Add a "fork" of musl as //fusl.

fusl/src/thread/pthread_once.c

Index: fusl/src/thread/pthread_once.c
diff --git a/fusl/src/thread/pthread_once.c b/fusl/src/thread/pthread_once.c
new file mode 100644
index 0000000000000000000000000000000000000000..a8f8aeb17a03995cba0ac62d75cd2d97d09ff0dc
--- /dev/null
+++ b/fusl/src/thread/pthread_once.c
@@ -0,0 +1,50 @@
+#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);