GODRIVER-895 check if strLength is 0 in ReadCodeWithScope (#311)

Author: iwysiu

bson/bsonrw/value_reader.go

@@ -450,6 +450,9 @@ func (vr *valueReader) ReadCodeWithScope() (code string, dr DocumentReader, err
 	if err != nil {
 		return "", nil, err
 	}
+	if strLength <= 0 {
+		return "", nil, fmt.Errorf("invalid string length: %d", strLength)
+	}
 	strBytes, err := vr.readBytes(strLength)
 	if err != nil {
 		return "", nil, err

bson/bsonrw/value_reader_test.go

@@ -259,6 +259,11 @@ func TestValueReader(t *testing.T) {
 			0x0A, 0x00, // null element, empty key
 			0x00, // document end
 		}
+		invalidCodeWithScope := []byte{
+			0x7, 0x00, 0x00, 0x00, // total length
+			0x0, 0x00, 0x00, 0x00, // string length = 0
+			0x05, 0x00, 0x00, 0x00, 0x00, // document
+		}
 		testCases := []struct {
 			name   string
 			data   []byte
@@ -308,6 +313,13 @@ func TestValueReader(t *testing.T) {
 				fmt.Errorf("length of CodeWithScope does not match lengths of components; total: %d; components: %d", 17, 19),
 				bsontype.CodeWithScope,
 			},
+			{
+				"invalid strLength",
+				invalidCodeWithScope,
+				0,
+				fmt.Errorf("invalid string length: %d", 0),
+				bsontype.CodeWithScope,
+			},
 		}
 
 		for _, tc := range testCases {