Add RequireSafeStrings to prevent unsafe string unmarshalling; see sh…

…amaton#25

decode.go

@@ -5,11 +5,11 @@ import "github.com/shamaton/msgpack/v2/internal/decoding"
 // UnmarshalAsMap decodes data that is encoded as map format.
 // This is the same thing that StructAsArray sets false.
 func UnmarshalAsMap(data []byte, v interface{}) error {
-	return decoding.Decode(data, v, false)
+	return decoding.Decode(data, v, false, RequireSafeStrings)
 }
 
 // UnmarshalAsArray decodes data that is encoded as array format.
 // This is the same thing that StructAsArray sets true.
 func UnmarshalAsArray(data []byte, v interface{}) error {
-	return decoding.Decode(data, v, true)
+	return decoding.Decode(data, v, true, RequireSafeStrings)
 }

internal/decoding/decoding.go

@@ -8,15 +8,16 @@ import (
 )
 
 type decoder struct {
-	data    []byte
-	asArray bool
+	data         []byte
+	asArray      bool
+	stringSafety bool
 	common.Common
 }
 
 // Decode analyzes the MessagePack-encoded data and stores
 // the result into the pointer of v.
-func Decode(data []byte, v interface{}, asArray bool) error {
-	d := decoder{data: data, asArray: asArray}
+func Decode(data []byte, v interface{}, asArray, stringSafety bool) error {
+	d := decoder{data: data, asArray: asArray, stringSafety: stringSafety}
 
 	rv := reflect.ValueOf(v)
 	if rv.Kind() != reflect.Ptr {

internal/decoding/string.go

@@ -47,6 +47,9 @@ func (d *decoder) asString(offset int, k reflect.Kind) (string, int, error) {
 		return emptyString, 0, err
 	}
 	bs, offset := d.asStringByte(offset, l, k)
+	if d.stringSafety {
+		return string(bs), offset, nil
+	}
 	return *(*string)(unsafe.Pointer(&bs)), offset, nil
 }
 

msgpack.go

@@ -13,6 +13,10 @@ import (
 // If this option sets true, default encoding sets to array-format.
 var StructAsArray = false
 
+// RequireSafeStrings disables the use of unsafe to decode strings.  This is much slower, but makes
+// unmarshaled strings immutable.
+var RequireSafeStrings = false
+
 // Marshal returns the MessagePack-encoded byte array of v.
 func Marshal(v interface{}) ([]byte, error) {
 	return encoding.Encode(v, StructAsArray)
@@ -21,7 +25,7 @@ func Marshal(v interface{}) ([]byte, error) {
 // Unmarshal analyzes the MessagePack-encoded data and stores
 // the result into the pointer of v.
 func Unmarshal(data []byte, v interface{}) error {
-	return decoding.Decode(data, v, StructAsArray)
+	return decoding.Decode(data, v, StructAsArray, RequireSafeStrings)
 }
 
 // AddExtCoder adds encoders for extension types.