decode: do not modify the source data in HookTranslateKeys

This was causing a 'fatal error: concurrent map iteration and map write' with gateways

lib/decode/decode.go

@@ -44,21 +44,28 @@ func HookTranslateKeys(_, to reflect.Type, data interface{}) (interface{}, error
 	}
 
 	rules := translationsForType(to)
+	// Avoid making a copy if there are no translation rules
+	if len(rules) == 0 {
+		return data, nil
+	}
+	result := make(map[string]interface{}, len(source))
 	for k, v := range source {
 		lowerK := strings.ToLower(k)
 		canonKey, ok := rules[lowerK]
 		if !ok {
+			result[k] = v
 			continue
 		}
-		delete(source, k)
 
 		// if there is a value for the canonical key then keep it
-		if _, ok := source[canonKey]; ok {
+		if canonValue, ok := source[canonKey]; ok {
+			// Assign the value for the case where canonKey == k
+			result[canonKey] = canonValue
 			continue
 		}
-		source[canonKey] = v
+		result[canonKey] = v
 	}
-	return source, nil
+	return result, nil
 }
 
 // TODO: could be cached if it is too slow

lib/decode/decode_test.go

@@ -178,6 +178,33 @@ func TestHookTranslateKeys_TargetStructHasPointerReceiver(t *testing.T) {
 	require.Equal(t, expected, target, "decode metadata: %#v", md)
 }
 
+func TestHookTranslateKeys_DoesNotModifySourceData(t *testing.T) {
+	raw := map[string]interface{}{
+		"S": map[string]interface{}{
+			"None":   "no translation",
+			"OldOne": "value1",
+			"oldtwo": "value2",
+		},
+	}
+
+	cfg := Config{}
+	decoder, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
+		DecodeHook: HookTranslateKeys,
+		Result:     &cfg,
+	})
+	require.NoError(t, err)
+	require.NoError(t, decoder.Decode(raw))
+
+	expected := map[string]interface{}{
+		"S": map[string]interface{}{
+			"None":   "no translation",
+			"OldOne": "value1",
+			"oldtwo": "value2",
+		},
+	}
+	require.Equal(t, raw, expected)
+}
+
 type translateExample struct {
 	FieldDefaultCanonical        string `alias:"first"`
 	FieldWithMapstructureTag     string `alias:"second" mapstructure:"field_with_mapstruct_tag"`