Downstream bug: https://github.com/openzfs/zfs/issues/16731

On FreeBSD, zfs send sometimes hangs in a deadlock after pthread_cancel(tid); pthread_join(tid, NULL); while the thread is in pause().

The program looks something like this:

#include <pthread.h>
#include <signal.h>
#include <stdbool.h>
#include <stdio.h>
#include <unistd.h>

static void send_progress_thread_act(int sig, siginfo_t *info, void *ucontext) {}

static void timer_delete_cleanup(void *timer) {}

void *send_progress_thread(void *arg1) {
  const struct sigaction signal_action =
      {.sa_sigaction = send_progress_thread_act, .sa_flags = SA_SIGINFO};
  sigaction(SIGUSR1, &signal_action, NULL);

  pthread_cleanup_push(timer_delete_cleanup, NULL);

  for(;;) {
    pause();  // <- thread blocked here
    // zfs_send_progress(), time(), localtime_r(), setproctitle(), fprintf()
    fprintf(stderr, "unpaused\n");
  }

  pthread_cleanup_pop(true);
  return NULL;
}

int main() {
  pthread_t tid;
  pthread_create(&tid, NULL, send_progress_thread, NULL);

  sigset_t new;
  sigemptyset(&new);
  sigaddset(&new, SIGUSR1);
  pthread_sigmask(SIG_BLOCK, &new, NULL);

  sleep(1);  // some sort of work

  pthread_cancel(tid);
  pthread_join(tid, NULL);  // <- thread blocked here
}

(In reality there's also a timer that may send SIGUSR1s and SIGINFO is also consumed but this is largely irrelevant.)

The backtrace looks like this:

Thread 2 (LWP 259843 of process 95307):
#0  0x000000082872c6aa in _sigsuspend () from /lib/libc.so.7
#1  0x0000000825f47fb6 in ?? () from /lib/libthr.so.3
#2  0x00000008286a2a65 in pause () from /lib/libc.so.7
#3  0x0000000820e3d7d5 in send_progress_thread (arg=0x8208cf2c0) at lib/libzfs/libzfs_sendrecv.c:1018
#4  0x0000000825f3eb05 in ?? () from /lib/libthr.so.3
#5  0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x82ce7a000

Thread 1 (LWP 100403 of process 95307):
#0  0x0000000825f3be2c in ?? () from /lib/libthr.so.3
#1  0x0000000825f40a2e in ?? () from /lib/libthr.so.3
#2  0x0000000820e3da4f in send_progress_thread_exit (hdl=0x349d6143c000, ptid=0x349d61412700, oldmask=oldmask@entry=0x8208cf1b0) at lib/libzfs/libzfs_sendrecv.c:1074
#3  0x0000000820e40367 in dump_snapshot (zhp=0x349d6145a500, arg=arg@entry=0x8208d0ec8) at lib/libzfs/libzfs_sendrecv.c:1291
#4  0x0000000820e3014a in zfs_iter_snapshots_sorted_v2 (zhp=zhp@entry=0x349d6145a000, flags=flags@entry=0, callback=0x820e3ff30 <dump_snapshot>, data=data@entry=0x8208d0ec8, min_txg=8889024, max_txg=8890857) at lib/libzfs/libzfs_iter.c:352
#5  0x0000000820e3fd34 in dump_filesystem (zhp=0x349d6145a000, sdd=0x8208d0ec8) at lib/libzfs/libzfs_sendrecv.c:1364
#6  0x0000000820e3ec99 in dump_filesystems (rzhp=0x349d61412700, rzhp@entry=0x349d6145a000, sdd=0x2, sdd@entry=0x8208d0ec8) at lib/libzfs/libzfs_sendrecv.c:1432
#7  0x0000000820e3e413 in zfs_send_cb_impl (zhp=0x349d6145a000, fromsnap=<optimized out>, tosnap=0x349d61419106 "zpair-02749186397145403432-cartman-674eaa54-537c2abe", flags=0x8208d16f0, outfd=1, filter_func=0x0, cb_arg=0x8208d16d0, debugnvp=0x0) at lib/libzfs/libzfs_sendrecv.c:2506
#8  0x0000000820e3c26e in zfs_send_cb (outfd=0, arg=<optimized out>) at lib/libzfs/libzfs_sendrecv.c:2559
#9  0x0000000820e3c237 in zfs_send (zhp=zhp@entry=0x349d6145a000, fromsnap=fromsnap@entry=0x349d614190c0 "zpair-02749186397145403432-cartman-674e8e34-16208f9f", tosnap=tosnap@entry=0x349d61419106 "zpair-02749186397145403432-cartman-674eaa54-537c2abe", flags=0x0, flags@entry=0x8208d16f0, outfd=0, outfd@entry=1, filter_func=0x0, cb_arg=0x8208d16d0, debugnvp=0x0) at lib/libzfs/libzfs_sendrecv.c:2577
#10 0x0000000000215754 in zfs_do_send (argc=<optimized out>, argv=<optimized out>) at cmd/zfs/zfs_main.c:5099
#11 0x000000000020fd18 in main (argc=6, argv=0x8208d1850) at cmd/zfs/zfs_main.c:9249

This is obviously wrong, since the thread is /sleeping in a cancellation point/. 
Adding pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, NULL); to the top of the thread doesn't change anything. In this case, it's obviously wrong because pthread_cancel()... didn't cancel the thread.