zipfs is an example of a bazil.org/fuse
filesystem that serves a Zip archive:
$ unzip -v archive.zip
Archive: archive.zip
Length Method Size Cmpr Date Time CRC-32 Name
-------- ------ ------- ---- ---------- ----- -------- ----
0 Stored 0 0% 2014-12-11 04:03 00000000 buried/
0 Stored 0 0% 2014-12-11 04:03 00000000 buried/deep/
5 Stored 5 0% 2014-12-11 04:03 2efcceec buried/deep/loot
13 Stored 13 0% 2014-12-11 04:03 f4247453 greeting
-------- ------- --- -------
18 18 0% 4 files
$ zipfs archive.zip mnt &
$ tree mnt
mnt
├── buried
│  └── deep
│  └── loot
└── greeting
2 directories, 2 files
$ cat mnt/greeting
hello, worldFUSE (Filesystem In Userpace) is a Linux kernel filesystem that sends the incoming requests over a file descriptor to userspace. Historically, these have been served with a C library of the same name, but ultimately FUSE is just a protocol. Since then, the protocol has been implemented for other platforms such as OS X, FreeBSD and OpenBSD.
bazil.org/fuse is a reimplementation of that protocol in pure Go.
Unix filesystems consist of inodes ("index nodes"). These nodes are files, directories, etc. Directories contain directory entries (dirent) that point to child inodes. A directory entry is identified by its name, and carries very little metadata. The inode manages both the metadata (including things like access control) and the content of the file.
Open files are identified in userspace with file descriptors, which are just safe references to kernel objects known as handles.
Our FUSE library is split into two parts. The low-level protocol is in
bazil.org/fuse while the
higher-level, optional, state machine keeping track of object
lifetimes is
bazil.org/fuse/fs.
Each file system has a root entry. The interface
fs.FS has a method
Root that returns an
fs.Node.
To access a file (see its metadata, open it, etc), the kernel looks it
up by name by sending a
fuse.LookupRequest
to the FUSE server, stating the parent directory and basename. This
request is served by a
Lookup
method on the parent
fs.Node. The method
returns an fs.Node, and
the result is cached in the kernel and reference counted. Dropping a
cache entry sends a
ForgetRequest, and
when the reference count reaches zero,
Forget gets
called.
Files are renamed with
Rename, deleted
with Remove, and
so on.
Kernel file handles are created for example by opening a file.
Opening an existing file sends an
OpenRequest, you
guessed it, served by
Open. All methods
creating new handles return a
Handle. Handles are
closed by a combination of
Flush and
Release.
The default Open
action, if the method is not implemented, is to use the
fs.Node also as a
Handle; this tends to
work well for stateless read-only files.
Reads from a Handle are
served by Read,
writes with
Write, and apart
from all the extra data available these look similar to
io.ReaderAt and
io.WriterAt. Note that file
size changes via
Setattr, not
based on Write,
and Attr needs to return
the correct Size.
Listing a directory happens by reading an open file handle that is a
directory. Instead of file contents, the read returns marshaled
directory entries. The
ReadDir method
implements a slightly higher-level API, where you return a slice of
directory entries.
And so on. Learning to write a file system requires a decent understanding of the kernel data structures and their state changes on an abstract level, but the actual Go parts of it are quite simple. So let's dive into the code.
As our example project, we'll write a filesystem that shows a read-only view of the contents of a Zip archive.
