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

fusl/src/thread/pthread_cancel.c

+#define _GNU_SOURCE
+#include <string.h>
+#include "pthread_impl.h"
+#include "syscall.h"
+#include "libc.h"
+
+__attribute__((__visibility__("hidden")))
+long __cancel(), __syscall_cp_asm(), __syscall_cp_c();
+
+long __cancel()
+{
+        pthread_t self = __pthread_self();
+        if (self->canceldisable == PTHREAD_CANCEL_ENABLE || self->cancelasync)
+                pthread_exit(PTHREAD_CANCELED);
+        self->canceldisable = PTHREAD_CANCEL_DISABLE;
+        return -ECANCELED;
+}
+
+long __syscall_cp_asm(volatile void *, syscall_arg_t,
+                      syscall_arg_t, syscall_arg_t, syscall_arg_t,
+                      syscall_arg_t, syscall_arg_t, syscall_arg_t);
+
+long __syscall_cp_c(syscall_arg_t nr,
+                    syscall_arg_t u, syscall_arg_t v, syscall_arg_t w,
+                    syscall_arg_t x, syscall_arg_t y, syscall_arg_t z)
+{
+        pthread_t self;
+        long r;
+        int st;
+
+        if ((st=(self=__pthread_self())->canceldisable)
+            && (st==PTHREAD_CANCEL_DISABLE || nr==SYS_close))
+                return __syscall(nr, u, v, w, x, y, z);
+
+        r = __syscall_cp_asm(&self->cancel, nr, u, v, w, x, y, z);
+        if (r==-EINTR && nr!=SYS_close && self->cancel &&
+            self->canceldisable != PTHREAD_CANCEL_DISABLE)
+                r = __cancel();
+        return r;
+}
+
+static void _sigaddset(sigset_t *set, int sig)
+{
+        unsigned s = sig-1;
+        set->__bits[s/8/sizeof *set->__bits] |= 1UL<<(s&8*sizeof *set->__bits-1);
+}
+
+__attribute__((__visibility__("hidden")))
+extern const char __cp_begin[1], __cp_end[1], __cp_cancel[1];
+
+static void cancel_handler(int sig, siginfo_t *si, void *ctx)
+{
+        pthread_t self = __pthread_self();
+        ucontext_t *uc = ctx;
+        uintptr_t pc = uc->uc_mcontext.MC_PC;
+
+        a_barrier();
+        if (!self->cancel || self->canceldisable == PTHREAD_CANCEL_DISABLE) return;
+
+        _sigaddset(&uc->uc_sigmask, SIGCANCEL);
+
+        if (self->cancelasync || pc >= (uintptr_t)__cp_begin && pc < (uintptr_t)__cp_end) {
+                uc->uc_mcontext.MC_PC = (uintptr_t)__cp_cancel;
+                return;
+        }
+
+        __syscall(SYS_tkill, self->tid, SIGCANCEL);
+}
+
+void __testcancel()
+{
+        pthread_t self = __pthread_self();
+        if (self->cancel && !self->canceldisable)
+                __cancel();
+}
+
+static void init_cancellation()
+{
+        struct sigaction sa = {
+                .sa_flags = SA_SIGINFO | SA_RESTART,
+                .sa_sigaction = cancel_handler
+        };
+        memset(&sa.sa_mask, -1, _NSIG/8);
+        __libc_sigaction(SIGCANCEL, &sa, 0);
+}
+
+int pthread_cancel(pthread_t t)
+{
+        static int init;
+        if (!init) {
+                init_cancellation();
+                init = 1;
+        }
+        a_store(&t->cancel, 1);
+        return pthread_kill(t, SIGCANCEL);
+}