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

fusl/src/mq/mq_notify.c

+#include <mqueue.h>
+#include <pthread.h>
+#include <errno.h>
+#include <sys/socket.h>
+#include <signal.h>
+#include <unistd.h>
+#include "syscall.h"
+
+struct args {
+        pthread_barrier_t barrier;
+        int sock;
+        const struct sigevent *sev;
+};
+
+static void *start(void *p)
+{
+        struct args *args = p;
+        char buf[32];
+        ssize_t n;
+        int s = args->sock;
+        void (*func)(union sigval) = args->sev->sigev_notify_function;
+        union sigval val = args->sev->sigev_value;
+
+        pthread_barrier_wait(&args->barrier);
+        n = recv(s, buf, sizeof(buf), MSG_NOSIGNAL|MSG_WAITALL);
+        close(s);
+        if (n==sizeof buf && buf[sizeof buf - 1] == 1)
+                func(val);
+        return 0;
+}
+
+int mq_notify(mqd_t mqd, const struct sigevent *sev)
+{
+        struct args args = { .sev = sev };
+        pthread_attr_t attr;
+        pthread_t td;
+        int s;
+        struct sigevent sev2;
+        static const char zeros[32];
+
+        if (!sev || sev->sigev_notify != SIGEV_THREAD)
+                return syscall(SYS_mq_notify, mqd, sev);
+
+        s = socket(AF_NETLINK, SOCK_RAW|SOCK_CLOEXEC, 0);
+        if (s < 0) return -1;
+        args.sock = s;
+
+        if (sev->sigev_notify_attributes) attr = *sev->sigev_notify_attributes;
+        else pthread_attr_init(&attr);
+        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
+        pthread_barrier_init(&args.barrier, 0, 2);
+
+        if (pthread_create(&td, &attr, start, &args)) {
+                __syscall(SYS_close, s);
+                errno = EAGAIN;
+                return -1;
+        }
+
+        pthread_barrier_wait(&args.barrier);
+        pthread_barrier_destroy(&args.barrier);
+
+        sev2.sigev_notify = SIGEV_THREAD;
+        sev2.sigev_signo = s;
+        sev2.sigev_value.sival_ptr = (void *)&zeros;
+
+        if (syscall(SYS_mq_notify, mqd, &sev2) < 0) {
+                pthread_cancel(td);
+                __syscall(SYS_close, s);
+                return -1;
+        }
+
+        return 0;
+}