fix parsing issues and nits

builder.go

@@ -38,7 +38,7 @@ func (p V0Builder) Sum(data []byte) (Cid, error) {
 	if err != nil {
 		return Undef, err
 	}
-	return NewCidV0(hash), nil
+	return Cid{string(hash)}, nil
 }
 
 func (p V0Builder) GetCodec() uint64 {

cid.go

@@ -137,25 +137,39 @@ var CodecToStr = map[uint64]string{
 	DashTx:             "dash-tx",
 }
 
+// tryNewCidV0 tries to convert a multihash into a CIDv0 CID and returns an
+// error on failure.
+func tryNewCidV0(mhash mh.Multihash) (Cid, error) {
+	// Need to make sure hash is valid for CidV0 otherwise we will
+	// incorrectly detect it as CidV1 in the Version() method
+	dec, err := mh.Decode(mhash)
+	if err != nil {
+		return Undef, err
+	}
+	if dec.Code != mh.SHA2_256 || dec.Length != 32 {
+		return Undef, fmt.Errorf("invalid hash for cidv0 %d-%d", dec.Code, dec.Length)
+	}
+	return Cid{string(mhash)}, nil
+}
+
 // NewCidV0 returns a Cid-wrapped multihash.
 // They exist to allow IPFS to work with Cids while keeping
 // compatibility with the plain-multihash format used used in IPFS.
 // NewCidV1 should be used preferentially.
+//
+// Panics if the multihash isn't sha2-256.
 func NewCidV0(mhash mh.Multihash) Cid {
-	// Need to make sure hash is valid for CidV0 otherwise we will
-	// incorrectly detect it as CidV1 in the Version() method
-	dec, err := mh.Decode(mhash)
+	c, err := tryNewCidV0(mhash)
 	if err != nil {
 		panic(err)
 	}
-	if dec.Code != mh.SHA2_256 || dec.Length != 32 {
-		panic("invalid hash for cidv0")
-	}
-	return Cid{string(mhash)}
+	return c
 }
 
 // NewCidV1 returns a new Cid using the given multicodec-packed
 // content type.
+//
+// Panics if the multihash is invalid.
 func NewCidV1(codecType uint64, mhash mh.Multihash) Cid {
 	hashlen := len(mhash)
 	// two 8 bytes (max) numbers plus hash
@@ -203,7 +217,7 @@ func Parse(v interface{}) (Cid, error) {
 	case []byte:
 		return Cast(v2)
 	case mh.Multihash:
-		return NewCidV0(v2), nil
+		return tryNewCidV0(v2)
 	case Cid:
 		return v2, nil
 	default:
@@ -234,7 +248,7 @@ func Decode(v string) (Cid, error) {
 			return Undef, err
 		}
 
-		return NewCidV0(hash), nil
+		return tryNewCidV0(hash)
 	}
 
 	_, data, err := mbase.Decode(v)
@@ -589,7 +603,7 @@ func PrefixFromBytes(buf []byte) (Prefix, error) {
 }
 
 func CidFromBytes(data []byte) (int, Cid, error) {
-	if len(data) > 2 && data[0] == 18 && data[1] == 32 {
+	if len(data) > 2 && data[0] == mh.SHA2_256 && data[1] == 32 {
 		if len(data) < 34 {
 			return 0, Undef, fmt.Errorf("not enough bytes for cid v0")
 		}
@@ -599,7 +613,7 @@ func CidFromBytes(data []byte) (int, Cid, error) {
 			return 0, Undef, err
 		}
 
-		return 34, NewCidV0(h), nil
+		return 34, Cid{string(h)}, nil
 	}
 
 	vers, n := binary.Uvarint(data)

cid_test.go

@@ -584,3 +584,14 @@ func TestReadCidsFromBuffer(t *testing.T) {
 		t.Fatal("had trailing bytes")
 	}
 }
+
+func TestBadParse(t *testing.T) {
+	hash, err := mh.Sum([]byte("foobar"), mh.SHA3_256, -1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = Parse(hash)
+	if err == nil {
+		t.Fatal("expected to fail to parse an invalid CIDv1 CID")
+	}
+}