The full source code is available at https://github.com/bazil/zipfs
Let's start with a skeleton with argument parsing:
package main
import (
"archive/zip"
"flag"
"fmt"
"io"
"log"
"os"
"path/filepath"
"strings"
"bazil.org/fuse"
"bazil.org/fuse/fs"
)
// We assume the zip file contains entries for directories too.
var progName = filepath.Base(os.Args[0])
func usage() {
fmt.Fprintf(os.Stderr, "Usage of %s:\n", progName)
fmt.Fprintf(os.Stderr, " %s ZIP MOUNTPOINT\n", progName)
flag.PrintDefaults()
}
func main() {
log.SetFlags(0)
log.SetPrefix(progName + ": ")
flag.Usage = usage
flag.Parse()
if flag.NArg() != 2 {
usage()
os.Exit(2)
}
path := flag.Arg(0)
mountpoint := flag.Arg(1)
if err := mount(path, mountpoint); err != nil {
log.Fatal(err)
}
}Mounting is a bit cumbersome due to OSXFUSE behaving very differently from Linux; there are several stages where errors may show up.
func mount(path, mountpoint string) error {
archive, err := zip.OpenReader(path)
if err != nil {
return err
}
defer archive.Close()
c, err := fuse.Mount(mountpoint)
if err != nil {
return err
}
defer c.Close()
filesys := &FS{
archive: &archive.Reader,
}
if err := fs.Serve(c, filesys); err != nil {
return err
}
// check if the mount process has an error to report
<-c.Ready
if err := c.MountError; err != nil {
return err
}
return nil
}On to the actual file system. We just hold a pointer to the zip archive:
type FS struct {
archive *zip.Reader
}And we need to provide the Root method:
var _ fs.FS = (*FS)(nil)
func (f *FS) Root() (fs.Node, fuse.Error) {
n := &Dir{
archive: f.archive,
}
return n, nil
}Zip files contain a list of files, but typical zip archivers include entries for the directories, with a name ending in a slash. We rely on this behavior later.
Let's define our Dir type, and implement the mandatory Attr
method. We use the *zip.File to serve directory metadata.
type Dir struct {
archive *zip.Reader
// nil for the root directory, which has no entry in the zip
file *zip.File
}
var _ fs.Node = (*Dir)(nil)
func zipAttr(f *zip.File) fuse.Attr {
return fuse.Attr{
Size: f.UncompressedSize64,
Mode: f.Mode(),
Mtime: f.ModTime(),
Ctime: f.ModTime(),
Crtime: f.ModTime(),
}
}
func (d *Dir) Attr() fuse.Attr {
if d.file == nil {
// root directory
return fuse.Attr{Mode: os.ModeDir | 0755}
}
return zipAttr(d.file)
}For our filesystem to contain anything useful, we need to be able to find entries by name. We just iterate over the zip entries, matching paths:
var _ = fs.NodeRequestLookuper(&Dir{})
func (d *Dir) Lookup(req *fuse.LookupRequest, resp *fuse.LookupResponse, intr fs.Intr) (fs.Node, fuse.Error) {
path := req.Name
if d.file != nil {
path = d.file.Name + path
}
for _, f := range d.archive.File {
switch {
case f.Name == path:
child := &File{
file: f,
}
return child, nil
case f.Name[:len(f.Name)-1] == path && f.Name[len(f.Name)-1] == '/':
child := &Dir{
archive: d.archive,
file: f,
}
return child, nil
}
}
return nil, fuse.ENOENT
}Our Lookup above returned File types when the matched entry did
not end in a slash. Let's define type File, using the same zipAttr
helper as for directories:
type File struct {
file *zip.File
}
var _ fs.Node = (*File)(nil)
func (f *File) Attr() fuse.Attr {
return zipAttr(f.file)
}Files are not very useful unless you can open them:
var _ = fs.NodeOpener(&File{})
func (f *File) Open(req *fuse.OpenRequest, resp *fuse.OpenResponse, intr fs.Intr) (fs.Handle, fuse.Error) {
r, err := f.file.Open()
if err != nil {
return nil, err
}
// individual entries inside a zip file are not seekable
resp.Flags |= fuse.OpenNonSeekable
return &FileHandle{r: r}, nil
}type FileHandle struct {
r io.ReadCloser
}
var _ fs.Handle = (*FileHandle)(nil)We hold an "open file" inside our handle. In this case, it's just a
helper type in archive/zip, but in another filesystem this might be
a *os.File, a network connection, or such. We should be careful to
close them:
var _ fs.HandleReleaser = (*FileHandle)(nil)
func (fh *FileHandle) Release(req *fuse.ReleaseRequest, intr fs.Intr) fuse.Error {
return fh.r.Close()
}And then let's handle actual Read operations:
var _ = fs.HandleReader(&FileHandle{})
func (fh *FileHandle) Read(req *fuse.ReadRequest, resp *fuse.ReadResponse, intr fs.Intr) fuse.Error {
// We don't actually enforce Offset to match where previous read
// ended. Maybe we should, but that would mean'd we need to track
// it. The kernel *should* do it for us, based on the
// fuse.OpenNonSeekable flag.
