Open Radar

 
If the .events field of a struct pollfd is 0, poll(2) doesn't detect POLLHUP on that fd.

Test case demonstrating the bug:

#include <sys/types.h>
#include <unistd.h>
#include <poll.h>
#include <stdio.h>
#include <sys/wait.h>

int monitor_process(int fd, short events) {
  struct pollfd spec = { .fd = fd, .events = events };
  int res = poll(&spec, 1, 5 /* seconds */ * 1000 /* milliseconds/second */);
  switch (res) {
  case -1:
    perror("waiting for events");
    return 1;
  case 0:
    fprintf(stderr, "poll timed out\n");
    return 1;
  default:
    return 0;
  }
}

int go(short events) {
  int pipe_fds[2];
  if (pipe(pipe_fds) == -1) {
    perror("making a pipe");
    return 1;
  }

  pid_t child = fork();
  switch (child) {
  case -1:
    perror("forking");
    return 1;
  case 0:
    close(pipe_fds[1]);
    _exit(monitor_process(pipe_fds[0], events));
  }
  close(pipe_fds[0]);
  close(pipe_fds[1]);
  int status;
  waitpid(child, &status, 0);
  if (WIFSIGNALED(status)) {
    fprintf(stderr, "child process died with signal: %d", WTERMSIG(status));
    return 1;
  }
  int ret = WEXITSTATUS(status);
  if (ret != 0)
    fprintf(stderr, "child process failed with status %d, POLLHUP %s specified\n", ret, events & POLLHUP ? "is" : "is not");
  return WEXITSTATUS(status);
}

int main(int argc, char ** argv) {
  int res0 = go(POLLHUP);
  int res1 = go(0);
  return res1 + res0;
}