Re: FEATURE REQUEST : shell option to automatically terminate child processes on parent death

[Corto Beau]

 I think that a loadable builtin might be overkill for the second case, 
because one could write a wrapper program to set PDEATHSIG and call 
execve on the command-line arguments.

This way you'd just have to run "noreparent foo" instead of writing a 
shell function that calls the noreparent builtin and exec into the child 
afterwards. It wouldn't allow to unset the flag though, but I can't 
think of a use-case for that.

It would also eliminate the need for a "fork_noreparent" builtin because 
"noreparent foo &" would do just that.

On the other hand, the shell option to automatically set PDEATHSIG on 
all newly created children allows to enable this behavior without 
prefixing every line with "noreparent". I would understand if this 
use-case was considered too narrow though.

On 11/11/2023 21:32, Corto Beau wrote:
> Do you mean something like a "fork_noreparent" builtin that would call 
> make_child and set PDEATHSIG afterwards, or a "noreparent" builtin 
> that the child would have to call ? I do agree that it wouldn't make 
> sense to implement the later as a shell option, but this particular 
> use of prctl is subject to a race condition that can only be avoided 
> by comparing the PID of the parent before calling fork and after 
> calling prctl (in case the process got reparented before setting 
> PDEATHSIG), because checking for ppid = 1 is not enough on systems 
> like Linux where the process might have been reparented by a subreaper.
>
> On 11/11/2023 20:49, Oğuz wrote:
> > On Saturday, November 11, 2023, Corto Beau <corto.beau@laposte.net> 
> > wrote:
> >
> >     Configuration Information [Automatically generated, do not change]:
> >     Machine: x86_64
> >     OS: linux-gnu
> >     Compiler: gcc
> >     Compilation CFLAGS: -g -O2
> >     uname output: Linux zinc 6.6.1-arch1-1 #1 SMP PREEMPT_DYNAMIC
> >     Wed, 08 Nov 2023 16:05:38 +0000 x86_64 GNU/Linux
> >     Machine Type: x86_64-pc-linux-gnu
> >
> >     Bash Version: 5.2 Patch
> >     Level: 21
> >     Release Status: release
> >
> >     Description:
> >     Hi,
> >
> >     I would like to suggest a new shell option to ensure child
> >     processes are automatically killed when the parent dies.
> >
> >     Right now, it's already possible to emulate this feature by
> >     setting a trap to kill all child processes on exit (trap "kill 0"
> >     EXIT), but obviously it doesn't work if the process is terminated
> >     by a signal that cannot be caught (like SIGKILL).
> >
> >     On Linux, it can be done by setting the PR_SET_PDEATHSIG flag to
> >     propagate the parent termination signal to the child, regardless
> >     of whether the signal can be caught or not.
> >
> >     The rationale for this feature is that scripts spawning
> >     background processes to listen to various events (udev events,
> >     window manager events, etc) often leave orphan processes behind
> >     when terminated forcefully.
> >
> >     I've attached a proof-of-concept patch.
> >
> >
> > I think it'd make more sense to implement this as a loadable builtin.
> >
> >
> > --
> > Oğuz