chainhash: add support to legacy-marshaled hashes

Recent commits 1d6e578 and 72ea23e introduced a change in the way Hashes
are serialized and deserialized. This change could cause errors in
downstream applications that persisted hashes serialized using the previous
methods.

This introduces support for legacy-serialized hashes unmarshaling and restores
the compatibility with previous versions.

Author: Beerosagos

chaincfg/chainhash/hash.go

@@ -118,6 +118,12 @@ func (hash Hash) MarshalJSON() ([]byte, error) {
 
 // UnmarshalJSON parses the hash with JSON appropriate string value.
 func (hash *Hash) UnmarshalJSON(input []byte) error {
+	// If the first byte indicates an array, the hash could have been marshalled
+	// using the legacy method and e.g. persisted.
+	if len(input) > 0 && input[0] == '[' {
+		return decodeLegacy(hash, input)
+	}
+
 	var sh string
 	err := json.Unmarshal(input, &sh)
 	if err != nil {
@@ -217,3 +223,17 @@ func Decode(dst *Hash, src string) error {
 
 	return nil
 }
+
+// decodeLegacy decodes an Hash that has been encoded with the legacy method
+// (i.e. represented as a bytes array) to a destination.
+func decodeLegacy(dst *Hash, src []byte) error {
+	var hashBytes []byte
+	err := json.Unmarshal(src, &hashBytes)
+	if err != nil {
+		return err
+	}
+	if len(hashBytes) != HashSize {
+		return ErrHashStrSize
+	}
+	return dst.SetBytes(hashBytes)
+}

chaincfg/chainhash/hash_test.go

@@ -199,6 +199,7 @@ func TestNewHashFromStr(t *testing.T) {
 // TestHashJsonMarshal tests json marshal and unmarshal.
 func TestHashJsonMarshal(t *testing.T) {
 	hashStr := "000000000003ba27aa200b1cecaad478d2b00432346c3f1f3986da1afd33e506"
+	legacyHashStr := []byte("[6,229,51,253,26,218,134,57,31,63,108,52,50,4,176,210,120,212,170,236,28,11,32,170,39,186,3,0,0,0,0,0]")
 
 	hash, err := NewHashFromStr(hashStr)
 	if err != nil {
@@ -219,4 +220,13 @@ func TestHashJsonMarshal(t *testing.T) {
 	if !hash.IsEqual(&newHash) {
 		t.Errorf("String: wrong hash string - got %v, want %v", newHash.String(), hashStr)
 	}
+
+	err = newHash.UnmarshalJSON(legacyHashStr)
+	if err != nil {
+		t.Errorf("Unmarshal legacy json error:%v, hash:%v", err, legacyHashStr)
+	}
+
+	if !hash.IsEqual(&newHash) {
+		t.Errorf("String: wrong hash string - got %v, want %v", newHash.String(), hashStr)
+	}
 }