rework cancellation weak alias logic not to depend on archive order

if the order of object files in the static archive libc.a was not
respected by the linker, the old logic could wrongly cause POSIX
symbols outside of the ISO C namespace to be pulled into pure C
programs. this should not happen with well-behaved linkers, but
relying on the link order was a bad idea anyway.

files are renamed to better reflect their contents now that they don't
need names to control their order as members in the archive file.

src/thread/pthread_cancel.c

@@ -0,0 +1,85 @@
+#include "pthread_impl.h"
+#include "syscall.h"
+
+void __cancel()
+{
+	pthread_t self = __pthread_self();
+	self->canceldisable = 1;
+	self->cancelasync = 0;
+	pthread_exit(PTHREAD_CANCELED);
+}
+
+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;
+
+	if (!libc.has_thread_pointer || (self = __pthread_self())->canceldisable)
+		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)
+		__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);
+}
+
+static void cancel_handler(int sig, siginfo_t *si, void *ctx)
+{
+	pthread_t self = __pthread_self();
+	ucontext_t *uc = ctx;
+	const char *ip = ((char **)&uc->uc_mcontext)[CANCEL_REG_IP];
+	extern const char __cp_begin[1], __cp_end[1];
+
+	if (!self->cancel || self->canceldisable) return;
+
+	_sigaddset(&uc->uc_sigmask, SIGCANCEL);
+
+	if (self->cancelasync || ip >= __cp_begin && ip < __cp_end) {
+		self->canceldisable = 1;
+		pthread_sigmask(SIG_SETMASK, &uc->uc_sigmask, 0);
+		__cancel();
+	}
+
+	__syscall(SYS_tkill, self->tid, SIGCANCEL);
+}
+
+void __testcancel()
+{
+	if (!libc.has_thread_pointer) return;
+	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
+	};
+	sigfillset(&sa.sa_mask);
+	__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);
+}