/*
 * Example usage in 3 terminals:
 * while :; do ./locktest; done
 * while :; do ./locktest; done
 * cd /dev/shm/locktest && while :; do od -tx1 target | grep 78; done
 * The latter command shouldn't detect any 78's, but it does with any of:
 * - flock() removed
 * - fstat() vs. stat() comparison removed
 * - close(fd); before the rename()
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/fcntl.h>
#include <sys/file.h>
#include <sys/stat.h>

#define PATH "/dev/shm/locktest"
#define TARGET_FILE PATH "/target"
#define TMP_FILE PATH "/tmp"

int main(int argc, char **argv)
{
	char tmp_filename[1024];
	int uid = argc > 1 ? atoi(argv[1]) : 0;
	snprintf(tmp_filename, sizeof(tmp_filename), "%s.%u", TMP_FILE, uid);

	srandom(getpid());

	if (mkdir(PATH, 0700) && errno != EEXIST) {
		perror("mkdir");
		return 1;
	}

	int fd = open(tmp_filename, O_CREAT | O_WRONLY, 0600);
	if (fd == -1) {
		perror("open");
		return 1;
	}

	usleep(random() & 0x3fff);

	if (flock(fd, LOCK_EX | LOCK_NB)) {
		perror("flock");
		return 1;
	}

	struct stat st1, st2;
	if (fstat(fd, &st1) || stat(tmp_filename, &st2)) {
		perror("[f]stat");
		/* unlink(tmp_filename); Probably already gone, but may have been recreated just now */
		return 1;
	}
	if (st1.st_dev != st2.st_dev || st1.st_ino != st2.st_ino) {
		fprintf(stderr, "Another invocation won race\n");
		return 1;
	}

	usleep(random() & 0x3fff);

	char buf[1024];
	memset(buf, 'x', sizeof(buf));
	if (write(fd, buf, sizeof(buf)) != sizeof(buf)) {
		fprintf(stderr, "Partial write\n");
		unlink(tmp_filename);
		return 1;
	}

	if (fsync(fd)) {
		perror("fsync");
		unlink(tmp_filename);
		return 1;
	}

	if ((random() & 0x1f) == 0) {
		fprintf(stderr, "Fault injection\n");
		unlink(tmp_filename);
		return 1;
	}

	usleep(random() & 0x3fff);

	if (lseek(fd, 0, SEEK_SET)) {
		perror("lseek");
		unlink(tmp_filename);
		return 1;
	}

	memset(buf, 'a', sizeof(buf));
	if (write(fd, buf, sizeof(buf)) != sizeof(buf)) {
		fprintf(stderr, "Partial write\n");
		unlink(tmp_filename);
		return 1;
	}

	if (fsync(fd)) {
		perror("fsync");
		unlink(tmp_filename);
		return 1;
	}

/* Don't!
	close(fd);
*/

	usleep(random() & 0x3fff);

	if (rename(tmp_filename, TARGET_FILE)) {
		perror("rename");
		unlink(tmp_filename);
		return 1;
	}

	return 0;
}