buf := make([]byte, req.Size)
n, err := fh.r.Read(buf)
resp.Data = buf[:n]
return err
}At this point, our files are accessible by cat and such, but you
need to know their names. Let's add support for ReadDir:
var _ = fs.HandleReadDirer(&Dir{})
func (d *Dir) ReadDir(intr fs.Intr) ([]fuse.Dirent, fuse.Error) {
prefix := ""
if d.file != nil {
prefix = d.file.Name
}
var res []fuse.Dirent
for _, f := range d.archive.File {
if !strings.HasPrefix(f.Name, prefix) {
continue
}
name := f.Name[len(prefix):]
if name == "" {
// the dir itself, not a child
continue
}
if strings.ContainsRune(name[:len(name)-1], '/') {
// contains slash in the middle -> is in a deeper subdir
continue
}
var de fuse.Dirent
if name[len(name)-1] == '/' {
// directory
name = name[:len(name)-1]
de.Type = fuse.DT_Dir
}
de.Name = name
res = append(res, de)
}
return res, nil
}Prepare a zip file:
$ mkdir -p data/buried/deep
$ echo hello, world >data/greeting
$ echo gold >data/buried/deep/loot
$ ( cd data && zip -r -q ../archive.zip . )Mount it:
$ mkdir mnt
$ zipfs archive.zip mnt &Lookup directory entries:
$ ls -ld mnt/greeting
-rw-r--r-- 1 root root 13 Dec 11 2014 mnt/greeting
$ ls -ld mnt/buried
drwxr-xr-x 1 root root 0 Dec 11 2014 mnt/buriedRead file contents:
$ cat mnt/greeting
hello, world
$ cat mnt/buried/deep/loot
goldReaddir (the "total 0" is not correct, but that doesn't matter):
$ ls -l mnt
total 0
drwxr-xr-x 1 root root 0 Dec 11 2014 buried
-rw-r--r-- 1 root root 13 Dec 11 2014 greeting
$ ls -l mnt/buried
total 0
drwxr-xr-x 1 root root 0 Dec 11 2014 deepUnmount (for OS X, use umount mnt):
$ fusermount -u mntThat's it! For a longer and more featureful examples to read, see https://github.com/bazil/bolt-mount (screencast of a code walkthrough) and all of the projects importing fuse.
-
Bazil is a distributed file system designed for single-person disconnected operation. It lets you share your files across all your computers, with or without cloud services.
-
FUSE is a Linux kernel filesystem that makes calls to userspace to serve filesystem content.
-
Confusingly also known as FUSE is the C library for implementing userspace FUSE filesystems.
-
bazil.org/fuse is a Go library for writing filesystems. See also GoDoc for
fuseandfuse/fs -
OSXFUSE is a FUSE kernel implementation for OS X.
-
bolt-mountis a more comprehensive example filesystem, including write operations. See also a screencast of a code walkthrough. -
Writing a file system in Go is an earlier talk that explains FUSE a bit more.
-
FUSE questions are welcome on the bazil-dev Google Group or on IRC channel #go-nuts on irc.freenode.net.