// SPDX-License-Identifier: GPL-2.0
/*
 * FUSE: Filesystem in Userspace
 * Copyright (C) 2026 Jingbo Xu <jefflexu@linux.alibaba.com>
 */

/** @file
 *
 * This example implements a file system with a single file whose
 * contents change dynamically: it always contains the current time.
 *
 * This example calls fuse_lowlevel_notify_prune() to notify the kernel
 * that the kernel's own dentry/inode cache has to be pruned, and thus
 * the corresponding FUSE_FORGET request will be sent to the FUSE server
 * with lookup_cnt decreased to 0.  The example updates the file's
 * content once the lookup_cnt decreased to 0 and the new file content
 * will be read when the file is lookued up and read for the next time.
 *
 * To see the effect, first start the file system with the
 *  ``--no-notify`` option:
 *
 *     $ notify_prune --update-interval=1 --no-notify mnt/
 *
 * Observe that the output never changes:
 *
 *     $ for i in 1 2 3 4 5; do
 *     >     cat mnt/current_time
 *     >     sleep 1
 *     > done
 *     The current time is 15:26:10
 *     The current time is 15:26:10
 *     The current time is 15:26:10
 *     The current time is 15:26:10
 *     The current time is 15:26:10
 *
 * If you instead enable the notification functions, the changes become
 * visible:
 *
 *      $ notify_prune --update-interval=1 mnt/
 *      $ for i in 1 2 3 4 5; do
 *      >     cat mnt/current_time
 *      >     sleep 1
 *      > done
 *      The current time is 15:30:20
 *      The current time is 15:30:24
 *      The current time is 15:30:25
 *      The current time is 15:30:26
 *      The current time is 15:30:27
 *
 * ## Compilation ##
 *
 *     gcc -Wall notify_prune.c `pkg-config fuse3 --cflags --libs` -o notify_prune
 *
 * ## Source code ##
 * \include notify_prune.c
 */

#define FUSE_USE_VERSION FUSE_MAKE_VERSION(3, 19)

#include <fuse_lowlevel.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <assert.h>
#include <stddef.h>
#include <unistd.h>
#include <pthread.h>
#include <stdbool.h>
#include <stdatomic.h>

/*
 * We can't actually tell the kernel that there is no
 * timeout, so we just send a big value.
 */
#define NO_TIMEOUT 500000

#define MAX_STR_LEN 128
#define FILE_INO 2
#define FILE_NAME "current_time"
static char file_contents[MAX_STR_LEN];
static int lookup_cnt;
static size_t file_size;
static _Atomic bool is_stop;

/* Command line parsing */
struct options {
	int no_notify;
	int update_interval;
};
static struct options options = {
	.no_notify = 0,
	.update_interval = 1,
};

#define OPTION(t, p) { t, offsetof(struct options, p), 1 }
static const struct fuse_opt option_spec[] = {
	OPTION("--no-notify", no_notify),
	OPTION("--update-interval=%d", update_interval), FUSE_OPT_END
};

static void update_fs(void)
{
	struct tm *now;
	time_t t = time(NULL);

	now = localtime(&t);
	assert(now != NULL);

	file_size = strftime(file_contents, MAX_STR_LEN,
			     "The current time is %H:%M:%S\n", now);
	assert(file_size != 0);
}

static int tfs_stat(fuse_ino_t ino, struct stat *stbuf)
{
	stbuf->st_ino = ino;
	if (ino == FUSE_ROOT_ID) {
		stbuf->st_mode = S_IFDIR | 0755;
		stbuf->st_nlink = 1;
	}

	else if (ino == FILE_INO) {
		stbuf->st_mode = S_IFREG | 0444;
		stbuf->st_nlink = 1;
		stbuf->st_size = file_size;
	}

	else
		return -1;

	return 0;
}

static void tfs_init(void *userdata, struct fuse_conn_info *conn)
{
	(void)userdata;

