// src/libmongoc/src/mongoc/mongoc-topology-scanner.c:749 (2.5.4; unchanged on master)
mongoc_handshake_t md = *_mongoc_handshake_get();

#include <mongoc/mongoc.h>
#include <pthread.h>
#include <stdio.h>

#define NTHREADS 16
#define ROUNDS 50

/* Minimal reusable barrier (macOS has no pthread_barrier_t). */
static pthread_mutex_t barrier_mtx = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t barrier_cv = PTHREAD_COND_INITIALIZER;
static int barrier_arrived = 0;
static int barrier_generation = 0;

static void
barrier_wait (void)
{
   pthread_mutex_lock (&barrier_mtx);
   int gen = barrier_generation;
   if (++barrier_arrived == NTHREADS) {
      barrier_arrived = 0;
      barrier_generation++;
      pthread_cond_broadcast (&barrier_cv);
   } else {
      while (gen == barrier_generation) {
         pthread_cond_wait (&barrier_cv, &barrier_mtx);
      }
   }
   pthread_mutex_unlock (&barrier_mtx);
}

static void *
thread_fn (void *arg)
{
   const int id = (int) (intptr_t) arg;

   for (int round = 0; round < ROUNDS; round++) {
      mongoc_client_t *client = mongoc_client_new ("mongodb://localhost:27017");

      barrier_wait ();
      /* Stagger the threads (busy-wait, so no sleep-based ordering is involved):
       * thread 0 reaches the struct copy while later threads are still about
       * to run _mongoc_handshake_freeze(). */
      for (volatile long spin = 0; spin < (long) id * 20000; spin++) {
      }

      if (!mongoc_client_append_metadata (client, "repro", "1.0", NULL)) {
         fprintf (stderr, "mongoc_client_append_metadata failed\n");
      }

      mongoc_client_destroy (client);
   }
   return NULL;
}

int
main (void)
{
   pthread_t threads[NTHREADS];

   mongoc_init ();
   for (int i = 0; i < NTHREADS; i++) {
      pthread_create (&threads[i], NULL, thread_fn, (void *) (intptr_t) i);
   }
   for (int i = 0; i < NTHREADS; i++) {
      pthread_join (threads[i], NULL);
   }
   mongoc_cleanup ();
   return 0;
}