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>

cmd/digest/main.go

@@ -4,14 +4,11 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"log"
 	"os"
-	"strings"
 
 	"github.com/docker/distribution/digest"
 	"github.com/docker/distribution/version"
-	"github.com/docker/docker/pkg/tarsum"
 )
 
 var (
@@ -80,36 +77,7 @@ func main() {
 	digestFn := algorithm.FromReader
 
 	if !algorithm.Available() {
-		// we cannot digest if is not available. An exception is made for
-		// tarsum.
-		if !strings.HasPrefix(algorithm.String(), "tarsum") {
-			unsupported()
-		}
-
-		var version tarsum.Version
-		if algorithm == "tarsum" {
-			// small hack: if we just have tarsum, use latest
-			version = tarsum.Version1
-		} else {
-			var err error
-			version, err = tarsum.GetVersionFromTarsum(algorithm.String())
-			if err != nil {
-				unsupported()
-			}
-		}
-
-		digestFn = func(rd io.Reader) (digest.Digest, error) {
-			ts, err := tarsum.NewTarSum(rd, true, version)
-			if err != nil {
-				return "", err
-			}
-
-			if _, err := io.Copy(ioutil.Discard, ts); err != nil {
-				return "", err
-			}
-
-			return digest.Digest(ts.Sum(nil)), nil
-		}
+		unsupported()
 	}
 
 	for _, job := range jobs {