Use resumable hash in digest package

Vendored resumable sha256/sha512 library. Digest package new exports a
resumable variant of the Digester.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

digest/digester.go

@@ -2,15 +2,20 @@ package digest
 
 import (
 	"crypto/sha256"
+	"fmt"
 	"hash"
+
+	"github.com/jlhawn/go-crypto"          // For ResumableHash
+	_ "github.com/jlhawn/go-crypto/sha256" // For Resumable SHA256
+	_ "github.com/jlhawn/go-crypto/sha512" // For Resumable SHA384, SHA512
 )
 
 // Digester calculates the digest of written data. It is functionally
 // equivalent to hash.Hash but provides methods for returning the Digest type
 // rather than raw bytes.
 type Digester struct {
-	alg  string
-	hash hash.Hash
+	alg string
+	hash.Hash
 }
 
 // NewDigester create a new Digester with the given hashing algorithm and instance
@@ -18,27 +23,58 @@ type Digester struct {
 func NewDigester(alg string, h hash.Hash) Digester {
 	return Digester{
 		alg:  alg,
-		hash: h,
+		Hash: h,
 	}
 }
 
 // NewCanonicalDigester is a convenience function to create a new Digester with
-// out default settings.
+// our default settings.
 func NewCanonicalDigester() Digester {
 	return NewDigester("sha256", sha256.New())
 }
 
-// Write data to the digester. These writes cannot fail.
-func (d *Digester) Write(p []byte) (n int, err error) {
-	return d.hash.Write(p)
-}
-
 // Digest returns the current digest for this digester.
 func (d *Digester) Digest() Digest {
-	return NewDigest(d.alg, d.hash)
+	return NewDigest(d.alg, d.Hash)
 }
 
-// Reset the state of the digester.
-func (d *Digester) Reset() {
-	d.hash.Reset()
+// ResumableDigester is a digester that can export its internal state and be
+// restored from saved state.
+type ResumableDigester struct {
+	alg string
+	crypto.ResumableHash
+}
+
+var resumableHashAlgs = map[string]crypto.Hash{
+	"sha256": crypto.SHA256,
+	"sha384": crypto.SHA384,
+	"sha512": crypto.SHA512,
+}
+
+// NewResumableDigester creates a new ResumableDigester with the given hashing
+// algorithm.
+func NewResumableDigester(alg string) (ResumableDigester, error) {
+	hash, supported := resumableHashAlgs[alg]
+	if !supported {
+		return ResumableDigester{}, fmt.Errorf("unsupported resumable hash algorithm: %s", alg)
+	}
+
+	return ResumableDigester{
+		alg:           alg,
+		ResumableHash: hash.New(),
+	}, nil
+}
+
+// NewCanonicalResumableDigester creates a ResumableDigester using the default
+// digest algorithm.
+func NewCanonicalResumableDigester() ResumableDigester {
+	return ResumableDigester{
+		alg:           "sha256",
+		ResumableHash: crypto.SHA256.New(),
+	}
+}
+
+// Digest returns the current digest for this resumable digester.
+func (d ResumableDigester) Digest() Digest {
+	return NewDigest(d.alg, d.ResumableHash)
 }