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

fusl/src/thread/sem_timedwait.c

+#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;
+}