// otherwise `nil`.
type removeFromFIFOFunc func() *request

// walkFunc is called for each request in the list in the
// oldest -> newest order.
// ok: if walkFunc returns false then the iteration stops immediately.
// walkFunc may remove the given request from the fifo,
// but may not mutate the fifo in any othe way.
type walkFunc func(*request) (ok bool)

// Internal interface to abstract out the implementation details

//   - callback walkFunc is invoked on root directory after visiting children dirs and files
func walkDeep(root string, walkFunc filepath.WalkFunc) error {
	info, err := os.Lstat(root)
	if err != nil {
		return walkFunc(root, nil, err)
	}
	return walk(root, info, walkFunc)
}

func walk(path string, info os.FileInfo, walkFunc filepath.WalkFunc) error {
	if !info.IsDir() {

	ReadFile(filename string) ([]byte, error)
	TempDir(dir, prefix string) (string, error)
	TempFile(dir, prefix string) (File, error)
	ReadDir(dirname string) ([]os.DirEntry, error)
	Walk(root string, walkFn filepath.WalkFunc) error
}

// File is an interface that we can use to mock various filesystem operations typically
// accessed through the File object from the "os" package
type File interface {

// is after a failed ReadDir and reports the error from ReadDir.
// (If ReadDir succeeds, there is no second call.)
//
// The differences between WalkDirFunc compared to [path/filepath.WalkFunc] are:
//
//   - The second argument has type [DirEntry] instead of [FileInfo].
//   - The function is called before reading a directory, to allow [SkipDir]

func (fs *DefaultFs) ReadDir(dirname string) ([]os.DirEntry, error) {
	return os.ReadDir(fs.prefix(dirname))
}

// Walk via filepath.Walk
func (fs *DefaultFs) Walk(root string, walkFn filepath.WalkFunc) error {
	return filepath.Walk(fs.prefix(root), walkFn)
}

// defaultFile implements File using same-named functions from "os"
type defaultFile struct {
	file *os.File
}

// Name via os.File.Name