	/* Disable the receiving and processing of FUSE_INTERRUPT requests */
	conn->no_interrupt = 1;
}

static void tfs_destroy(void *userarg)
{
	(void)userarg;

	is_stop = true;
}

static void tfs_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
{
	struct fuse_entry_param e;

	memset(&e, 0, sizeof(e));

	if (parent != FUSE_ROOT_ID)
		goto err_out;
	else if (strcmp(name, FILE_NAME) == 0) {
		e.ino = FILE_INO;
		lookup_cnt++;
	} else
		goto err_out;

	e.attr_timeout = NO_TIMEOUT;
	e.entry_timeout = NO_TIMEOUT;
	if (tfs_stat(e.ino, &e.attr) != 0)
		goto err_out;
	fuse_reply_entry(req, &e);
	return;

err_out:
	fuse_reply_err(req, ENOENT);
}

static void tfs_forget(fuse_req_t req, fuse_ino_t ino, uint64_t nlookup)
{
	(void)req;
	if (ino == FILE_INO) {
		lookup_cnt -= nlookup;
		if (!lookup_cnt)
			update_fs();
	} else {
		assert(ino == FUSE_ROOT_ID);
	}
	fuse_reply_none(req);
}

static void tfs_getattr(fuse_req_t req, fuse_ino_t ino,
			struct fuse_file_info *fi)
{
	struct stat stbuf;

	(void)fi;

	memset(&stbuf, 0, sizeof(stbuf));
	if (tfs_stat(ino, &stbuf) != 0)
		fuse_reply_err(req, ENOENT);
	else
		fuse_reply_attr(req, &stbuf, NO_TIMEOUT);
}

struct dirbuf {
	char *p;
	size_t size;
};

static void dirbuf_add(fuse_req_t req, struct dirbuf *b, const char *name,
		       fuse_ino_t ino)
{
	struct stat stbuf;
	size_t oldsize = b->size;

	b->size += fuse_add_direntry(req, NULL, 0, name, NULL, 0);
	b->p = (char *)realloc(b->p, b->size);
	memset(&stbuf, 0, sizeof(stbuf));
	stbuf.st_ino = ino;
	fuse_add_direntry(req, b->p + oldsize, b->size - oldsize, name, &stbuf,
			  b->size);
}

#define min(x, y) ((x) < (y) ? (x) : (y))

static int reply_buf_limited(fuse_req_t req, const char *buf, size_t bufsize,
			     off_t off, size_t maxsize)
{
	if (off < bufsize)
		return fuse_reply_buf(req, buf + off,
				      min(bufsize - off, maxsize));
	else
		return fuse_reply_buf(req, NULL, 0);
}

static void tfs_readdir(fuse_req_t req, fuse_ino_t ino, size_t size, off_t off,
			struct fuse_file_info *fi)
{
	(void)fi;

	if (ino != FUSE_ROOT_ID)
		fuse_reply_err(req, ENOTDIR);
	else {
		struct dirbuf b;

		memset(&b, 0, sizeof(b));
		dirbuf_add(req, &b, FILE_NAME, FILE_INO);
		reply_buf_limited(req, b.p, b.size, off, size);
		free(b.p);
	}
}

static void tfs_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
{
	/*
	 * Make cache persistent even if file is closed,
	 * this makes it easier to see the effects.
	 */
	fi->keep_cache = 1;

	if (ino == FUSE_ROOT_ID)
		fuse_reply_err(req, EISDIR);
	else if ((fi->flags & O_ACCMODE) != O_RDONLY)
		fuse_reply_err(req, EACCES);
	else if (ino == FILE_INO)
		fuse_reply_open(req, fi);
	else {
		/* This should not happen */
		fprintf(stderr, "Got open for non-existing inode!\n");
		fuse_reply_err(req, ENOENT);
	}
}

static void tfs_read(fuse_req_t req, fuse_ino_t ino, size_t size, off_t off,
		     struct fuse_file_info *fi)
{
	(void)fi;

	assert(ino == FILE_INO);
	reply_buf_limited(req, file_contents, file_size, off, size);
}

