A FileSystem Abstraction System for Go

Overview

Afero is a filesystem framework providing a simple, uniform and universal API interacting with any filesystem, as an abstraction layer providing interfaces, types and methods. Afero has an exceptionally clean interface and simple design without needless constructors or initialization methods.

Afero is also a library providing a base set of interoperable backend filesystems that make it easy to work with afero while retaining all the power and benefit of the os and ioutil packages.

Afero provides significant improvements over using the os package alone, most notably the ability to create mock and testing filesystems without relying on the disk.

It is suitable for use in any situation where you would consider using the OS package as it provides an additional abstraction that makes it easy to use a memory backed file system during testing. It also adds support for the http filesystem for full interoperability.

Afero Features

• A single consistent API for accessing a variety of filesystems
• Interoperation between a variety of file system types
• A set of interfaces to encourage and enforce interoperability between backends
• An atomic cross platform memory backed file system
• Support for compositional (union) file systems by combining multiple file systems acting as one
• Specialized backends which modify existing filesystems (Read Only, Regexp filtered)
• A set of utility functions ported from io, ioutil & hugo to be afero aware
• Wrapper for go 1.16 filesystem abstraction io/fs.FS

Using Afero

Afero is easy to use and easier to adopt.

A few different ways you could use Afero:

• Use the interfaces alone to define your own file system.
• Wrapper for the OS packages.
• Define different filesystems for different parts of your application.
• Use Afero for mock filesystems while testing

Step 1: Install Afero

First use go get to install the latest version of the library.

$ go get github.com/spf13/afero

Next include Afero in your application.

import "github.com/spf13/afero"

Step 2: Declare a backend

First define a package variable and set it to a pointer to a filesystem.

var AppFs = afero.NewMemMapFs()

or

var AppFs = afero.NewOsFs()

It is important to note that if you repeat the composite literal you will be using a completely new and isolated filesystem. In the case of OsFs it will still use the same underlying filesystem but will reduce the ability to drop in other filesystems as desired.

Step 3: Use it like you would the OS package

Throughout your application use any function and method like you normally would.

So if my application before had:

os.Open("/tmp/foo")

We would replace it with:

AppFs.Open("/tmp/foo")

AppFs being the variable we defined above.

List of all available functions

File System Methods Available:

Chmod(name string, mode os.FileMode) : error
Chown(name string, uid, gid int) : error
Chtimes(name string, atime time.Time, mtime time.Time) : error
Create(name string) : File, error
Mkdir(name string, perm os.FileMode) : error
MkdirAll(path string, perm os.FileMode) : error
Name() : string
Open(name string) : File, error
OpenFile(name string, flag int, perm os.FileMode) : File, error
Remove(name string) : error
RemoveAll(path string) : error
Rename(oldname, newname string) : error
Stat(name string) : os.FileInfo, error

File Interfaces and Methods Available:

io.Closer
io.Reader
io.ReaderAt
io.Seeker
io.Writer
io.WriterAt

Name() : string
Readdir(count int) : []os.FileInfo, error
Readdirnames(n int) : []string, error
Stat() : os.FileInfo, error
Sync() : error
Truncate(size int64) : error
WriteString(s string) : ret int, err error

In some applications it may make sense to define a new package that simply exports the file system variable for easy access from anywhere.

Using Afero's utility functions

Afero provides a set of functions to make it easier to use the underlying file systems. These functions have been primarily ported from io & ioutil with some developed for Hugo.

The afero utilities support all afero compatible backends.

The list of utilities includes:

DirExists(path string) (bool, error)
Exists(path string) (bool, error)
FileContainsBytes(filename string, subslice []byte) (bool, error)
GetTempDir(subPath string) string
IsDir(path string) (bool, error)
IsEmpty(path string) (bool, error)
ReadDir(dirname string) ([]os.FileInfo, error)
ReadFile(filename string) ([]byte, error)
SafeWriteReader(path string, r io.Reader) (err error)
TempDir(dir, prefix string) (name string, err error)
TempFile(dir, prefix string) (f File, err error)
Walk(root string, walkFn filepath.WalkFunc) error
WriteFile(filename string, data []byte, perm os.FileMode) error
WriteReader(path string, r io.Reader) (err error)

For a complete list see Afero's GoDoc

They are available under two different approaches to use. You can either call them directly where the first parameter of each function will be the file system, or you can declare a new Afero, a custom type used to bind these functions as methods to a given filesystem.

