Use a build flag to disable resumable digests.

Signed-off-by: Andy Goldstein <agoldste@redhat.com>

digester.go

@@ -2,12 +2,7 @@ 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
@@ -38,43 +33,22 @@ func (d *Digester) Digest() Digest {
 	return NewDigest(d.alg, d.Hash)
 }
 
-// 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(),
-	}
+// ResumableHash is the common interface implemented by all resumable hash
+// functions.
+type ResumableHash interface {
+	// ResumableHash is a superset of hash.Hash
+	hash.Hash
+	// Len returns the number of bytes written to the Hash so far.
+	Len() uint64
+	// State returns a snapshot of the state of the Hash.
+	State() ([]byte, error)
+	// Restore resets the Hash to the given state.
+	Restore(state []byte) error
 }
 
-// Digest returns the current digest for this resumable digester.
-func (d ResumableDigester) Digest() Digest {
-	return NewDigest(d.alg, d.ResumableHash)
+// ResumableDigester is a digester that can export its internal state and be
+// restored from saved state.
+type ResumableDigester interface {
+	ResumableHash
+	Digest() Digest
 }

digester_resumable.go

@@ -0,0 +1,52 @@
+// +build !noresumabledigest
+
+package digest
+
+import (
+	"fmt"
+
+	"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
+)
+
+// resumableDigester implements ResumableDigester.
+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)
+}