Re: [PATCH] Allow enabling job control without a tty in non-interactive mode..

From: Steffen Nurpmeso <steffen@sdaoden.eu>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: dash@vger.kernel.org,
	Ganael Laplanche <ganael.laplanche@martymac.org>,
	Jilles Tjoelker <jilles@stack.nl>
Subject: Re: [PATCH] Allow enabling job control without a tty in non-interactive mode..
Date: Sun, 07 Apr 2024 01:07:04 +0200	[thread overview]
Message-ID: <20240406230704.Tr-SBJb2@steffen%sdaoden.eu> (raw)
In-Reply-To: <ZhD4JcqNct+rmxca@gondor.apana.org.au>

Isn't that standardized answer a bit misplaced for that patch?
It was about Ganael Laplanche having problems with creating
process groups in backgrounded jobs (2023-01-12):

  The problem is when the main script is started *in the
  background*. Can your test script successfully be run in the
  background with dash if $JOBMON is not empty ? I think you'll
  face the same problem as I do.

  I could not find Jilles' commit (cd60e2c) equivalent in dash
  code, so I presume enabling job control without a tty in
  non-interactive mode is just not possible with dash, am I right

And i stated that another indirection fixes that problem and he
then said

  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 requirement.  The following example shows
  that the new process' PGID remains the same as the initial
  shell:

which let me baffled

  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.

and

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

and Ganael ended (without the patch) like

  Yes, I am afraid it's the only syntax that works (but it is not
  very convenient).

etc. etc. etc.

Full stop.

Please consider this script:

  cat > t.sh <<'_EOT'
  set -m
  (
  sleep 1
  ) &
  i=$!
  set +m
  echo >&2 "inner Main shell has: $(ps -o pid,pgid $$)"
  echo >&2 "inner Sub-shell has: $(ps -o pid,pgid $i)"
  wait $i
  _EOT

Current dash:

  #?0|kent:dash.git$ dash t.sh </dev/null >.X 2>&1 &
  [3] 28719
  #?0|kent:dash.git$
^RETURN

  [3]+  Stopped                 dash t.sh < /dev/null > .X 2>&1
  #?0|kent:dash.git$ fg
  dash t.sh < /dev/null > .X 2>&1

Patched dash:

  #?0|kent:dash.git$ src/dash t.sh </dev/null >.X 2>&1 &
  [3] 28749
  #?0|kent:dash.git$
^RETURN
  [3]   Done                    src/dash t.sh < /dev/null > .X 2>&1

Comparison:

  #?0|kent:dash.git$ bash t.sh </dev/null >.X 2>&1 &
  [3] 28766
  #?0|kent:dash.git$
^RETURN
  [3]   Done                    bash t.sh < /dev/null > .X 2>&1

A nice Sunday everybody!
Ciao from Germany,

--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)