/*
  FUSE: Filesystem in Userspace
  Copyright (C) 2001-2007  Miklos Szeredi <miklos@szeredi.hu>

  This program can be distributed under the terms of the GNU GPL.
  See the file COPYING.
*/

/* gcc -Wall fuse_lo.c `pkg-config fuse --cflags --libs` -o fuse_lo */

#define _GNU_SOURCE
#define FUSE_USE_VERSION 30

#include <fuse_lowlevel.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>
#include <stdbool.h>
#include <string.h>
#include <dirent.h>
#include <assert.h>
#include <errno.h>
#include <err.h>

struct lo_inode {
	struct lo_inode *next;
	struct lo_inode *prev;
	int fd;
	ino_t ino;
	dev_t dev;
	uint64_t nlookup;
};

struct lo_data {
	int debug;
	struct lo_inode root;
};

static struct lo_data *lo_data(fuse_req_t req)
{
	return (struct lo_data *) fuse_req_userdata(req);
}

static struct lo_inode *lo_inode(fuse_req_t req, fuse_ino_t ino)
{
	if (ino == FUSE_ROOT_ID)
		return &lo_data(req)->root;
	else
		return (struct lo_inode *) (uintptr_t) ino; 
}

static int lo_fd(fuse_req_t req, fuse_ino_t ino)
{
	return lo_inode(req, ino)->fd;
}

static bool lo_debug(fuse_req_t req)
{
	return lo_data(req)->debug != 0;
}

static void lo_getattr(fuse_req_t req, fuse_ino_t ino,
			     struct fuse_file_info *fi)
{
	int res;
	struct stat buf;

	res = fstatat(lo_fd(req, ino), "", &buf, AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW);
	if (res == -1)
		return (void) fuse_reply_err(req, errno);

	fuse_reply_attr(req, &buf, 1.0);
}

static struct lo_inode *lo_find(fuse_req_t req, struct stat *st)
{
	struct lo_data *lo = lo_data(req);
	struct lo_inode *p;

	for (p = lo->root.next; p != &lo->root; p = p->next) {
		if (p->ino == st->st_ino && p->dev == st->st_dev)
			return p;
	}
	return NULL;
}

static void lo_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
{
	int newfd;
	int res;
	int saverr;
	struct lo_inode *inode;
	struct fuse_entry_param e = {
		e.attr_timeout = 1.0,
		e.entry_timeout = 1.0,
	};

	newfd = openat(lo_fd(req, parent), name, O_PATH | O_NOFOLLOW);
	if (newfd == -1)
		goto out_err;

	res = fstatat(newfd, "", &e.attr, AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW);
	if (res == -1)
		goto out_err;

	inode = lo_find(req, &e.attr);
	if (inode) {
		close(newfd);
		newfd = -1;
	} else {
		struct lo_inode *prev = &lo_data(req)->root;
		struct lo_inode *next = prev->next;
		saverr = ENOMEM;
		inode = calloc(1, sizeof(struct lo_inode));
		if (!inode)
			goto out_err;

		inode->fd = newfd;
		inode->ino = e.attr.st_ino;
		inode->dev = e.attr.st_dev;

		next->prev = inode;
		inode->next = next;
		inode->prev = prev;
		prev->next = inode;
	}
	inode->nlookup++;
	e.ino = (uintptr_t) inode;

	if (lo_debug(req))
		fprintf(stderr, "  %lli/%s -> %lli\n",
			(unsigned long long) parent, name, (unsigned long long) e.ino);

	fuse_reply_entry(req, &e);
	return;

out_err:
	saverr = errno;
	if (newfd != -1)
		close(newfd);
	fuse_reply_err(req, saverr);
}

static void lo_free(struct lo_inode *inode)
{
	struct lo_inode *prev = inode->prev;
	struct lo_inode *next = inode->next;

	next->prev = prev;
	prev->next = next;
	close(inode->fd);
	free(inode);
}

static void lo_forget(fuse_req_t req, fuse_ino_t ino, uint64_t nlookup)
{
	struct lo_inode *inode = lo_inode(req, ino);

	if (lo_debug(req)) {
		fprintf(stderr, "  forget %lli %lli -%lli\n",
			(unsigned long long) ino, (unsigned long long) inode->nlookup,
			(unsigned long long) nlookup);
	}

	assert(inode->nlookup >= nlookup);
	inode->nlookup -= nlookup;

	if (!inode->nlookup)
		lo_free(inode);

	fuse_reply_none(req);
}

static void lo_readlink(fuse_req_t req, fuse_ino_t ino)
{
	char buf[PATH_MAX + 1];
	int res;

	res = readlinkat(lo_fd(req, ino), "", buf, sizeof(buf));
	if (res == -1)
		return (void) fuse_reply_err(req, errno);

	if (res == sizeof(buf))
		return (void) fuse_reply_err(req, ENAMETOOLONG);

	buf[res] = '\0';

	fuse_reply_readlink(req, buf);
}

struct lo_dirp {
	DIR *dp;
	struct dirent *entry;
	off_t offset;
};

static struct lo_dirp *lo_dirp(struct fuse_file_info *fi)
{
	return (struct lo_dirp *) (uintptr_t) fi->fh;
}

static void lo_opendir(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
{
	int fd = -1;
	int error = ENOMEM;
	struct lo_dirp *d = calloc(1, sizeof(struct lo_dirp));
	if (d == NULL)
		goto out_err;

	fd = openat(lo_fd(req, ino), ".", O_RDONLY);
	if (fd == -1)
		goto out_errno;

	d->dp = fdopendir(fd);
	if (d->dp == NULL)
		goto out_errno;

	d->offset = 0;
	d->entry = NULL;

	fi->fh = (uintptr_t) d;
	fuse_reply_open(req, fi);
	return;

out_errno:
	error = errno;
out_err:
	free(d);
	if (fd != -1)
		close(fd);
	fuse_reply_err(req, error);
}

static void lo_readdir(fuse_req_t req, fuse_ino_t ino, size_t size, off_t offset,
		       struct fuse_file_info *fi)
{
	struct lo_dirp *d = lo_dirp(fi);
	char *buf;
	char *p;
	size_t rem;

