fg in zsh sometimes fails to restore stopped job

[Matt Day]

To reproduce the problem:

1. Run zsh on Ubuntu 22.04 LTS (also happens on 20.04 LTS)
2. Run vim and hit Control-Z to stop it
3. Enter "fg" to restore the stopped job

I expect fg to always be able to restore a stopped vim. Instead, sometimes vim stops itself again immediately after entering fg. The problem seems to be timing or scheduling related because it doesn't always occur.

This is due to a known bug in zsh versions 5.7, 5.7.1, 5.8, and 5.8.1. It was reported to the zsh project by Erik Paulson in June 2021, who also provided a one-line fix. The zsh project accepted Paulson's fix, which is now available in zsh 5.9.

The bug causes zsh to sometimes send two SIGCONT signals to the stopped job instead of only one. Certain programs behave unexpectedly when processing the second SIGCONT. vim stops itself. According to Paulson's bug report, emacsclient crashes.

Paulson's original bug report with patch:
https://www.zsh.org/mla/workers/2021/msg01255.html

The git commit that introduced the bug in Sept 2018:
https://github.com/zsh-users/zsh/commit/0a20f4e5a64106d1793dd44d2fa0113417225d0f

The git commit of Paulson's fix:
https://github.com/zsh-users/zsh/commit/f8ec33c453159ad848d9b1a28782c5df7f0cc944

The test program in this Stack Overflow question is a good way to reproduce the problem. With this program it is easy to see the two SIGCONT signals being sent sometimes by zsh fg:
https://stackoverflow.com/questions/64531411/why-do-i-receive-two-sigcont-signals

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: zsh 5.8.1-1
ProcVersionSignature: Ubuntu 5.15.0-46.49-generic 5.15.39
Uname: Linux 5.15.0-46-generic x86_64
ApportVersion: 2.20.11-0ubuntu82.1
Architecture: amd64
CasperMD5CheckResult: unknown
Date: Sun Aug 28 14:06:07 2022
ProcEnviron:
 TERM=screen.xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=zsh
SourcePackage: zsh
UpgradeStatus: No upgrade log present (probably fresh install)

[Matt Day (fjarlq)]

The attached patch for this bug applies to Ubuntu's zsh package 5.8.1-1. Original author is Erik Paulson. This fix is already in zsh 5.9.

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "zsh-fg.diff" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

[This is an automated message performed by a Launchpad user owned by ~brian-murray, for any issues please contact him.]