// NewChecker ensures that content with the specified length is read from rc.
// Calling Close on this will close upstream.
func NewChecker(rc io.ReadCloser, h hash.Hash, wantSum []byte, length int64) *Checker {
	return &Checker{c: rc, r: ioutil.HardLimitReader(rc, length), h: h, want: wantSum}
}

// Read satisfies io.Reader
func (c Checker) Read(p []byte) (n int, err error) {
	n, err = c.r.Read(p)
	if n > 0 {
		c.h.Write(p[:n])
	}