	(void) ino;

	buf = calloc(size, 1);
	if (!buf)
		return (void) fuse_reply_err(req, ENOMEM);

	if (offset != d->offset) {
		seekdir(d->dp, offset);
		d->entry = NULL;
		d->offset = offset;
	}
	p = buf;
	rem = size;
	while (1) {
		size_t entsize;
		struct stat st;
		off_t nextoff;

		if (!d->entry) {
			errno = 0;
			d->entry = readdir(d->dp);
			if (!d->entry) {
				if (errno && rem == size) {
					int saverr = errno;
					free(buf);
					return (void) fuse_reply_err(req, saverr);
				}

				break;
			}
		}

		memset(&st, 0, sizeof(st));
		st.st_ino = d->entry->d_ino;
		st.st_mode = d->entry->d_type << 12;
		nextoff = telldir(d->dp);

		entsize = fuse_add_direntry(req, p, rem, d->entry->d_name, &st, nextoff);
		if (entsize > rem)
			break;

		p += entsize;
		rem -= entsize;

		d->entry = NULL;
		d->offset = nextoff;
	}

	fuse_reply_buf(req, buf, size - rem);
	free(buf);
}

static void lo_releasedir(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
{
	struct lo_dirp *d = lo_dirp(fi);
	(void) ino;
	closedir(d->dp);
	free(d);
	fuse_reply_err(req, 0);
}

static void lo_open(fuse_req_t req, fuse_ino_t ino,
			  struct fuse_file_info *fi)
{
	int fd;
	char buf[64];

	sprintf(buf, "/proc/self/fd/%i", lo_fd(req, ino));
	fd = open(buf, fi->flags & ~O_NOFOLLOW);
	if (fd == -1)
		return (void) fuse_reply_err(req, errno);

	fi->fh = fd;
	fuse_reply_open(req, fi);
}

static void lo_release(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
{
	close(fi->fh);
	fuse_reply_err(req, 0);
}

static void lo_read(fuse_req_t req, fuse_ino_t ino, size_t size,
			  off_t offset, struct fuse_file_info *fi)
{
	struct fuse_bufvec buf = FUSE_BUFVEC_INIT(size);

	(void) ino;

	buf.buf[0].flags = FUSE_BUF_IS_FD | FUSE_BUF_FD_SEEK;
	buf.buf[0].fd = fi->fh;
	buf.buf[0].pos = offset;

	fuse_reply_data(req, &buf, FUSE_BUF_SPLICE_MOVE);
}

static struct fuse_lowlevel_ops lo_oper = {
	.lookup		= lo_lookup,
	.forget		= lo_forget,
	.getattr	= lo_getattr,
	.readlink	= lo_readlink,
	.opendir	= lo_opendir,
	.readdir	= lo_readdir,
	.releasedir	= lo_releasedir,
	.open		= lo_open,
	.release	= lo_release,
	.read		= lo_read,
};

#define LO_OPT(t, p, v) { t, offsetof(struct lo_data, p), v }

static const struct fuse_opt lo_opts[] = {
	FUSE_OPT_KEY("debug",		      FUSE_OPT_KEY_KEEP),
	FUSE_OPT_KEY("-d",		      FUSE_OPT_KEY_KEEP),
	LO_OPT("debug",		      debug, 1),
	LO_OPT("-d",		      debug, 1),
	FUSE_OPT_END
};

int main(int argc, char *argv[])
{
	struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
	struct fuse_chan *ch;
	char *mountpoint;
	int ret = -1;
	struct lo_data lo = { };

	if (fuse_opt_parse(&args, &lo, lo_opts, NULL) == -1)
		exit(1);
	lo.root.next = lo.root.prev = &lo.root;
	lo.root.fd = open("/", O_PATH);
	lo.root.nlookup = 2;
	if (lo.root.fd == -1)
		err(1, "open(\"/\", O_PATH)");

	if (fuse_parse_cmdline(&args, &mountpoint, NULL, NULL) != -1 &&
	    (ch = fuse_mount(mountpoint, &args)) != NULL) {
		struct fuse_session *se;
		se = fuse_lowlevel_new(&args, &lo_oper, sizeof(lo_oper), &lo);
		if (se != NULL) {
			if (fuse_set_signal_handlers(se) != -1) {
				fuse_session_add_chan(se, ch);
				ret = fuse_session_loop(se);
				fuse_remove_signal_handlers(se);
				fuse_session_remove_chan(ch);
			}
			fuse_session_destroy(se);
		}
		fuse_unmount(mountpoint, ch);
		free(mountpoint);
	}
	fuse_opt_free_args(&args);

	while (lo.root.next != &lo.root)
		lo_free(lo.root.next);

	return ret ? 1 : 0;
}