#include "pthread_impl.h"

static void undo(void* control) {
  /* Wake all waiters, since the waiter status is lost when
   * resetting control to the initial state. */
  if (a_swap(control, 0) == 3)
    __wake(control, -1, 1);
}

int __pthread_once_full(pthread_once_t* control, void (*init)(void)) {
  /* Try to enter initializing state. Four possibilities:
   *  0 - we're the first or the other cancelled; run init
   *  1 - another thread is running init; wait
   *  2 - another thread finished running init; just return
   *  3 - another thread is running init, waiters present; wait */

  for (;;)
    switch (a_cas(control, 0, 1)) {
      case 0:
        pthread_cleanup_push(undo, control);
        init();
        pthread_cleanup_pop(0);

        if (a_swap(control, 2) == 3)
          __wake(control, -1, 1);
        return 0;
      case 1:
        /* If this fails, so will __wait. */
        a_cas(control, 1, 3);
      case 3:
        __wait(control, 0, 3, 1);
        continue;
      case 2:
        return 0;
    }
}

int __pthread_once(pthread_once_t* control, void (*init)(void)) {
  /* Return immediately if init finished before, but ensure that
   * effects of the init routine are visible to the caller. */
  if (*(volatile int*)control == 2) {
    a_barrier();
    return 0;
  }
  return __pthread_once_full(control, init);
}

weak_alias(__pthread_once, pthread_once);