Add a notification mechanism for signaling when the mountpoint is ready

Add the notify_pipe option which allows the user to specify the path to
a named pipe. When the mountpoint is ready, a message will be written to
this pipe: 's' for success and 'f' for failure. To avoid blocking the
fuse process until the pipe is read, a new process is spawned which
performs a blocking operation on the pipe. In case of mount failure, no
additional process is created and the main process blocks until the pipe
is read.

An example of operation is provided below:
```
set -x
FIFO=$(mktemp -u)
mkfifo "$FIFO"
./squashfuse_ll -o notify_pipe="$FIFO" -f /path/to/squashfs/archive /tmp/squash&
STATUS=$(head -c1 "$FIFO")
if [ "$STATUS" = "s" ]; then
	echo "Mountpoint contains:"
	ls /tmp/squash
else
	echo "Mounting squashfuse on /tmp/squash failed"
fi
```

Fixes vasi#49

fs.h

@@ -39,6 +39,7 @@ struct sqfs {
 	int uid;
 	int gid;
 	struct squashfs_super_block sb;
+	const char *notify_pipe;
 	sqfs_table id_table;
 	sqfs_table frag_table;
 	sqfs_table export_table;

fuseprivate.c

@@ -29,6 +29,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 #include "nonstd.h"
 
@@ -78,6 +79,7 @@ void sqfs_usage(char *progname, bool fuse_usage, bool ll_usage) {
 	fprintf(stderr, "\n%s options:\n", progname);
 	fprintf(stderr, "    -o offset=N            offset N bytes into ARCHIVE to mount\n");
 	fprintf(stderr, "    -o subdir=PATH         mount subdirectory PATH of ARCHIVE\n");
+	fprintf(stderr, "    -o notify_pipe=PATH    named pipe that will receive 's' or 'f' when the mount is ready\n");
 	if (ll_usage) {
 		fprintf(stderr, "    -o timeout=N           idle N seconds for automatic unmount\n");
 		fprintf(stderr, "    -o uid=N               set file owner to uid N\n");
@@ -148,3 +150,34 @@ int sqfs_statfs(sqfs *sq, struct statvfs *st) {
 
 	return 0;
 }
+
+void notify_mount_ready(const char *notify_pipe, int status) {
+	char message[] = "sf";
+	struct stat stat_buf = {};
+
+	if (stat(notify_pipe, &stat_buf)) {
+		fprintf(stderr, "Cannot stat file \"%s\" (%d)\n", notify_pipe, errno);
+		goto err;
+	}
+
+	if (!(stat_buf.st_mode & S_IFIFO)) {
+		fprintf(stderr, "\"%s\" is not a pipe\n", notify_pipe);
+		goto err;
+	}
+
+	sqfs_fd_t pipe_fd = open(notify_pipe, O_WRONLY);
+	if(pipe_fd < 0) {
+		fprintf(stderr, "Open fifo failed \"%s\" (%d)\n", notify_pipe, errno);
+		goto err;
+	}
+
+	if (write(pipe_fd, &message[!!status], 1) < 0) {
+		fprintf(stderr, "Write to fifo failed \"%s\" (%d)\n", notify_pipe, errno);
+		goto err;
+	}
+
+	close(pipe_fd);
+
+err:
+	return;
+}

fuseprivate.h

@@ -47,11 +47,13 @@ typedef struct {
 	unsigned int idle_timeout_secs;
 	int uid;
 	int gid;
+	const char *notify_pipe;
 } sqfs_opts;
 int sqfs_opt_proc(void *data, const char *arg, int key,
 	struct fuse_args *outargs);
 
 /* Get filesystem super block info */
 int sqfs_statfs(sqfs *sq, struct statvfs *st);
+void notify_mount_ready(const char *notify_pipe, int status);
 
 #endif

hl.c

@@ -33,6 +33,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 
 typedef struct sqfs_hl sqfs_hl;
@@ -72,7 +73,25 @@ static void *sqfs_hl_op_init(struct fuse_conn_info *conn
 			     ,struct fuse_config *cfg
 #endif
 			     ) {
-	return fuse_get_context()->private_data;
+	sqfs_hl *hl = fuse_get_context()->private_data;
+
+	if (!hl->fs.notify_pipe) {
+		goto out;
+	}
+
+	pid_t pid = fork();
+	if (pid < 0) {
+		fprintf(stderr, "Fork Failed");
+	}
+	else if (pid == 0) { /* child process */
+		notify_mount_ready(hl->fs.notify_pipe, 0);
+		exit(0);
+	}
+	else { /* parent process */
+	}
+
+out:
+	return hl;
 }
 
 static int sqfs_hl_op_getattr(const char *path, struct stat *st
@@ -297,6 +316,7 @@ int main(int argc, char *argv[]) {
 	struct fuse_opt fuse_opts[] = {
 		{"offset=%zu", offsetof(sqfs_opts, offset), 0},
 		{"subdir=%s", offsetof(sqfs_opts, subdir), 0},
+		{"notify_pipe=%s", offsetof(sqfs_opts, notify_pipe), 0},
 		FUSE_OPT_END
 	};
 
