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libc, librthread: _twait: subtraction is not comparison

Compare the current time with the absolute timeout before computing
the relative timeout to avoid arithmetic overflow.  Fixes a bug where
large negative absolute timeouts are subtracted into large positive
relative timeouts and incorrectly cause the caller to block.

While here, use timespeccmp(3) and timespecsub(3) to simplify the
code.

Thread: https://marc.info/?l=openbsd-tech&m=169945962503129&w=2

/*	$OpenBSD: synch.h,v 1.10 2024/01/07 19:44:28 cheloha Exp $ */
/*
 * Copyright (c) 2017 Martin Pieuchot
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/atomic.h>
#include <sys/time.h>
#include <sys/futex.h>

static inline int
_wake(volatile uint32_t *p, int n)
{
	return futex(p, FUTEX_WAKE, n, NULL, NULL);
}

static inline int
_twait(volatile uint32_t *p, int val, clockid_t clockid, const struct timespec *abs)
{
	struct timespec now, rel;
	int saved_errno = errno;
	int error;

	if (abs == NULL) {
		error = futex(p, FUTEX_WAIT, val, NULL, NULL);
		if (error == -1) {
			error = errno;
			errno = saved_errno;
		}
		return error;
	}

	if (!timespecisvalid(abs) || clock_gettime(clockid, &now))
		return EINVAL;

	if (timespeccmp(abs, &now, <=))
		return ETIMEDOUT;
	timespecsub(abs, &now, &rel);

	error = futex(p, FUTEX_WAIT, val, &rel, NULL);
	if (error == -1) {
		error = errno;
		errno = saved_errno;
	}
	return error;
}

static inline int
_requeue(volatile uint32_t *p, int n, int m, volatile uint32_t *q)
{
	return futex(p, FUTEX_REQUEUE, n, (void *)(long)m, q);
}