cygwin 3.6.0: Signals may fail permanently if received after SIGSTOP

[Christian Franke via Cygwin]

Found with 'stress-ng --cpu-sched 1':

Testcase (attached):
$ uname -r
3.6.0-0.387.g8cebbb2b42bf.x86_64

$ gcc -o timersig timersig.c

$ ./timersig
638: fork()=639
!!!!!!!!!!!!!...!!!!!!!!!!!!!SIGSTOP: Permission denied
    0 [itimer] timersig 639 sig_send: error sending signal 14, pid 639, 
pipe handle 0x14C, nb 0, packsize 192, Win32 error 0
SIGKILL: Permission denied

$ kill 639
-bash: kill: (639) - Permission denied

$ kill -9 639
-bash: kill: (639) - Permission denied

$ /bin/kill --force 639

$ /bin/kill --force 639
kill: 639: No such process


A similar problem, but without the "error sending signal" message, 
occurs if the timer is not used but the parent process issues SIGSTOP 
SIGALRM SIGCONT ... sequences.

--
Regards,
Christian

#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <sys/wait.h>

static sig_atomic_t sigcnt;

static void sighandler(int sig)
{
  (void)sig;
  ++sigcnt;
  write(1, "!", 1);
}

int main()
{
  pid_t pid = fork();
  if (pid == (pid_t)-1) {
    perror("fork"); return 1;
  }

  if (!pid) {
    signal(SIGALRM, sighandler);
    timer_t tid;
    if (timer_create(CLOCK_REALTIME, NULL, &tid)) {
      perror("timer_create"); _exit(1);
    }
    struct itimerspec t = {};
    t.it_value.tv_nsec = t.it_interval.tv_nsec = 100;
    timer_settime(tid, 0, &t, NULL); 

    while (sigcnt < 1000)
      sched_yield();

    printf("%d: exit\n", (int)getpid()); fflush(stdout);
    _exit(0);
  }

  printf("%d: fork()=%d\n", (int)getpid(), (int)pid);
  sleep(1);

  int wp, status;
  while (!(wp = waitpid(pid, &status, WNOHANG))) {
    sched_yield();
    if (kill(pid, SIGSTOP)) {
      perror("SIGSTOP"); break;
    }
    sched_yield();
    if (kill(pid, SIGCONT)) {
      perror("SIGCONT"); break;
    }
  }

  if (wp < 0)
    perror("waitpid");
  else if (!wp && kill(pid, SIGKILL))
    perror("SIGKILL");
  return (wp <= 0);
}
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