GODRIVER-1504 Restrict top-level keys from being parsed as extjson (#340)

Author: Divjot Arora

bson/bsonrw/extjson_parser.go

@@ -86,6 +86,7 @@ func newExtJSONParser(r io.Reader, canonical bool) *extJSONParser {
 func (ejp *extJSONParser) peekType() (bsontype.Type, error) {
 	var t bsontype.Type
 	var err error
+	initialState := ejp.s
 
 	ejp.advanceState()
 	switch ejp.s {
@@ -113,6 +114,9 @@ func (ejp *extJSONParser) peekType() (bsontype.Type, error) {
 		case jpsInvalidState:
 			err = ejp.err
 		case jpsSawKey:
+			if initialState == jpsStartState {
+				return bsontype.EmbeddedDocument, nil
+			}
 			t = wrapperKeyBSONType(ejp.k)
 
 			switch t {

bson/bsonrw/extjson_parser_test.go

@@ -155,6 +155,10 @@ func TestExtJSONParserPeekType(t *testing.T) {
 	for _, tc := range cases {
 		t.Run(tc.desc, func(t *testing.T) {
 			ejp := newExtJSONParser(strings.NewReader(tc.input), true)
+			// Manually set the parser's starting state to jpsSawColon so peekType will read ahead to find the extjson
+			// type of the value. If not set, the parser will be in jpsStartState and advance to jpsSawKey, which will
+			// cause it to return without peeking the extjson type.
+			ejp.s = jpsSawColon
 
 			for i, eTyp := range tc.typs {
 				errF := tc.errFs[i]

bson/extjson_prose_test.go

@@ -27,6 +27,7 @@ func TestExtJSON(t *testing.T) {
 		{"timestamp - negative int32 value", `{"":{"$timestamp":{"t":0,"i":-1}}}`, false, timestampNegativeInt32Err},
 		{"timestamp - negative int64 value", `{"":{"$timestamp":{"t":0,"i":-2147483649}}}`, false, timestampNegativeInt64Err},
 		{"timestamp - value overflows uint32", `{"":{"$timestamp":{"t":0,"i":4294967296}}}`, false, timestampLargeValueErr},
+		{"top level key is not treated as special", `{"$code": "foo"}`, false, nil},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {