waffle-mole/main.c

/*
 * Copyright (C) 2022 Camden Dixie O'Brien
 *
 * This program is free software: you can redistribute it and/or
 * modify it under the terms of the GNU Affero General Public License
 * as published by the Free Software Foundation, either version 3 of
 * the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public
 * License along with this program.  If not, see
 * <https://www.gnu.org/licenses/>.
 */

#include <errno.h>
#include <netinet/in.h>
#include <signal.h>
#include <stdbool.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/socket.h>
#include <unistd.h>

#define SBUF_SIZE 1024

static bool exit_requested = false;

void handle_exit_signal(int signum)
{
	(void)signum;
	exit_requested = true;
}

int main(int argc, char *argv[])
{
	(void)argc;
	(void)argv;

	int res = EXIT_SUCCESS;

	/*
	 * Register signal handler for SIGTERM and SIGINT.
	 *
	 * The behaviour of signal() is not entirely consistent across
	 * different implementations, so this should ideally be rewritten
	 * to use signaction() instead at some point.
	 */
	if (signal(SIGTERM, handle_exit_signal) == SIG_ERR) {
		fprintf(stderr, "Failed to register SIGTERM signal handler\n");
		return EXIT_FAILURE;
	}
	if (signal(SIGINT, handle_exit_signal) == SIG_ERR) {
		fprintf(stderr, "Failed to register SIGINT signal handler\n");
		return EXIT_FAILURE;
	}

	/*
	 * Initialise socket.
	 *
	 * Currently only supporting IPv6, and hard-coding address and
	 * port. Should eventully get address and port from arguments, and
	 * support IPv4 as well.
	 */
	int sfd = socket(AF_INET6, SOCK_STREAM, 0);
	if (sfd == -1) {
		fprintf(stderr, "Failed to open socket\n");
		return EXIT_FAILURE;
	}
	const struct sockaddr_in6 haddr = {
		.sin6_family = AF_INET6,
		.sin6_port = htons(7070),
		.sin6_addr = IN6ADDR_LOOPBACK_INIT,
	};
	if (bind(sfd, (const struct sockaddr *)&haddr, sizeof(haddr)) == -1) {
		fprintf(stderr, "Error binding socket to address\n");
		res = EXIT_FAILURE;
		goto close_server_socket;
	}
	if (listen(sfd, 8) == -1) {
		fprintf(stderr, "Error attempting to listen on socket\n");
		res = EXIT_FAILURE;
		goto close_server_socket;
	}

	struct sockaddr_in6 paddr;
	socklen_t paddr_size = sizeof(paddr);
	int cfd;
	char sbuf[SBUF_SIZE];
	ssize_t n, slen;
	while (!exit_requested) {
		/*
		 * Accept incoming connection.
		 *
		 * If this is interrupted we go to the next loop iteration
		 * rather than retrying directly in case the received signal
		 * was requesting that the server exits.
		 *
		 * System calls after this point should be retried before
		 * checking if we need to exit, as we don't want to leave the
		 * client hanging.
		 */
		cfd = accept(sfd, (struct sockaddr *)&paddr, &paddr_size);
		if (cfd == -1) {
			if (errno != EINTR)
				fprintf(stderr, "Error accepting connection\n");
			continue;
		}

		/*
		 * Read selector from client socket.
		 *
		 * For now, assuming that the selector is less than RDBUF
		 * bytes long.
		 */
		do {
			errno = 0;
			n = read(cfd, sbuf, SBUF_SIZE);
		} while (errno == EINTR);
		if (n == -1) {
			fprintf(stderr, "Error reading selector from client\n");
			goto close_client_socket;
		}

		/*
		 * Locate the end of the selector.
		 *
		 * The end of the selector string is indicated with CRLF. Most
		 * of the time this will be at the end of the received data,
		 * so might as well start from there.
		 */
		slen = n;
		for (unsigned i = n - 2; i < n; --i) {
			if (sbuf[i] == 0x0d && sbuf[i + 1] == 0x0a)
				slen = i;
		}
		if (slen == n) {
			fprintf(stderr, "Received invalid selector (no CRLF)\n");
			goto close_client_socket;
		}

		/*
		 * Write an empty response to the client.
		 */
		do {
			errno = 0;
			n = write(cfd, ".\r\n", 3);
		} while (errno == EINTR);
		if (n == -1) {
			fprintf(stderr, "Error sending response to client\n");
			goto close_client_socket;
		}

	close_client_socket:
		close(cfd);
	}

close_server_socket:
	close(sfd);
	return res;
}