Re: [glibc] posix: Add terminal control setting support for posix_spawn

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella via Glibc-cvs:

> commit 342cc934a3bf74ac618e2318d738f22ac93257ba
> Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> Date:   Mon Jun 14 14:41:31 2021 -0300
>
>     posix: Add terminal control setting support for posix_spawn
>     
>     Currently there is no proper way to set the controlling terminal through
>     posix_spawn in race free manner [1].  This forces shell implementations
>     to keep using fork+exec when launching background process groups,
>     even when using posix_spawn yields better performance.
>     
>     This patch adds a new GNU extension so the creating process can
>     configure the created process terminal group.  This is done with a new
>     flag, POSIX_SPAWN_TCSETPGROUP, along with two new attribute functions:
>     posix_spawnattr_tcsetpgrp_np, and posix_spawnattr_tcgetpgrp_np.
>     The function sets a new attribute, spawn-tcgroupfd, that references to
>     the controlling terminal.
>     
>     The controlling terminal is set after the spawn-pgroup attribute, and
>     uses the spawn-tcgroupfd along with current creating process group
>     (so it is composable with POSIX_SPAWN_SETPGROUP).
>     
>     To create a process and set the controlling terminal, one can use the
>     following sequence:
>     
>         posix_spawnattr_t attr;
>         posix_spawnattr_init (&attr);
>         posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP);
>         posix_spawnattr_tcsetpgrp_np (&attr, tcfd);
>     
>     If the idea is also to create a new process groups:
>     
>         posix_spawnattr_t attr;
>         posix_spawnattr_init (&attr);
>         posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP
>                                          | POSIX_SPAWN_SETPGROUP);
>         posix_spawnattr_tcsetpgrp_np (&attr, tcfd);
>         posix_spawnattr_setpgroup (&attr, 0);

Sorry, I did not look at this in detail so far.  A file descriptor
outside the file actions API makes me quite nervous.

Advanced Programming in the UNIX Environment says (like POSIX), “If the
process has a controlling terminal, the process can call tcsetpgrp to
set the foreground process group ID […]”.  This suggests to me that a
file action may be required to establish a controlling terminal.
Callers of posix_spawn might restrict that to the spawned process.  As
far as I know, Linux follows SVR4, and opening the terminal device under
/dev/pts/ in a session leader that has no controlling terminal
establishes one.  This translate to a posix_spawn file action.  So the
action established by posix_spawnattr_tcsetpgrp_np must come after the
file actions.

But the descriptor used by posix_spawnattr_tcsetpgrp_np must still be
open.  So it would have to come before a closefrom file action (or a
regular close).

I think this means that the action established by
posix_spawnattr_tcsetpgrp_np needs to be sequenced among the file
actions, and needs to be a file action itself.

Thanks,
Florian