static const struct fuse_lowlevel_ops tfs_oper = {
	.init = tfs_init,
	.destroy = tfs_destroy,
	.lookup = tfs_lookup,
	.getattr = tfs_getattr,
	.readdir = tfs_readdir,
	.open = tfs_open,
	.read = tfs_read,
	.forget = tfs_forget,
};

static void *update_fs_loop(void *data)
{
	struct fuse_session *se = (struct fuse_session *)data;

	while (!is_stop) {
		if (!options.no_notify && lookup_cnt) {
			fuse_ino_t nodeids = FILE_INO;
			/* Only send notification if the kernel is aware of the inode */

			/* Some errors (ENOENT, EBADF, ENODEV) have to be accepted as they
			 * might come up during umount, when kernel side already releases
			 * all inodes, but does not send FUSE_DESTROY yet.
			 */
			int ret = fuse_lowlevel_notify_prune(se, &nodeids, 1);

			if ((ret != 0 && !is_stop) && ret != -ENOENT &&
			     ret != -EBADF && ret != -ENODEV) {
				fprintf(stderr,
					"ERROR: fuse_lowlevel_notify_store() failed with %s (%d)\n",
					strerror(-ret), -ret);
				abort();
			}
		}
		sleep(options.update_interval);
	}
	return NULL;
}

static void show_help(const char *progname)
{
	printf("usage: %s [options] <mountpoint>\n\n", progname);
	printf("File-system specific options:\n"
	       "    --update-interval=<secs>  Update-rate of file system contents\n"
	       "    --no-notify            Disable kernel notifications\n"
	       "\n");
}

int main(int argc, char *argv[])
{
	struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
	struct fuse_session *se;
	struct fuse_cmdline_opts opts;
	struct fuse_loop_config *config;
	pthread_t updater;
	int ret = -1;

	if (fuse_opt_parse(&args, &options, option_spec, NULL) == -1)
		return 1;

	if (fuse_parse_cmdline(&args, &opts) != 0) {
		ret = 1;
		goto err_out1;
	}

	if (opts.show_help) {
		show_help(argv[0]);
		fuse_cmdline_help();
		fuse_lowlevel_help();
		ret = 0;
		goto err_out1;
	} else if (opts.show_version) {
		printf("FUSE library version %s\n", fuse_pkgversion());
		fuse_lowlevel_version();
		ret = 0;
		goto err_out1;
	}

	/* Initial contents */
	update_fs();

	se = fuse_session_new(&args, &tfs_oper, sizeof(tfs_oper), NULL);
	if (se == NULL)
		goto err_out1;

	if (fuse_set_signal_handlers(se) != 0)
		goto err_out2;

	if (fuse_session_mount(se, opts.mountpoint) != 0)
		goto err_out3;

	fuse_daemonize(opts.foreground);

	/* Start thread to update file contents */
	ret = pthread_create(&updater, NULL, update_fs_loop, (void *)se);
	if (ret != 0) {
		fprintf(stderr, "pthread_create failed with %s\n",
			strerror(ret));
		goto err_out3;
	}

	/* Block until ctrl+c or fusermount -u */
	if (opts.singlethread)
		ret = fuse_session_loop(se);
	else {
		config = fuse_loop_cfg_create();
		fuse_loop_cfg_set_clone_fd(config, opts.clone_fd);
		fuse_loop_cfg_set_max_threads(config, opts.max_threads);
		ret = fuse_session_loop_mt(se, config);
		fuse_loop_cfg_destroy(config);
		config = NULL;
	}

	fuse_session_unmount(se);
err_out3:
	fuse_remove_signal_handlers(se);
err_out2:
	fuse_session_destroy(se);
err_out1:
	fuse_opt_free_args(&args);
	free(opts.mountpoint);

	return ret ? 1 : 0;
}

/**
 * Local Variables:
 * mode: c
 * indent-tabs-mode: nil
 * c-basic-offset: 4
 * End:
 */
