}
func (sr sparseFileReader) physicalRemaining() int64 {
	return sr.fr.physicalRemaining()
}

type zeroReader struct{}

func (zeroReader) Read(b []byte) (int, error) {
	clear(b)
	return len(b), nil
}

// mustReadFull is like io.ReadFull except it returns
// io.ErrUnexpectedEOF when io.EOF is hit before len(b) bytes are read.
func mustReadFull(r io.Reader, b []byte) (int, error) {