Job-control not fully working in subshells since v4.4 ?

[Luca Landi]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -Wno-parentheses -Wno-format-security
uname output: Linux ubuntu-14-04-4 4.2.0-42-generic #49~14.04.1-Ubuntu SMP Wed 
Jun 29 20:22:11 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.0
Patch Level: 7
Release Status: release

Description:

It appears that, since ca. v4.4, job-control from within an interactive subshell
is still allowed but does not work fully because the shell won't hand over
foreground-ness.

Please consider the following demonstrative session run on a vanilla 5.0.7.
The first execution is from a non-interactive shell which will hand over
foreground-ness to the subshell and below, while the second execution is
from an interactive shell which will not do it:

$ ./bash --norc --noprofile -c 'uname -a; echo $BASH_VERSION; (set -bm; echo 
$-; sleep 3 & ps -s '$$' -o pid,ppid,pgid,tpgid,sid,s,cmd; wait); echo end'
Linux ubuntu-14-04-4 4.2.0-42-generic #49~14.04.1-Ubuntu SMP Wed Jun 29 
20:22:11 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
5.0.7(1)-release
bhmBc
   PID   PPID   PGID  TPGID    SID S CMD
  1339   1270   1339  30211   1339 S -bash
 30207   1339  30207  30211   1339 S ./bash --norc --noprofile -c uname -a; 
echo $BASH_VERSION; (set -bm; echo $-
 30209  30207  30207  30211   1339 S ./bash --norc --noprofile -c uname -a; 
echo $BASH_VERSION; (set -bm; echo $-
 30210  30209  30210  30211   1339 S sleep 3
 30211  30209  30211  30211   1339 R ps -s 1339 -o pid,ppid,pgid,tpgid,sid,s,cmd
[1]+  Done                    sleep 3
end
$
$ ./bash --norc --noprofile
bash-5.0$
bash-5.0$ uname -a; echo $BASH_VERSION; (set -bm; echo $-; sleep 3 & ps -s 1339 
-o pid,ppid,pgid,tpgid,sid,s,cmd; wait); echo end
Linux ubuntu-14-04-4 4.2.0-42-generic #49~14.04.1-Ubuntu SMP Wed Jun 29 
20:22:11 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
5.0.7(1)-release
bhimBHs
   PID   PPID   PGID  TPGID    SID S CMD
  1339   1270   1339  30214   1339 S -bash
 30212   1339  30212  30214   1339 S ./bash --norc --noprofile
 30214  30212  30214  30214   1339 S ./bash --norc --noprofile
 30215  30214  30215  30214   1339 S sleep 3
 30216  30214  30216  30214   1339 R ps -s 1339 -o pid,ppid,pgid,tpgid,sid,s,cmd
end
bash-5.0$
$

I get same result on v4.4.20(1) stock on Ubuntu 18.04 as well as on 5.0.3(1)
on Ubuntu 19.04.

On Bash 4.3 stock on Ubuntu 16.04 (and 14.04 also tested) instead, everything
seems all right:

$ bash -c 'uname -a; echo $BASH_VERSION; (set -bm; echo $-; sleep 3 & ps -s 
'$$' -o pid,ppid,pgid,tpgid,sid,s,cmd; wait); echo end'
Linux ubuntu-16-04 4.4.0-154-generic #181-Ubuntu SMP Tue Jun 25 05:29:03 UTC 
2019 x86_64 x86_64 x86_64 GNU/Linux
4.3.48(1)-release
bhmBc
   PID   PPID   PGID  TPGID    SID S CMD
  1885   1879   1885   2667   1885 S bash
  2663   1885   2663   2667   1885 S bash -c uname -a; echo $BASH_VERSION; (set 
-bm; echo $-; sleep 3 & ps -s 1885 -o pid,ppid,pgid,tpgid,sid,s,cmd;
  2665   2663   2663   2667   1885 S bash -c uname -a; echo $BASH_VERSION; (set 
-bm; echo $-; sleep 3 & ps -s 1885 -o pid,ppid,pgid,tpgid,sid,s,cmd;
  2666   2665   2666   2667   1885 S sleep 3
  2667   2665   2667   2667   1885 R ps -s 1885 -o pid,ppid,pgid,tpgid,sid,s,cmd
[1]+  Completato              sleep 3
end
$
$ uname -a; echo $BASH_VERSION; (set -bm; echo $-; sleep 3 & ps -s $$ -o 
pid,ppid,pgid,tpgid,sid,s,cmd; wait); echo end
Linux ubuntu-16-04 4.4.0-154-generic #181-Ubuntu SMP Tue Jun 25 05:29:03 UTC 
2019 x86_64 x86_64 x86_64 GNU/Linux
4.3.48(1)-release
bhimBH
   PID   PPID   PGID  TPGID    SID S CMD
  1885   1879   1885   2698   1885 S bash
  2696   1885   2696   2698   1885 S bash
  2697   2696   2697   2698   1885 S sleep 3
  2698   2696   2698   2698   1885 R ps -s 1885 -o pid,ppid,pgid,tpgid,sid,s,cmd
end
$

I suppose v4.3's behavior is the correct one ?

Repeat-By:

Sample sequence of commands as in below (mind the $$ of course):

echo start; (set -bm; echo $-; sleep 3 & ps -s $$ -o 
pid,ppid,pgid,tpgid,sid,s,cmd; wait); echo end