An interface exposing all exported methods of *os.File.

In an internal API that makes heavy use of file operations, it can be helpful to mock the behaviour of a file for testing, or inject functionality for debugging.

In particular, testing code to be resilient against various infrequent OS errors can be difficult because these errors can't be easily simulated.

Godoc: http:https://pkg.go.dev/github.com/iamcalledrob/fi

Use fi.File in place of *os.File in internal APIs.

func wrangle(f fi.File) error {
	// ...
}

fi.MockFile implements fi.File, and allows for a fully mocked file

f := &fi.MockFile{
	ReadFn: func(p []byte) (int, error) {
		// ...
	},
	WriteFn: func(b []byte) (int, error) {
		// ...
	}
}

wrangle(f)

fi.MockFileFromOS() delegates all functionality to an underlying *os.File. Behaviour can then be tweaked on a per-method basis.

For example, to test write/close behaviour when the disk is out of space

// raw is *os.File
raw, err := os.Open("/path/to/file")
if err != nil {
	// ...
}

f := fi.MockFileFromOS(raw)

f.CloseFn = func() error {
	return &os.PathError{
		Op:   "close",
		Path: f.Name(),
		Err:  syscall.ENOSPC,
	}
}

f.Write([]byte{1, 2, 3})
f.Close() // "out of space"