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

fusl/src/unistd/setxid.c

+#include <unistd.h>
+#include <errno.h>
+#include "syscall.h"
+#include "libc.h"
+#include "pthread_impl.h"
+
+struct ctx {
+        int id, eid, sid;
+        int nr, err;
+};
+
+static void do_setxid(void *p)
+{
+        struct ctx *c = p;
+        if (c->err>0) return;
+        int ret = -__syscall(c->nr, c->id, c->eid, c->sid);
+        if (ret && !c->err) {
+                /* If one thread fails to set ids after another has already
+                 * succeeded, forcibly killing the process is the only safe
+                 * thing to do. State is inconsistent and dangerous. Use
+                 * SIGKILL because it is uncatchable. */
+                __block_all_sigs(0);
+                __syscall(SYS_kill, __syscall(SYS_getpid), SIGKILL);
+        }
+        c->err = ret;
+}
+
+int __setxid(int nr, int id, int eid, int sid)
+{
+        /* err is initially nonzero so that failure of the first thread does not
+         * trigger the safety kill above. */
+        struct ctx c = { .nr = nr, .id = id, .eid = eid, .sid = sid, .err = -1 };
+        __synccall(do_setxid, &c);
+        if (c.err) {
+                if (c.err>0) errno = c.err;
+                return -1;
+        }
+        return 0;
+}