The Billion Dollar Mistake

To be fair, it only happens here because this is a C-style API that was
converted to Go but keeping its general shape. The proper fix, to be
done eventually, is to replace all these boolean error flags by proper
error types.

Fixes #665

decode_test.go

@@ -947,6 +947,7 @@ var unmarshalErrorTests = []struct {
 	{"%TAG !%79! tag:yaml.org,2002:\n---\nv: !%79!int '1'", "yaml: did not find expected whitespace"},
 	{"a:\n  1:\nb\n  2:", ".*could not find expected ':'"},
 	{"a: 1\nb: 2\nc 2\nd: 3\n", "^yaml: line 3: could not find expected ':'$"},
+	{"#\n-\n{", "yaml: line 3: could not find expected ':'"}, // Issue #665
 	{"0: [:!00 \xef", "yaml: incomplete UTF-8 octet sequence"}, // Issue #666
 	{
 		"a: &a [00,00,00,00,00,00,00,00,00]\n" +

parserc.go

@@ -687,6 +687,9 @@ func yaml_parser_parse_node(parser *yaml_parser_t, event *yaml_event_t, block, i
 func yaml_parser_parse_block_sequence_entry(parser *yaml_parser_t, event *yaml_event_t, first bool) bool {
 	if first {
 		token := peek_token(parser)
+		if token == nil {
+			return false
+		}
 		parser.marks = append(parser.marks, token.start_mark)
 		skip_token(parser)
 	}
@@ -786,7 +789,7 @@ func yaml_parser_split_stem_comment(parser *yaml_parser_t, stem_len int) {
 	}
 
 	token := peek_token(parser)
-	if token.typ != yaml_BLOCK_SEQUENCE_START_TOKEN && token.typ != yaml_BLOCK_MAPPING_START_TOKEN {
+	if token == nil || token.typ != yaml_BLOCK_SEQUENCE_START_TOKEN && token.typ != yaml_BLOCK_MAPPING_START_TOKEN {
 		return
 	}
 
@@ -813,6 +816,9 @@ func yaml_parser_split_stem_comment(parser *yaml_parser_t, stem_len int) {
 func yaml_parser_parse_block_mapping_key(parser *yaml_parser_t, event *yaml_event_t, first bool) bool {
 	if first {
 		token := peek_token(parser)
+		if token == nil {
+			return false
+		}
 		parser.marks = append(parser.marks, token.start_mark)
 		skip_token(parser)
 	}
@@ -922,6 +928,9 @@ func yaml_parser_parse_block_mapping_value(parser *yaml_parser_t, event *yaml_ev
 func yaml_parser_parse_flow_sequence_entry(parser *yaml_parser_t, event *yaml_event_t, first bool) bool {
 	if first {
 		token := peek_token(parser)
+		if token == nil {
+			return false
+		}
 		parser.marks = append(parser.marks, token.start_mark)
 		skip_token(parser)
 	}