in synccall, ignore the signal before any threads' signal handlers return

this protects against deadlock from spurious signals (e.g. sent by
another process) arriving after the controlling thread releases the
other threads from the sync operation.

src/thread/synccall.c

@@ -71,6 +71,10 @@ void __synccall(void (*func)(void *), void *ctx)
 	sigqueue(self->pid, SIGSYNCCALL, (union sigval){0});
 	while (sem_wait(&chaindone));
 
+	sa.sa_flags = 0;
+	sa.sa_handler = SIG_IGN;
+	__libc_sigaction(SIGSYNCCALL, &sa, 0);
+
 	for (cur=head; cur; cur=cur->next) {
 		sem_post(&cur->sem);
 		while (sem_wait(&cur->sem2));
@@ -82,10 +86,6 @@ void __synccall(void (*func)(void *), void *ctx)
 		sem_post(&cur->sem);
 	}
 
-	sa.sa_flags = 0;
-	sa.sa_handler = SIG_IGN;
-	__libc_sigaction(SIGSYNCCALL, &sa, 0);
-
 	__syscall(SYS_rt_sigprocmask, SIG_SETMASK,
 		&oldmask, 0, _NSIG/8);