Re: Monitor mode handling (bug ?)

[Steffen Nurpmeso <steffen@xxxxxxxxxx>]

Hello Ganael.

Ganael Laplanche wrote in
 <2616938.TYJnH3iKXO@xxxxxxxxxxxxxxxxx>:
 |On Friday, January 13, 2023 10:03:35 PM CET Steffen Nurpmeso wrote:
 |> Well i mean, in spirit of Wheeler's "anything is possible with
 |> (one more) indirection" it could be possible nonetheless?
 ...
 |It is not clear for me why that extra fork fixes the problem, but it \
 |works, 
 |thanks!
 |
 |Unfortunately, my initial goal is to get a new process group for later 
 |children processes, but FreeBSD's sh (as well as dash) requires 'set \
 |-m' to be 
 |executed from the first process [1]. The extra fork breaks that requirem\
 |ent. 
 |The following example shows that the new process' PGID remains the \
 |same as the 
 |initial shell:
 |
 |$ cat test_pgid.sh
 |(
 |  set -m
 |  (
 |    sleep 1
 |  ) &
 |  i=$!
 |  echo "Main shell has: $(ps -o pid,pgid $$)"
 |  echo "Sub-shell has: $(ps -o pid,pgid $i)"
 |  wait $i
 |)
 |$ dash test_pgid.sh &
 |Main shell has:   PID  PGID
 |10362 10362
 |Sub-shell has:   PID  PGID
 |10364 10362
 |[1]  + done       dash test_pgid.sh
 |
 |So I am afraid the 'extra fork' tip is not useable for me...

Thanks for this information, that i did not expect.
Indeed .. it seems no new process group is used for the child
shell.  That thoroughly i have not looked.

  ...
 |[1] Discussed here: https://lists.freebsd.org/archives/freebsd-hackers/2\
 |022-March/000924.html

Very interesting.  With the correction (fwiw) that

  * with bash, it hangs (as expected with a new process group)

is not true with bash 5.2.15 on Linux 6.1 with glibc

  $ bash -c '(set -m; echo "echo from background" & wait "$!"); echo "wait returns $?"'
  echo from background
  [1]+  Done                    echo "echo from background"
  wait returns 0

(Well the following "hangs" in that it does not print a prompt)

  $ bash -c '(set -m; echo "echo from background $$" & echo $!; wait "$!"); echo "wait returns $?"' &
  [1] 21123
  #?0|kent:nail.git$ 21125
  echo from background 21123
  [1]+  Done                    echo "echo from background $$"
  wait returns 0

So i think i am out of ideas except doing what Jilles suggested
in the message, enwrapping the inner thing with sh -c '..'.
And that seems to work a bit as

  #!/bin/dash
  #(
    sh -c '
    set -m
    (
      sleep 1
    ) &
    i=$!
    echo >&2 "inner Main shell has: $(ps -o pid,pgid $$)"
    echo >&2 "inner Sub-shell has: $(ps -o pid,pgid $i)"
    wait $i
    ' </dev/null >/dev/null &
    echo "Main shell has: $(ps -o pid,pgid $$)"
  #)
    echo "outer Main shell has: $(ps -o pid,pgid $$)"

ends up as

  #?148|kent:tmp$ ./t.sh
  Main shell has:   PID  PGID
  22300 22300
  inner Main shell has:   PID  PGID
  22301 22301
  outer Main shell has:   PID  PGID
  22300 22300
  #?0|kent:tmp$ inner Sub-shell has:   PID  PGID
  22303 22303
  sh: 9: Cannot set tty process group (No such process)

  logout
  There are stopped jobs.

Ciao!

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)