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Rather than running status-left, status-right and window title #() with popen
immediately every redraw, queue them up and run them in the background,
starting each once every status-interval. The actual status line uses the
output from the last run.

This brings several advantages:

- tmux itself may be called from inside #() without causing the server to hang;
- likewise, sleep or similar doesn't cause the server to block;
- commands aren't run excessively often when redrawing;
- commands shared by status-left and status-right, or used multiple times, will
  only be run once.

run-shell and if-shell still use system()/popen() but will be changed over to
use this too later.

/* $OpenBSD: buffer-poll.c,v 1.3 2009/10/10 15:03:01 nicm Exp $ */

/*
 * Copyright (c) 2007 Nicholas Marriott <nicm@users.sourceforge.net>
 *
 * 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 MIND, 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/types.h>

#include <errno.h>
#include <unistd.h>

#include "tmux.h"

/* Fill buffers from socket based on poll results. */
int
buffer_poll(struct pollfd *pfd, struct buffer *in, struct buffer *out)
{
	ssize_t	n;

	if (pfd->revents & (POLLERR|POLLNVAL))
		return (-1);
	if (in != NULL && pfd->revents & POLLIN) {
		buffer_ensure(in, BUFSIZ);
		n = read(pfd->fd, BUFFER_IN(in), BUFFER_FREE(in));
		if (n == 0)
			return (-1);
		if (n == -1) {
			if (errno != EINTR && errno != EAGAIN)
				return (-1);
		} else
			buffer_add(in, n);
	} else if (pfd->revents & POLLHUP)
		return (-1);
	if (out != NULL && BUFFER_USED(out) > 0 && pfd->revents & POLLOUT) {
		n = write(pfd->fd, BUFFER_OUT(out), BUFFER_USED(out));
		if (n == -1) {
			if (errno != EINTR && errno != EAGAIN)
				return (-1);
		} else
			buffer_remove(out, n);
	}
	return (0);
}