Our /bin/sh and process group IDs

Hello,

I am trying to fork a sub-shell with its own process group id through a 
function that must itself be executed in the background.

It should work with the following code:

#----
#!/bin/sh

# Parent shell IDs
ps -o pid,ppid,pgid,comm -p $$

test_func () {
   set -m
   { /bin/sh -c 'sleep 1' ; } &
   # Forked shell IDs (pgid should be different from parent,
   # but it is not)
   ps -o pid,ppid,pgid,comm -p $!
}

# The following does not work:
test_func &
# ...but it works when function is not executed in the background:
#test_func

sleep 2
exit 0
#----

Unfortunately, with our /bin/sh, the sleeping process gets the *same* 
process group ID as its parent.

I've tested several shell implementations; it works with :

/usr/local/bin/bash --posix 'test.sh'     # from bash-5.1.16
/usr/local/bin/zsh --emulate sh 'test.sh' # from zsh-5.8.1
/usr/local/bin/ksh93 'test.sh'            # from ksh93-devel-2020.06.30
/usr/local/bin/mksh 'test.sh'             # from mksh-59c
/usr/local/bin/ksh 'test.sh'              # from pdksh-5.2.14p2_6

but not with :

/bin/sh 'test.sh'                         # on 13.0-RELEASE-p8
/usr/local/bin/dash 'test.sh'             # from dash-0.5.11.5

am I missing something ?

Any help welcome :)

Best regards,

-- 
Ganael LAPLANCHE <ganael.laplanche@martymac.org>
http://www.martymac.org | http://contribs.martymac.org
FreeBSD: martymac <martymac@FreeBSD.org>, http://www.FreeBSD.org