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repair exit codes, slightly broken after last commit (please do not go
to an enum, that's just alien).  Documents the return values in each
function and unifies the protocol incompatibility error message.
from kristaps

/*	$Id: server.c,v 1.9 2019/03/23 00:20:55 deraadt Exp $ */
/*
 * Copyright (c) 2019 Kristaps Dzonsons <kristaps@bsd.lv>
 *
 * 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/stat.h>

#include <assert.h>
#include <fcntl.h>
#include <inttypes.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <err.h>

#include "extern.h"

static int
fcntl_nonblock(struct sess *sess, int fd)
{
	int	 fl;

	if ((fl = fcntl(fd, F_GETFL, 0)) == -1)
		ERR(sess, "fcntl: F_GETFL");
	else if (fcntl(fd, F_SETFL, fl|O_NONBLOCK) == -1)
		ERR(sess, "fcntl: F_SETFL");
	else
		return 1;

	return 0;
}

/*
 * The server (remote) side of the system.
 * This parses the arguments given it by the remote shell then moves
 * into receiver or sender mode depending upon those arguments.
 * Returns exit code 0 on success, 1 on failure, 2 on failure with
 * incompatible protocols.
 */
int
rsync_server(const struct opts *opts, size_t argc, char *argv[])
{
	struct sess	 sess;
	int		 fdin = STDIN_FILENO,
			 fdout = STDOUT_FILENO, rc = 1;

	if (pledge("stdio unix rpath wpath cpath dpath fattr chown getpw unveil",
	    NULL) == -1)
		err(1, "pledge");

	memset(&sess, 0, sizeof(struct sess));
	sess.opts = opts;

	/* Begin by making descriptors non-blocking. */

	if (!fcntl_nonblock(&sess, fdin) ||
	     !fcntl_nonblock(&sess, fdout)) {
		ERRX1(&sess, "fcntl_nonblock");
		goto out;
	}

	/* Standard rsync preamble, server side. */

	sess.lver = RSYNC_PROTOCOL;
	sess.seed = arc4random();

	if (!io_read_int(&sess, fdin, &sess.rver)) {
		ERRX1(&sess, "io_read_int");
		goto out;
	} else if (!io_write_int(&sess, fdout, sess.lver)) {
		ERRX1(&sess, "io_write_int");
		goto out;
	} else if (!io_write_int(&sess, fdout, sess.seed)) {
		ERRX1(&sess, "io_write_int");
		goto out;
	}

	sess.mplex_writes = 1;

	if (sess.rver < sess.lver) {
		ERRX(&sess,
		    "remote protocol %d is older than our own %d: unsupported",
		    sess.rver, sess.lver);
		rc = 2;
		goto out;
	}

	LOG2(&sess, "server detected client version %" PRId32
		", server version %" PRId32 ", seed %" PRId32,
		sess.rver, sess.lver, sess.seed);

	if (sess.opts->sender) {
		LOG2(&sess, "server starting sender");

		/*
		 * At this time, I always get a period as the first
		 * argument of the command line.
		 * Let's make it a requirement until I figure out when
		 * that differs.
		 * rsync [flags] "." <source> <...>
		 */

		if (strcmp(argv[0], ".")) {
			ERRX(&sess, "first argument must "
				"be a standalone period");
			goto out;
		}
		argv++;
		argc--;
		if (argc == 0) {
			ERRX(&sess, "must have arguments");
			goto out;
		}

		if (!rsync_sender(&sess, fdin, fdout, argc, argv)) {
			ERRX1(&sess, "rsync_sender");
			goto out;
		}
	} else {
		LOG2(&sess, "server starting receiver");

		/*
		 * I don't understand why this calling convention
		 * exists, but we must adhere to it.
		 * rsync [flags] "." <destination>
		 */

		if (argc != 2) {
			ERRX(&sess, "server receiver mode "
				"requires two argument");
			goto out;
		} else if (strcmp(argv[0], ".")) {
			ERRX(&sess, "first argument must "
				"be a standalone period");
			goto out;
		}

		if (!rsync_receiver(&sess, fdin, fdout, argv[1])) {
			ERRX1(&sess, "rsync_receiver");
			goto out;
		}
	}

#if 0
	/* Probably the EOF. */
	if (io_read_check(&sess, fdin))
		WARNX(&sess, "data remains in read pipe");
#endif

	rc = 0;
out:
	return rc;
}