Make tag scanning code slightly cleaner.

The current code uses "length" as a proxy for "has a tag"
but the length doesn't actually match the length of the
tag when there's a %-encoded character, so use a boolean
instead.

Also fix a panic found when trying to write a test for the change.
Aside: seems that %-encoded tag handles are not allowed by
the YAML spec.

decode_test.go

@@ -610,7 +610,8 @@ type inlineC struct {
 }
 
 func (s *S) TestUnmarshal(c *C) {
-	for _, item := range unmarshalTests {
+	for i, item := range unmarshalTests {
+		c.Logf("test %d: %q", i, item.data)
 		t := reflect.ValueOf(item.value).Type()
 		var value interface{}
 		switch t.Kind() {
@@ -654,6 +655,7 @@ var unmarshalErrorTests = []struct {
 	{"a: !!binary ==", "yaml: !!binary value contains invalid base64 data"},
 	{"{[.]}", `yaml: invalid map key: \[\]interface \{\}\{"\."\}`},
 	{"{{.}}", `yaml: invalid map key: map\[interface\ \{\}\]interface \{\}\{".":interface \{\}\(nil\)\}`},
+	{"%TAG !%79! tag:yaml.org,2002:\n---\nv: !%79!int '1'", "yaml: did not find expected whitespace"},
 }
 
 func (s *S) TestUnmarshalErrors(c *C) {

scannerc.go

@@ -1944,7 +1944,7 @@ func yaml_parser_scan_tag_handle(parser *yaml_parser_t, directive bool, start_ma
 	} else {
 		// It's either the '!' tag or not really a tag handle.  If it's a %TAG
 		// directive, it's an error.  If it's a tag token, it must be a part of URI.
-		if directive && !(s[0] == '!' && s[1] == 0) {
+		if directive && string(s) != "!" {
 			yaml_parser_set_scanner_tag_error(parser, directive,
 				start_mark, "did not find expected '!'")
 			return false
@@ -1959,12 +1959,12 @@ func yaml_parser_scan_tag_handle(parser *yaml_parser_t, directive bool, start_ma
 func yaml_parser_scan_tag_uri(parser *yaml_parser_t, directive bool, head []byte, start_mark yaml_mark_t, uri *[]byte) bool {
 	//size_t length = head ? strlen((char *)head) : 0
 	var s []byte
-	length := len(head)
+	hasTag := len(head) > 0
 
 	// Copy the head if needed.
 	//
 	// Note that we don't copy the leading '!' character.
-	if length > 0 {
+	if len(head) > 1 {
 		s = append(s, head[1:]...)
 	}
 
@@ -1997,15 +1997,14 @@ func yaml_parser_scan_tag_uri(parser *yaml_parser_t, directive bool, head []byte
 			}
 		} else {
 			s = read(parser, s)
-			length++
 		}
 		if parser.unread < 1 && !yaml_parser_update_buffer(parser, 1) {
 			return false
 		}
+		hasTag = true
 	}
 
-	// Check if the tag is non-empty.
-	if length == 0 {
+	if !hasTag {
 		yaml_parser_set_scanner_tag_error(parser, directive,
 			start_mark, "did not find expected tag URI")
 		return false