// if sha256Hex is not the empty string.
//
// If size resp. actualSize is unknown at the time of calling
// NewReader then it should be set to -1.
// When size is >=0 it *must* match the amount of data provided by r.
//
// NewReader may try merge the given size, MD5 and SHA256 values
// into src - if src is a Reader - to avoid computing the same
// checksums multiple times.

	src io.Reader

	md5      hash.Hash
	checksum ETag

	readN int64
}

// NewReader returns a new Reader that computes the
// MD5 checksum of the content read from r as ETag.
//
// If the provided etag is not nil the returned
// Reader compares the etag with the computed
// MD5 sum once the r returns io.EOF.
func NewReader(ctx context.Context, r io.Reader, etag ETag, forceMD5 []byte) *Reader {