Re: [PATCH] posix_spawn blocks SIGCHLD in spawned processes

[Jürgen Hötzel]

Robert Pluim <rpluim@gmail.com> writes:

>>>>>> On Fri, 04 Mar 2022 09:38:25 +0100, Jürgen Hötzel <juergen@hoetzel.info> 
>>>>>> said:
>     Jürgen> IMO the correct way to fix this issue is to use the oldset passed 
> by
>     Jürgen> emacs_spawn (patch enclosed) just like the fork/exec 
> implementation does.
>
> Sure, that would work as well.
>
>     Jürgen> I guess without a fix many forking commands (that don't examine 
> and
>     Jürgen> change mask of blocked signals) will "hang" in Emacs.
>
> Not really. Itʼs a code path thatʼs non-default, since
> process-connection-type defaults to t => use pty's, and the command in

‘call-process’ invokes emacs_spawn 'call-process' with pty set to NULL
so ‘process-connection-type’ doesn't have an effect here.

A minimal C example to reproduce the HANG is

--8<---------------cut here---------------start hang.c------------->8---
#include <signal.h>
#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>

void chld_handler(int signum) { printf("\nInside CHLD handler function\n"); }

int main(int argc, char *argv[]) {
  sigset_t blocked, old;

  signal(SIGCHLD, chld_handler); // Setup Child handler
  sigemptyset(&blocked);
  sigaddset(&blocked, SIGCHLD);
  sigprocmask(SIG_BLOCK, &blocked, &old);
  if (!fork()) {
    return 0; /* child signals SIGCHLD */
  }
  sigsuspend(&old); /* WAITS for ever when SIGCHILD was originally blocked */
  printf("Parent terminates\n");
  return 0;
}
--8<---------------cut here---------------end hang.c--------------->8---


(call-process "chldtest" nil t nil) will never return.

Regards, Jürgen