From 27909acb584aed231d757d1d63c6c62b57c8e152 Mon Sep 17 00:00:00 2001
From: Vincent Mihalkovic <vmihalko@redhat.com>
Date: Mon, 3 Jan 2022 14:03:20 +0100
Subject: [PATCH] Fix race conditions running external commands with job
 control on

When ksh is compiled with SHOPT_SPAWN (the default), which uses
posix_spawn(3) or vfork(2) (via sh_ntfork()) to launch external
commands, at least two race conditions occur when launching
external commands while job control is active. See:
https://bugs.launchpad.net/ubuntu/+source/ksh/+bug/1887863/comments/3
https://www.mail-archive.com/ast-developers@research.att.com/msg00717.html

The basic issue is that this performance optimisation is
incompatible with job control, because it uses a spawning mechanism
that doesn't copy the parent process' memory pages into the child
process, therefore no state that involves memory can be set before
exec-ing the external program. This makes it impossible to
correctly set the terminal's process group ID in the child process,
something that is essential for job control to work.

src/cmd/ksh93/sh/xec.c:
- Use sh_fork() instead of sh_ntfork() if job control is active.
  This uses fork(2), which is 30%-ish slower on most sytems, but
  allows for correctly setting the terminal process group.

Fixes: https://github.com/ksh93/ksh/issues/79
---
 src/cmd/ksh93/sh/xec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/cmd/ksh93/sh/xec.c b/src/cmd/ksh93/sh/xec.c
index ae13178..be57a6b 100644
--- a/src/cmd/ksh93/sh/xec.c
+++ b/src/cmd/ksh93/sh/xec.c
@@ -1737,7 +1737,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 #else
 #if SHOPT_SPAWN
 #   ifdef _lib_fork
-				if(com)
+				if(com && !job.jobcontrol)
 					parent = sh_ntfork(shp,t,com,&jobid,ntflag);
 				else
 					parent = sh_fork(shp,type,&jobid);
-- 
2.31.1