#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <poll.h>
#include <err.h>

struct index {
	struct index *next;
	struct index *prev;
	const char *line;
};

struct state {
	size_t bufsize;
	char *buf;
	size_t index_size;
	struct index *index;
	struct index head;
};

static void read_mountinfo(struct pollfd *pfd, char *buf, size_t bufsize)
{
	int readcnt, backoff = 0, retry = 0;
	size_t len;
	ssize_t res;

retry:
	if (lseek(pfd->fd, 0, SEEK_SET) == (off_t) -1)
		err(1, "lseek");
	len = 0;
	readcnt = 0;
	do {
		if (len >= bufsize - 4096)
			errx(1, "buffer overrun");
		res = read(pfd->fd, buf + len, bufsize - len);
		if (res == -1)
			err(1, "read");
		len += res;

		if (!res || !(++readcnt % 16)) {
			if (poll(pfd, 1, 0) == -1)
				err(1, "poll/0");
			if (pfd->revents & POLLPRI) {
				if (!backoff) {
					backoff++;
					goto retry;
				}
				if (!retry) {
					fprintf(stderr, "retry.");
					retry = 1;
				}
				do {
					usleep(backoff * 1000);
					if (backoff < 128)
						backoff *= 2;
					if (poll(pfd, 1, 0) == -1)
						err(1, "poll/0");
				} while (pfd->revents & POLLPRI);
				goto retry;
			}
		}
	} while (res);
	buf[len] = '\0';

	if (retry) {
		fprintf(stderr, "..\n");
		retry = 0;
	}
}

static void add_index(struct state *s, struct index *this, const char *line)
{
	struct index *prev = s->head.prev, *next = &s->head;

	if (this->line)
		errx(1, "index corruption");

	this->line = line;
	this->next = next;
	this->prev = prev;
	prev->next = next->prev = this;
}

static void del_index(struct index *this)
{
	struct index *prev = this->prev, *next = this->next;

	this->line = NULL;
	prev->next = next;
	next->prev = prev;
}

static void diff_mountinfo(struct state *old, struct state *cur)
{
	char *line, *end;
	struct index *this;
	int mntid;

	cur->head.next = cur->head.prev = &cur->head;
	for (line = cur->buf; line[0]; line = end + 1) {
		end = strchr(line, '\n');
		if (!end)
			errx(1, "parsing (1)");
		*end = '\0';
		if (sscanf(line, "%i", &mntid) != 1)
			errx(1, "parsing (2)");
		if (mntid < 0 || (size_t) mntid >= cur->index_size)
			errx(1, "index overflow");
		add_index(cur, &cur->index[mntid], line);

		this = &old->index[mntid];
		if (this->line) {
			if (strcmp(this->line, line))
				printf("* %s\n", line);
			del_index(this);
		} else {
			printf("+ %s\n", line);
		}
	}
	while (old->head.next != &old->head) {
		this = old->head.next;
		printf("- %s\n", this->line);
		del_index(this);
	}
	fflush(stdout);
}

int main(void)
{
	struct state state[2], *old = &state[0], *cur = &state[1], *tmp;
	struct pollfd pfd = { .events = POLLPRI };

	old->index_size = cur->index_size = 131072;
	old->bufsize = cur->bufsize = cur->index_size * 128;
	old->index = calloc(old->index_size, sizeof(struct index));
	cur->index = calloc(cur->index_size, sizeof(struct index));
	old->buf = malloc(old->bufsize);
	cur->buf = malloc(cur->bufsize);
	if (!old->index || !cur->index || !old->buf || !cur->buf)
		err(1, "allocating buffers");

	old->buf[0] = '\0';
	old->head.prev = old->head.next = &old->head;

	pfd.fd = open("/proc/self/mountinfo", O_RDONLY);
	if (pfd.fd == -1)
		err(1, "open");

	while (1) {
		read_mountinfo(&pfd, cur->buf, cur->bufsize);
		diff_mountinfo(old, cur);

		tmp = old;
		old = cur;
		cur = tmp;

		if (poll(&pfd, 1, -1) == -1)
			err(1, "poll/inf");
	}
}