sys/pal/unix/sync/mutex: Fix Mutex::new() on NuttX

[no1wudi]

PTHREAD_MUTEX_INITIALIZER is highly configurable on NuttX, making it difficult to use a hardcoded initializer from libc crate in Rust way.

Instead, call pthread_mutex_init() to initialize the mutex with default attributes, ensuring consistent behavior with PTHREAD_MUTEX_INITIALIZER.

• Added conditional compilation for NuttX target
• Replaced PTHREAD_MUTEX_INITIALIZER with pthread_mutex_init() for NuttX
• Ensured consistent mutex initialization behavior across platforms

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[bjorn3]

NuttX allows moving a mutex after calling pthread_mutex_init? POSIX makes that UB as the mutex could eg be self-referential.

[no1wudi]

Yes, in this aspect, the situation is somewhat different. When initializing a mutex, NuttX only sets the flags:

int pthread_mutex_init(FAR pthread_mutex_t *mutex,
                       FAR const pthread_mutexattr_t *attr)
{
  int status;

  sinfo("mutex=%p attr=%p\n", mutex, attr);

  if (!mutex)
    {
      return EINVAL;
    }

  /* Initialize the mutex like a semaphore with initial count = 1 */

  status = mutex_init(&mutex->mutex);
  if (status < 0)
    {
      return -status;
    }

  /* Were attributes specified?  If so, use them */

#ifdef CONFIG_PTHREAD_MUTEX_TYPES
      mutex->type  = attr ? attr->type : PTHREAD_MUTEX_DEFAULT;
#endif
#ifndef CONFIG_PTHREAD_MUTEX_UNSAFE
      mutex->flink = NULL;
#  ifdef CONFIG_PTHREAD_MUTEX_BOTH
      mutex->flags = attr && attr->robust == PTHREAD_MUTEX_ROBUST ?
                     _PTHREAD_MFLAGS_ROBUST : 0;
#  else
      mutex->flags = 0;
#  endif
#endif

#if defined(CONFIG_PRIORITY_INHERITANCE) || defined(CONFIG_PRIORITY_PROTECT)
  if (attr)
    {
      status = mutex_set_protocol(&mutex->mutex, attr->proto);
      if (status < 0)
        {
          mutex_destroy(&mutex->mutex);
          return -status;
        }

#  ifdef CONFIG_PRIORITY_PROTECT
      if (attr->proto == PTHREAD_PRIO_PROTECT)
        {
          status = mutex_setprioceiling(&mutex->mutex, attr->ceiling, NULL);
          if (status < 0)
            {
              mutex_destroy(&mutex->mutex);
              return -status;
            }
        }
#  endif
    }
#endif

  return 0;
}

[bjorn3]

Is this a documented guarantee or does NuttX reserve the right to change the implementation in such a way that this becomes UB?

[no1wudi]

Unfortunately I can't find such a document, I think I can add it to the test cases on the relevant side and incorporate it into the NuttX side to remind developers and prevent accidental changes to this behavior.