@@ -326,6 +346,7 @@ int main(int argc, char *argv[]) {
 	opts.subdir = NULL;
 	opts.mountpoint = 0;
 	opts.offset = 0;
+	opts.notify_pipe = NULL;
 	if (fuse_opt_parse(&args, &opts, fuse_opts, sqfs_opt_proc) == -1)
 		sqfs_usage(argv[0], true, false);
 	if (!opts.image)
@@ -334,9 +355,16 @@ int main(int argc, char *argv[]) {
 	hl = sqfs_hl_open(opts.image, opts.offset, opts.subdir);
 	if (!hl)
 		return -1;
+
+	hl->fs.notify_pipe = opts.notify_pipe;
 
 	fuse_opt_add_arg(&args, "-s"); /* single threaded */
 	ret = fuse_main(args.argc, args.argv, &sqfs_hl_ops, hl);
+	if (ret) {
+		if (hl->fs.notify_pipe) {
+			notify_mount_ready(hl->fs.notify_pipe, -1);
+		}
+	}
 	fuse_opt_free_args(&args);
 	return ret;
 }

ll.c

@@ -432,6 +432,25 @@ void stfs_ll_op_statfs(fuse_req_t req, fuse_ino_t ino) {
 	}
 }
 
+void sqfs_ll_op_init(void *userdata, struct fuse_conn_info *conn) {
+	sqfs_ll *ll = userdata;
+
+	if (!ll->fs.notify_pipe) {
+		return;
+	}
+
+	pid_t pid = fork();
+	if (pid < 0) {
+		fprintf(stderr, "Fork Failed");
+	}
+	else if (pid == 0) { /* child process */
+		notify_mount_ready(ll->fs.notify_pipe, 0);
+		exit(0);
+	}
+	else { /* parent process */
+	}
+}
+
 /* Helpers to abstract out FUSE 2.5 vs 3.0+ differences */
 
 #if FUSE_USE_VERSION >= 30

ll.h

@@ -111,6 +111,8 @@ void sqfs_ll_op_getxattr(fuse_req_t req, fuse_ino_t ino,
 void sqfs_ll_op_forget(fuse_req_t req, fuse_ino_t ino,
 		unsigned long nlookup);
 
+void sqfs_ll_op_init(void *userdata, struct fuse_conn_info *conn);
+
 void stfs_ll_op_statfs(fuse_req_t req, fuse_ino_t ino);
 
 

ll_main.c

@@ -142,6 +142,7 @@ int main(int argc, char *argv[]) {
 		{"uid=%d", offsetof(sqfs_opts, uid), 0},
 		{"gid=%d", offsetof(sqfs_opts, gid), 0},
 		{"subdir=%s", offsetof(sqfs_opts, subdir), 0},
+		{"notify_pipe=%s", offsetof(sqfs_opts, notify_pipe), 0},
 		FUSE_OPT_END
 	};
 
@@ -161,6 +162,7 @@ int main(int argc, char *argv[]) {
 	sqfs_ll_ops.getxattr	= sqfs_ll_op_getxattr;
 	sqfs_ll_ops.forget		= sqfs_ll_op_forget;
 	sqfs_ll_ops.statfs    = stfs_ll_op_statfs;
+	sqfs_ll_ops.init    = sqfs_ll_op_init;
 
 	/* PARSE ARGS */
 	args.argc = argc;
@@ -175,6 +177,7 @@ int main(int argc, char *argv[]) {
 	opts.uid = 0;
 	opts.gid = 0;
 	opts.subdir = NULL;
+	opts.notify_pipe = NULL;
 	if (fuse_opt_parse(&args, &opts, fuse_opts, sqfs_opt_proc) == -1)
 		sqfs_usage(argv[0], true, true);
 
@@ -220,6 +223,8 @@ int main(int argc, char *argv[]) {
 	if (!err) {
 		ll->fs.uid = opts.uid;
 		ll->fs.gid = opts.gid;
+		ll->fs.notify_pipe = opts.notify_pipe;
+
 		sqfs_ll_chan ch;
 		err = -1;
 		if (sqfs_ll_mount(
@@ -259,6 +264,10 @@ int main(int argc, char *argv[]) {
 			}
 			sqfs_ll_destroy(ll);
 			sqfs_ll_unmount(&ch, fuse_cmdline_opts.mountpoint);
+		} else {
+			if (ll->fs.notify_pipe) {
+				notify_mount_ready(ll->fs.notify_pipe, -1);
+			}
 		}
 	}
 	fuse_opt_free_args(&args);