echoed - A very questionable daemon for converting upper-case and lower-case

echoed.c

/*
 * echoed - Pipedeamon which changes to lowercase and uppercase
 * Copyright (C) 2019  Harald Seiler
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
#include <ctype.h>
#include <errno.h>
#include <fcntl.h>
#include <poll.h>
#include <signal.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/signalfd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#define FIFO_IN_PATH "/tmp/fifo-in"
#define FIFO_OUT_PATH "/tmp/fifo-out"

static __attribute__((noreturn)) int bail(char *reason);
static int echoed_create_fifo(char *name);
static int echoed_create_signalfd(void);
static int echoed_read_signal(int fd_sig);
static int echoed_poll(int fd_sig, int fd_pipe, bool is_output);

int main(void)
{
	int fd_sig, fd_in, fd_out;

	if (echoed_create_fifo(FIFO_IN_PATH) < 0)
		bail("Failed to create read fifo");
	if (echoed_create_fifo(FIFO_OUT_PATH) < 0)
		bail("Failed to create write fifo");

	if ((fd_sig = echoed_create_signalfd()) < 0)
		bail("Failed to initialize signalfd");

	/*
	 * Open the pipes as read-write.  This has two advantages:
	 *
	 * 1. There will always be a reader/writer open for the pipe which means
	 *    that we will never run into a SIGPIPE on the output pipe or EOF on
	 *    the input pipe.
	 * 2. Because we want to open them with O_NONBLOCK, if no reader were
	 *    open for the output pipe, open() would fail with ENXIO (see fifo(7)).
	 *    Opening read-write circumvents this issue (Linux specific).
	 */
	if ((fd_in = open(FIFO_IN_PATH, O_RDWR | O_NONBLOCK)) < 0)
		bail("Failed to open read fifo");
	if ((fd_out = open(FIFO_OUT_PATH, O_RDWR | O_NONBLOCK)) < 0)
		bail("Failed to open write fifo");

	char buf[1024], *bufptr;
	int (*converter_func)(int) = toupper;
	ssize_t bytes = 0;

	while (1) {
		int ev;
		if (bytes > 0) {
			/* Some data left, which needs to be flushed. */
			ev = echoed_poll(fd_sig, fd_out, true);
		} else {
			/* Wait for new data to come in. */
			ev = echoed_poll(fd_sig, fd_in, false);
		}

		if (ev == fd_sig) {
			int signo = echoed_read_signal(fd_sig);
			switch (signo) {
			case SIGINT:
				goto exit;
			case SIGUSR1:
				converter_func = toupper;
				break;
			case SIGUSR2:
				converter_func = tolower;
				break;
			default:
				bail("Catched an unknown signal");
			}
		} else if (ev == fd_out && bytes > 0) {
			ssize_t written = write(fd_out, buf, bytes);
			if (written < 0)
				bail("Failed writing to fifo");

			bufptr += written;
			bytes -=written;
		} else if (ev == fd_in && bytes == 0) {
			bytes = read(fd_in, buf, sizeof(buf));
			if (bytes < 0)
				bail("Failed to read from fifo");

			for (ssize_t i = 0; i < bytes; i++) {
				buf[i] = converter_func(buf[i]);
			}

			bufptr = buf;
		}
	}

exit:
	unlink(FIFO_IN_PATH);
	unlink(FIFO_OUT_PATH);
	exit(EXIT_SUCCESS);
}

/*
 * Abort because of an unrecoverable error.
 */
static __attribute__((noreturn)) int bail(char *reason)
{
	char buf[512];
	snprintf(buf, sizeof(buf), "echoed: %s", reason);
	perror(buf);
	exit(EXIT_FAILURE);
}

/*
 * Create a named pipe.  In case the file previously existed, remove it and
 * retry.
 */
static int echoed_create_fifo(char *name)
{
	if (mkfifo(name, 0666) < 0) {
		if (errno == EEXIST) {
			/*
			 * If the named fifo already exists, remove it and try
			 * again.
			 */
			unlink(name);
			return mkfifo(name, 0666);
		}

		return -1;
	}
	return 0;
}

/*
 * Initialize a signalfd(2).  We will use it to monitor for:
 *
 * - SIGINT: Keyboard-Interrupt; cleanup & exit
 * - SIGUSR1: Change to uppercase mode
 * - SIGUSR2: Change to lowercase mode
 */
static int echoed_create_signalfd(void)
{
	sigset_t mask;
	sigemptyset(&mask);
	sigaddset(&mask, SIGINT);
	sigaddset(&mask, SIGUSR1);
	sigaddset(&mask, SIGUSR2);

	/* Block default handlers so they won't get triggered */
	if (sigprocmask(SIG_BLOCK, &mask, NULL) < 0) {
		return -1;
	}

	return signalfd(-1, &mask, 0);
}

/*
 * Read a signal from the signalfd and returns its signal number.
 */
static int echoed_read_signal(int fd_sig) {
	struct signalfd_siginfo fdsi;

	if (read(fd_sig, &fdsi, sizeof(fdsi)) != sizeof(fdsi))
		bail("signalfd read wrong size");

	return fdsi.ssi_signo;
}

/*
 * Poll for either a signal or the pipe.  If `is_output` is true, wait for
 * fd_pipe to become writable otherwise wait for it to become readable.
 *
 * Returns the file descriptor which triggered the event.
 */
static int echoed_poll(int fd_sig, int fd_pipe, bool is_output)
{
	struct pollfd poll_fds[] = {
		{.fd = fd_sig,  .events = POLLIN},
		{.fd = fd_pipe, .events = is_output ? POLLOUT : POLLIN},
	};

	int ret = poll(poll_fds, sizeof(poll_fds) / sizeof(struct pollfd), -1);
	if (ret < 0)
		bail("Poll failed");

	if (poll_fds[0].revents & POLLIN) {
		return fd_sig;
	} else if (poll_fds[1].revents & (POLLIN | POLLOUT)) {
		return fd_pipe;
	}

	bail("Poll did not return any event");
}