Calling utilities directly

fs := new(afero.MemMapFs)
f, err := afero.TempFile(fs,"", "ioutil-test")

Calling via Afero

fs := afero.NewMemMapFs()
afs := &afero.Afero{Fs: fs}
f, err := afs.TempFile("", "ioutil-test")

Using Afero for Testing

There is a large benefit to using a mock filesystem for testing. It has a completely blank state every time it is initialized and can be easily reproducible regardless of OS. You could create files to your heart’s content and the file access would be fast while also saving you from all the annoying issues with deleting temporary files, Windows file locking, etc. The MemMapFs backend is perfect for testing.

• Much faster than performing I/O operations on disk
• Avoid security issues and permissions
• Far more control. 'rm -rf /' with confidence
• Test setup is far more easier to do
• No test cleanup needed

One way to accomplish this is to define a variable as mentioned above. In your application this will be set to afero.NewOsFs() during testing you can set it to afero.NewMemMapFs().

It wouldn't be uncommon to have each test initialize a blank slate memory backend. To do this I would define my appFS = afero.NewOsFs() somewhere appropriate in my application code. This approach ensures that Tests are order independent, with no test relying on the state left by an earlier test.

Then in my tests I would initialize a new MemMapFs for each test:

func TestExist(t *testing.T) {
	appFS := afero.NewMemMapFs()
	// create test files and directories
	appFS.MkdirAll("src/a", 0755)
	afero.WriteFile(appFS, "src/a/b", []byte("file b"), 0644)
	afero.WriteFile(appFS, "src/c", []byte("file c"), 0644)
	name := "src/c"
	_, err := appFS.Stat(name)
	if os.IsNotExist(err) {
		t.Errorf("file \"%s\" does not exist.\n", name)
	}
}

Available Backends

Operating System Native

OsFs

The first is simply a wrapper around the native OS calls. This makes it very easy to use as all of the calls are the same as the existing OS calls. It also makes it trivial to have your code use the OS during operation and a mock filesystem during testing or as needed.

appfs := afero.NewOsFs()
appfs.MkdirAll("src/a", 0755)

Memory Backed Storage

MemMapFs

Afero also provides a fully atomic memory backed filesystem perfect for use in mocking and to speed up unnecessary disk io when persistence isn’t necessary. It is fully concurrent and will work within go routines safely.

mm := afero.NewMemMapFs()
mm.MkdirAll("src/a", 0755)

InMemoryFile

As part of MemMapFs, Afero also provides an atomic, fully concurrent memory backed file implementation. This can be used in other memory backed file systems with ease. Plans are to add a radix tree memory stored file system using InMemoryFile.

Network Interfaces

SftpFs

Afero has experimental support for secure file transfer protocol (sftp). Which can be used to perform file operations over a encrypted channel.

GCSFs

Afero has experimental support for Google Cloud Storage (GCS). You can either set the GOOGLE_APPLICATION_CREDENTIALS_JSON env variable to your JSON credentials or use opts in NewGcsFS to configure access to your GCS bucket.

Some known limitations of the existing implementation:

• No Chmod support - The GCS ACL could probably be mapped to *nix style permissions but that would add another level of complexity and is ignored in this version.
• No Chtimes support - Could be simulated with attributes (gcs a/m-times are set implicitly) but that's is left for another version.
• Not thread safe - Also assumes all file operations are done through the same instance of the GcsFs. File operations between different GcsFs instances are not guaranteed to be consistent.

Filtering Backends

BasePathFs

The BasePathFs restricts all operations to a given path within an Fs. The given file name to the operations on this Fs will be prepended with the base path before calling the source Fs.

bp := afero.NewBasePathFs(afero.NewOsFs(), "/base/path")

ReadOnlyFs

A thin wrapper around the source Fs providing a read only view.

fs := afero.NewReadOnlyFs(afero.NewOsFs())
_, err := fs.Create("/file.txt")
// err = syscall.EPERM

RegexpFs

A filtered view on file names, any file NOT matching the passed regexp will be treated as non-existing. Files not matching the regexp provided will not be created. Directories are not filtered.

fs := afero.NewRegexpFs(afero.NewMemMapFs(), regexp.MustCompile(`\.txt$`))
_, err := fs.Create("/file.html")
// err = syscall.ENOENT

HttpFs

Afero provides an http compatible backend which can wrap any of the existing backends.

The Http package requires a slightly specific version of Open which returns an http.File type.

Afero provides an httpFs file system which satisfies this requirement. Any Afero FileSystem can be used as an httpFs.

httpFs := afero.NewHttpFs()
fileserver := http.FileServer(httpFs.Dir(