Mdthbs

We recently found an interesting case where posix_spawnp hangs until the child process it spawned terminates on Debian 9. This is not reproducible on other distros like Ubuntu(18.04) or CentOS(7.3). You could use the snippet at the end to reproduce it. Just compile it and run ./test_posix_spawnp sleep 30, assuming you named the executable as test_posix_spawnp. We passed in sleep 30 just to keep the child process running for a short while. You would see PID of child: xxx is not printed immediately as an indicator.

The sample code below is to mock our real code, the key of which is to close all the file descriptors in the child process except stdin/stdout/stderr and the one that opened for logging and redirect stdout/stderr to the logging file. In either the real case or this mocking one, it seems that the child process has been spawned and it has started running the executable being passed.

Our questions:

Has anyone came across this issue before? Does it sound like a libc(2.24) bug? If not, how can we fix our code? If so, what are we supposed to do?

P.S. not sure if it matters but we observed that when it's reproducible or when on Debian, there is an extra pipe created during the execution of posix_spawnp, the parent has the read end and the child the write end.

#include <sys/types.h>

#include <sys/stat.h>

#include <fcntl.h>

#include <spawn.h>

#include <stdio.h>

#include <unistd.h>

#include <stdlib.h>

#include <string.h>

#include <wait.h>

#include <errno.h>



#define errExit(msg)    do { perror(msg); 

                            exit(EXIT_FAILURE); } while (0)



#define errExitEN(en, msg) 

                       do { errno = en; perror(msg); 

                            exit(EXIT_FAILURE); } while (0)



char **environ;



int

main(int argc, char *argv[])

{

  pid_t child_pid;

  int s, status;

  sigset_t mask;

  posix_spawnattr_t attr;

  posix_spawnattr_t *attrp;

  posix_spawn_file_actions_t file_actions;

  posix_spawn_file_actions_t *file_actionsp;



  attrp = NULL;

  file_actionsp = NULL;

  long open_max = sysconf(_SC_OPEN_MAX);

  printf("sysconf says: max open file descriptor %ldn", open_max);

  if (open_max > 32768) {

    open_max = 32768;

    printf("bump max open file desriptor to %ldn", open_max);

  }

  int flags = O_WRONLY | O_CREAT | O_APPEND;

  mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IRGRP;

  int log_fd = open("test_posix_spawnp.log", flags, mode);



  printf("opened output file "test_posix_spawnp.log", fd=%dn", log_fd);



  /* Close all fds except log_fd to which stdout and stderr are redirected */



  s = posix_spawn_file_actions_init(&file_actions);

  if (s != 0)

    errExitEN(s, "posix_spawn_file_actions_init");



  s = posix_spawn_file_actions_adddup2(&file_actions, log_fd, STDOUT_FILENO);

  if (s != 0)

    errExitEN(s, "posix_spawn_file_actions_adddup2");



  s = posix_spawn_file_actions_adddup2(&file_actions, log_fd, STDERR_FILENO);

  if (s != 0)

    errExitEN(s, "posix_spawn_file_actions_adddup2");



  for (int i = 3; i < open_max; ++i) {

    if (i == log_fd) continue;

    s = posix_spawn_file_actions_addclose(&file_actions, i);

    if (s != 0)

      errExitEN(s, "posix_spawn_file_actions_addclose");

  }



  file_actionsp = &file_actions;



  s = posix_spawnp(&child_pid, argv[optind], file_actionsp, attrp,

                   &argv[optind], environ);

  if (s != 0)

    errExitEN(s, "posix_spawn");



  printf("PID of child: %ldn", (long) child_pid);



  /* Clean up after ourselves */



  if (file_actionsp != NULL) {

    s = posix_spawn_file_actions_destroy(file_actionsp);

    if (s != 0)

      errExitEN(s, "posix_spawn_file_actions_destroy");

  }



  exit(EXIT_SUCCESS);

}