digest: remove error return from Digest.Verifier

Signed-off-by: Stephen J Day <stephen.day@docker.com>

digest/digest.go

@@ -104,16 +104,17 @@ func (d Digest) Validate() error {
 		return ErrDigestInvalidFormat
 	}
 
-	switch algorithm := Algorithm(s[:i]); algorithm {
-	case SHA256, SHA384, SHA512:
-		if algorithm.Size()*2 != len(s[i+1:]) {
-			return ErrDigestInvalidLength
-		}
-		break
-	default:
+	algorithm := Algorithm(s[:i])
+	if !algorithm.Available() {
 		return ErrDigestUnsupported
 	}
 
+	// Digests much always be hex-encoded, ensuring that their hex portion will
+	// always be size*2
+	if algorithm.Size()*2 != len(s[i+1:]) {
+		return ErrDigestInvalidLength
+	}
+
 	return nil
 }
 
@@ -124,17 +125,12 @@ func (d Digest) Algorithm() Algorithm {
 }
 
 // Verifier returns a writer object that can be used to verify a stream of
-// content against the digest. If the digest is invalid, an error will be
-// returned.
-func (d Digest) Verifier() (Verifier, error) {
-	if err := d.Validate(); err != nil {
-		return nil, err
-	}
-
+// content against the digest. If the digest is invalid, the method will panic.
+func (d Digest) Verifier() Verifier {
 	return hashVerifier{
 		hash:   d.Algorithm().Hash(),
 		digest: d,
-	}, nil
+	}
 }
 
 // Hex returns the hex digest portion of the digest. This will panic if the
@@ -151,7 +147,7 @@ func (d Digest) sepIndex() int {
 	i := strings.Index(string(d), ":")
 
 	if i < 0 {
-		panic("could not find ':' in digest: " + d)
+		panic(fmt.Sprintf("no ':' separator in digest %q", d))
 	}
 
 	return i