implement POSIX timers

this implementation is superior to the glibc/nptl implementation, in
that it gives true realtime behavior. there is no risk of timer
expiration events being lost due to failed thread creation or failed
malloc, because the thread is created as time creation time, and
reused until the timer is deleted.

src/time/timer_create.c

@@ -0,0 +1,110 @@
+#include <time.h>
+#include "pthread_impl.h"
+
+struct ksigevent {
+	union sigval sigev_value;
+	int sigev_signo;
+	int sigev_notify;
+	int sigev_tid;
+};
+
+struct start_args {
+	pthread_barrier_t b;
+	struct sigevent *sev;
+	timer_t t;
+};
+
+static void sighandler(int sig, siginfo_t *si, void *ctx)
+{
+	int st;
+	timer_t t = si->si_value.sival_ptr;
+	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &st);
+	t->notify(t->val);
+	pthread_setcancelstate(st, 0);
+}
+
+static void killtimer(void *arg)
+{
+	timer_t t = arg;
+	if (t->timerid >= 0) __syscall(SYS_timer_delete, t->timerid);
+}
+
+static void *start(void *arg)
+{
+	struct start_args *args = arg;
+	struct __timer t = {
+		.notify = args->sev->sigev_notify_function,
+		.val = args->sev->sigev_value,
+	};
+
+	args->t = &t;
+
+	pthread_barrier_wait(&args->b);
+
+	pthread_cleanup_push(killtimer, &t);
+	pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, 0);
+	/* Loop on async-signal-safe cancellation point */
+	for (;;) sleep(1);
+	pthread_cleanup_pop(1);
+	return 0;
+}
+
+int timer_create(clockid_t clk, struct sigevent *evp, timer_t *res)
+{
+	struct sigevent sev = { 
+		.sigev_notify = SIGEV_SIGNAL,
+		.sigev_signo = SIGALRM
+	};
+	pthread_t td;
+	pthread_attr_t attr;
+	int r;
+	struct start_args args;
+	timer_t t;
+	struct ksigevent ksev;
+
+	if (evp) sev = *evp;
+
+	switch (sev.sigev_notify) {
+	case SIGEV_NONE:
+	case SIGEV_SIGNAL:
+		if (!(t = calloc(1, sizeof *t)))
+			return -1;
+		ksev.sigev_value = evp ? sev.sigev_value
+			: (union sigval){.sival_ptr=t};
+		ksev.sigev_signo = sev.sigev_signo;
+		ksev.sigev_notify = sev.sigev_notify;
+		ksev.sigev_tid = 0;
+		break;
+	case SIGEV_THREAD:
+		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.b, 0, 2);
+		args.sev = &sev;
+		r = pthread_create(&td, &attr, start, &args);
+		if (r) {
+			errno = r;
+			return -1;
+		}
+		pthread_barrier_wait(&args.b);
+		t = args.t;
+		t->thread = td;
+		ksev.sigev_value.sival_ptr = t;
+		ksev.sigev_signo = SIGCANCEL;
+		ksev.sigev_notify = 4; /* SIGEV_THREAD_ID */
+		ksev.sigev_tid = td->tid;
+		if (!libc.sigtimer) libc.sigtimer = sighandler;
+		break;
+	}
+
+	t->timerid = -1;
+	if (syscall(SYS_timer_create, clk, &ksev, &t->timerid) < 0) {
+		timer_delete(t);
+		return -1;
+	}
+
+	*res = t;
+	return 0;
+}