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To:	cygwin <cygwin AT cygwin DOT com>
Subject:	Synchronization problem with posix_spawn
Message-ID:	<b1992e8b-d2e8-9c44-8f93-a270d5a879ed@cornell.edu>
Date:	Wed, 29 Jul 2020 16:17:14 -0400
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From:	Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Ken Brown <kbrown AT cornell DOT edu>
Sender:	"Cygwin" <cygwin-bounces AT cygwin DOT com>

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posix_spawn(p) returns before the spawned process is fully up and running.  As a 
result, the spawned process can fail to receive signals.  The attached test case 
illustrates the problem.  It spawns a sleep process and then tries to kill it. 
On exit, the sleep process is still running.

The following excerpts from the strace output show the issue: The SIGTERM signal 
is sent after the main program has forked a subprocess (and posix_spawnp has 
returned), but before the forked subprocess has exec'd the sleep process:

   559   32069 [main] spawn_test 4125 vfork: stub called
   257   48437 [main] spawn_test 4125 dofork: 4126 = fork()
   754    9511 [main] spawn_test 4126 dofork: 0 = fork()
    66   48503 [main] spawn_test 4125 kill0: kill (4126, 15)
    44    9555 [main] spawn_test 4126 find_exec: find_exec (/usr/bin/sleep)
    42   10835 [main] spawn_test 4126 spawnve: spawnve (/usr/bin/sleep, sleep, 
0x8000281A0)
    45    3149 [main] sleep 4126 child_info::ready: signalled 0x164 that I was ready
  6475   21055 [main] spawn_test 4126! child_info::sync: pid 45028, WFMO 
returned 0, exit_code 0x103, res 1
--- Process 45028 (pid: 4126) thread 41444 created

Ken

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#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <spawn.h>
#include <signal.h>

extern char **environ;

pid_t
run_sleep ()
{
  pid_t pid;
  char *argv[] = { "sleep", "3600", NULL };
  int err = posix_spawnp (&pid, "/usr/bin/sleep", NULL, NULL, argv, environ);
  if (err == 0)
    return pid;
  else
    {
      printf ("posix_spawnp: %s\n", strerror (err));
      exit (1);
    }
}

int
main ()
{
  pid_t pid = run_sleep ();
  kill (pid, SIGTERM);
}

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