Mocking a file system in Go

Intro

In this post we’ll try to mock some features of a file system in Go. We’ll focus mainly on file tree structure instead on IO operations on files itself. If you are interested in full abstract file system for Go check out afero package.

If your application uses a file system (FS) a bit beyond just reading a single file then you should probably want to mock a FS in order to test it. Without mocking it only way to write unit tests is to actually create files, performs tests and clean up afterwards. It’s slow (reaching to hard drive) and ineffective.

Let’s say we want to implement reading a file tree structure. For me natural representation of such a tree is the following:

type Dir struct {
    Path    string,
    Files   []os.FileInfo,
    SubDirs map[string]Dir
}

Structure Dir have a Path path to root catalog, Files which is a list of meta information of files and a dictionary which maps a sub catalog name into actual sub catalog - another Dir. Which gives us recursive tree-like structure representing a file tree. I called it “natural” representation because it’s usually the case shown in file explorers. Current catalog with files and other sub catalogs. Another representation could be just an array of os.FileInfo extended by full path but it less natural for most people. It’s more natural for computers though.

Now I’d like to have some functionality which scans actual OS file system to provide Dir representation. On other hand I want to prepare mock FS in order to test that functionality without manual manipulation of actual FS.

func Scan(...) (Dir, error) {
    ...
}

Interface

I have not stated Scan function arguments yet. I’d like to use this function both on actual FS and also on mocked FS, so what should be an argument for that function? Yep, an interface. We should define minimal interface which would let us write recursive algorithm for scanning file tree.

For given directory this interface should have functionalities to give us a list of files and sub directories, path of current location and something to reproduce or move itself for sub catalogs.

type DirReader interface {
    DirPath() string
    Readdir() ([]os.FileInfo, error)
    New(string) DirReader
}

The DirReader interface is my proposition to meet those requirements. It’s composed of three methods - DirPath returns current location, Readdir reads content of current catalog and New produce another DirReader (assumable) for sub catalogs. Furthermore let’s also note that type os.FileInfo from the standard library is an interface. Details can be found here.

Now we can finally pronounce signature of Scan function

func Scan(reader DirReader) (Dir, error) {
    ...
}

At this point we could even provide a full implementation of Scan function using only DirReader interface definition. I’ll leave this implementation in the Appendix to not distract us from main goal which is mocking a FS.

In the next two sections we’ll focus on implementing DirReader interface for actual OS file system using cross-platform os package and for mocked in-memory Go objects. This pattern, having an interface depending on interface rather then concrete type, is very broadly used despite particular programming language. I believe it originated in Simula with virtual functions and was later adopted by Bjarne Stroustrup in C++ in early 1980s. In C++ we could define abstract base class with methods similar to DirReader.

Actual implementation

The actual implementation of DirReader which would use an actual OS file system is rather straightforward. Let’s start from defining a new type which will represent single (non-recursive) directory - FlatDir.

type FlatDir struct {
    Path string
}

To satisfy DirReader interface we’ve got to implement required methods. Getting directory’s path and create a new FlatDir are trivial methods in this case:

func (fd FlatDir) DirPath() string {
    return fd.Path
}

func (fd FlatDir) New(path string) DirReader {
    return FlatDir{Path: path}
}

All of the work is done in Readdir method. We’ll use functionalities from Go os package to implement reading content of OS file system directory.

func (fd FlatDir) Readdir() ([]os.FileInfo, error) {
    currentDir, err := os.Open(fd.Path)
    if err != nil {
        return nil, err
    }

    fileInfos, err := currentDir.Readdir(-1)
    if err != nil {
        return nil, err
    }

    return fileInfos, nil
}

That method is really just a wrapper on Readdir method which is cross-platform and do the heavy lifting for us.

After that type FlatDir satisfies DirReader interface. Now, in order to scan file tree for given root path, we could run tree, err := Scan(FlatDir{path}).

Mock

In order to prepare a mock for a file system we have to define an object which behaves like a file system and at the same time satisfies DirReader interface. As I stated at the beginning Dir is natural representation of a file system for me. Because of that my mock object will be very similar to Dir:

type MockDir struct {
    Path    string
    Files   []os.FileInfo
    SubDirs map[string]*MockDir
}

Defined MockDir represents our mock FS. Using map[string]*MockDir instead of map[string]MockDir is just for my convenience while building mock trees. In this version I can modify sub trees of the tree which was already initialized.

To satisfy DirReader interface we, once again, have to provide implementation of methods DirPath, Readdir and New. The first one in this case also just returns MockDir.Path. Method Readdir returns all files from MockDir.Files and based on SubDirs keys - sub catalog names also list of catalogs as []os.FileInfo. Method New is a bit tricky in context of the mock object. This method suppose to return a new DirReader for a given path. In this case we can do that only for sub catalogs of current tree, because there isn’t any other valid path outside sub trees. Implementation of those three methods can be found in the Appendix.

Wait but what about MockDir.Files? As we saw earlier os.FileInfo is an interface from Go standard library. We also have to mock this interface in order to produce MockDir objects. Let’s define MockFileInfo type which would satisfy os.FileInfo interface:

type MockFileInfo struct {
    FileName    string
    IsDirectory bool
}

func (mfi MockFileInfo) Name() string       { return mfi.FileName }
func (mfi MockFileInfo) Size() int64        { return int64(8) }
func (mfi MockFileInfo) Mode() os.FileMode  { return os.ModePerm }
func (mfi MockFileInfo) ModTime() time.Time { return time.Now() }
func (mfi MockFileInfo) IsDir() bool        { return mfi.IsDirectory }
func (mfi MockFileInfo) Sys() interface{}   { return nil }

The actual object contains only information about file name and flag stating whenever this file is a catalog. In this particular case I don’t need other file information like size or last modification date.

Up to this point we’ve defined function Scan which scans a file tree based on DirReader interface. We’ve got implementation of DirReader for actual OS file system and also implementation of mock DirReader. Hence we can use both FlatDir and MockDir in Scan function.

One last piece left is convenient building MockDir object for unit testing. I’ve found very useful to have a variadic function to produce my mock file trees. In this situation we have a plenty of options but my first attempt was implement a helper function with the following signature

func NewMockDir(rootPath string, files ...string) MockDir {
    ...
}

This function have an assumption about files. If single file name contains _ then it would be put inside sub catalog. For example

mockTree := NewMockDir("~/Downloads/", "f1.go", "f2.cpp", 
                       "sub1_g.txt", "sub2_h.html")

would create a file tree which looks like

In case when I need deeper trees I would use NewMockDir functions to create sub catalogs (and sub catalogs of sub catalogs…).

Summary

Created mockTree is of type MockDir which satisfies DirReader interface. Hence we could use Scan(mockTree), to produce actual file tree representation - Dir. That means now we are able to test every functionality which depends on Dir object. And that’s great! We can easily and safely test manipulations of a file system. In praticular now we can setup scenarios which could be very difficult to create on actual OS file system like creating 1 << 30 files or tree with very large depth.

The idea of mocking a file system using an “interface” can be used in most of programming languages. What Go saves us is having already defined interface os.FileInfo representing meta file information. In other languages we usually have to wrap it to abstract class or an interface and mock these afterwards.

References

1. Afero
2. Go ‘os’ package
3. dorg

Appendix

Source code of Scan function:

func Scan(reader DirReader) (Dir, error) {
    fileInfos, err := reader.Readdir()
    if err != nil {
        return Dir{}, err
    }
    dir := New(reader.DirPath(), 100) // Produces a new empty Dir

    for _, fileInfo := range fileInfos {
        name := fileInfo.Name()
        if !fileInfo.IsDir() {
            dir.Files = append(dir.Files, fileInfo)
            continue
        }

        newPath := filepath.Join(reader.DirPath(), name)
        newReader := reader.New(newPath)
        subDir, err := Scan(newReader)
        if err != nil {
            return Dir{}, err
        }
        dir.SubDirs[name] = subDir
    }
    return dir, nil
}

MockDir methods

func (md MockDir) DirPath() string {
    return md.Path
}

func (md MockDir) Readdir() ([]os.FileInfo, error) {
    files := make([]os.FileInfo, 0, 10)

    for _, file := range md.Files {
        files = append(files, file)
    }

    for dirName := range md.SubDirs {
        files = append(files, NewMockFileInfo(dirName, true))
    }
    return files, nil
}

func (md MockDir) New(path string) DirReader {
    for _, dir := range md.SubDirs {
        if dir.Path == path {
            return dir
        }
    }
    emptyDirs := make(map[string]*MockDir)
    return MockDir{path, make([]os.FileInfo, 0), emptyDirs}
}

MockFileInfo

func NewMockFileInfo(name string, isDir bool) MockFileInfo {
    return MockFileInfo{
        FileName:    name,
        IsDirectory: isDir,
    }
}

func NewMockDir(rootPath string, files ...string) MockDir {
    topFiles := make([]os.FileInfo, 0, 10)
    subDirs := make(map[string]*MockDir)

    for _, file := range files {
        if strings.Contains(file, "_") {
            parts := strings.Split(file, "_")
            dirName := parts[0]
            subFile := NewMockFileInfo(parts[1], false)

            if _, exist := subDirs[dirName]; exist {
                (*subDirs[dirName]).Files = append((*subDirs[dirName]).Files, subFile)
                continue
            }

            subDirs[dirName] = &MockDir{filepath.Join(rootPath, dirName), []os.FileInfo{subFile}, nil}
            continue
        }
        topFiles = append(topFiles, NewMockFileInfo(file, false))
    }

    return MockDir{rootPath, topFiles, subDirs}
}