 #21842

See merge request !6194

CHANGELOG

@@ -132,6 +132,7 @@ 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