Remove tarsum support for digest package

tarsum is not actually used by the registry. Remove support for it.

Convert numerous uses in unit tests to SHA256.

Update docs to remove mentions of tarsums (which were often inaccurate).

Remove tarsum dependency.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

digest/verifiers.go

@@ -3,9 +3,6 @@ package digest
 import (
 	"hash"
 	"io"
-	"io/ioutil"
-
-	"github.com/docker/docker/pkg/tarsum"
 )
 
 // Verifier presents a general verification interface to be used with message
@@ -27,70 +24,10 @@ func NewDigestVerifier(d Digest) (Verifier, error) {
 		return nil, err
 	}
 
-	alg := d.Algorithm()
-	switch alg {
-	case "sha256", "sha384", "sha512":
-		return hashVerifier{
-			hash:   alg.Hash(),
-			digest: d,
-		}, nil
-	default:
-		// Assume we have a tarsum.
-		version, err := tarsum.GetVersionFromTarsum(string(d))
-		if err != nil {
-			return nil, err
-		}
-
-		pr, pw := io.Pipe()
-
-		// TODO(stevvooe): We may actually want to ban the earlier versions of
-		// tarsum. That decision may not be the place of the verifier.
-
-		ts, err := tarsum.NewTarSum(pr, true, version)
-		if err != nil {
-			return nil, err
-		}
-
-		// TODO(sday): Ick! A goroutine per digest verification? We'll have to
-		// get the tarsum library to export an io.Writer variant.
-		go func() {
-			if _, err := io.Copy(ioutil.Discard, ts); err != nil {
-				pr.CloseWithError(err)
-			} else {
-				pr.Close()
-			}
-		}()
-
-		return &tarsumVerifier{
-			digest: d,
-			ts:     ts,
-			pr:     pr,
-			pw:     pw,
-		}, nil
-	}
-}
-
-// NewLengthVerifier returns a verifier that returns true when the number of
-// read bytes equals the expected parameter.
-func NewLengthVerifier(expected int64) Verifier {
-	return &lengthVerifier{
-		expected: expected,
-	}
-}
-
-type lengthVerifier struct {
-	expected int64 // expected bytes read
-	len      int64 // bytes read
-}
-
-func (lv *lengthVerifier) Write(p []byte) (n int, err error) {
-	n = len(p)
-	lv.len += int64(n)
-	return n, err
-}
-
-func (lv *lengthVerifier) Verified() bool {
-	return lv.expected == lv.len
+	return hashVerifier{
+		hash:   d.Algorithm().Hash(),
+		digest: d,
+	}, nil
 }
 
 type hashVerifier struct {
@@ -105,18 +42,3 @@ func (hv hashVerifier) Write(p []byte) (n int, err error) {
 func (hv hashVerifier) Verified() bool {
 	return hv.digest == NewDigest(hv.digest.Algorithm(), hv.hash)
 }
-
-type tarsumVerifier struct {
-	digest Digest
-	ts     tarsum.TarSum
-	pr     *io.PipeReader
-	pw     *io.PipeWriter
-}
-
-func (tv *tarsumVerifier) Write(p []byte) (n int, err error) {
-	return tv.pw.Write(p)
-}
-
-func (tv *tarsumVerifier) Verified() bool {
-	return tv.digest == Digest(tv.ts.Sum(nil))
-}