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

fusl/src/thread/sem_timedwait.c

Index: fusl/src/thread/sem_timedwait.c
diff --git a/fusl/src/thread/sem_timedwait.c b/fusl/src/thread/sem_timedwait.c
new file mode 100644
index 0000000000000000000000000000000000000000..8132eb1bf7a276d9b2dd2c9c978bbc0f0b29b7f4
--- /dev/null
+++ b/fusl/src/thread/sem_timedwait.c
@@ -0,0 +1,31 @@
+#include <semaphore.h>
+#include "pthread_impl.h"
+
+static void cleanup(void *p)
+{
+	a_dec(p);
+}
+
+int sem_timedwait(sem_t *restrict sem, const struct timespec *restrict at)
+{
+	pthread_testcancel();
+
+	if (!sem_trywait(sem)) return 0;
+
+	int spins = 100;
+	while (spins-- && sem->__val[0] <= 0 && !sem->__val[1]) a_spin();
+
+	while (sem_trywait(sem)) {
+		int r;
+		a_inc(sem->__val+1);
+		a_cas(sem->__val, 0, -1);
+		pthread_cleanup_push(cleanup, (void *)(sem->__val+1));
+		r = __timedwait_cp(sem->__val, -1, CLOCK_REALTIME, at, sem->__val[2]);
+		pthread_cleanup_pop(1);
+		if (r && r != EINTR) {
+			errno = r;
+			return -1;
+		}
+	}
+	return 0;
+}