fix some semaphore wait semantics (race condition deadlock and error checking)

[musl] / src / thread / sem_trywait.c

#include <semaphore.h>
#include "pthread_impl.h"

int sem_trywait(sem_t *sem)
{
        int val = a_fetch_add(sem->__val, -1);
        if (val > 0) return 0;
        if (!a_fetch_add(sem->__val, 1))
                __wake(sem->__val, 1, 0);
        errno = EAGAIN;
        return -1;
}