Re: supporting terminal ownership assignment (tcsetpgrp()) in posix_spawn

[Godmar Back <godmar@gmail.com>]

On Tue, Jun 8, 2021 at 9:50 AM Adhemerval Zanella <
adhemerval.zanella@linaro.org> wrote:

>
> So one options might be to add something as the one I suggested before,
> but as generic extension instead of a file extension (which does not make
> sense in fact):
>
>   int posix_spawnattr_tcsetpgrp_np (posix_spawnattr_t *__attr, int fd,
> pid_t pgrp);
>
>   Similar to tcgetpgrp, it make the created process group with process
> group
>   instructed by the PGRP argument the foreground process group on the
> terminal
>   associated to FD.  If PGRP is 0, the current group obtained with
> getpgrp()
>   will be used, otherwise PGRP will be used.
>
>   This is done after just after setting the process group ID
>   (POSIX_SPAWN_SETPGROUP) and right before setting the effective user and
>   group id (POSIX_SPAWN_RESETIDS).
>
> So if the called want to create a new session id, it can issue:
>
>   posix_spawnattr_t attr;
>   posix_spawnattr_setflags (&attr, POSIX_SPAWN_SETSID);
>   posix_spawnattr_tcsetpgrp_np (&attr, fd, 0);
>
> And the created process will issue the following in the order:
>
>   setsid ();
>   tcsetpgrp (fd, getpgid (0));
>
>
After setsid(), the caller is a new session leader and a new process group
leader, but it doesn't have a controlling terminal.
(setsid(2) says: "The calling process will be the only process in the new
process group and in the new session. Initially, the new session has no
controlling terminal.")

The man page of tcsetpgrp however states that the fd passed: "must be the
controlling terminal of the calling process."
So if you implemented it like that in a library, it should fail based on
the description in the man pages.

I'm not sure if POSIX_SPAWN_SETSID needs to be able to be combined with
setting the terminal's foreground process group. Those are different use
cases. In the case of setsid, the spawned process must be prepared to
become a new session leader (open a new controlling terminal, etc.) so it
would not generally assume to already be a member of the terminal's fg
process group (since it doesn't have a controlling terminal yet).


>
> If the caller already has group it want to use, it can issue instead:
>
>   posix_spawnattr_t attr;
>   posix_spawnattr_setpgroup (&attr, groupid);
>   posix_spawnattr_tcsetpgrp_np (&attr, fd, groupid);
>
> Which in turn will make the created process to issue:
>
>   setpgid (0, groupid);
>   tcsetpgrp (fd, groupid);
>

For this use case, as long as it supports groupid == 0, this should work as
it is what shells currently do.

 - Godmar