encoding/xml: allow ]]> in attribute values

This is permitted by the XML specification.

Fixes: #68387

src/encoding/xml/xml.go

@@ -1004,8 +1004,9 @@ Input:
 		}
 
 		// <![CDATA[ section ends with ]]>.
-		// It is an error for ]]> to appear in ordinary text.
-		if b0 == ']' && b1 == ']' && b == '>' {
+		// It is an error for ]]> to appear in ordinary text,
+		// but it is allowed in quoted strings.
+		if quote < 0 && b0 == ']' && b1 == ']' && b == '>' {
 			if cdata {
 				trunc = 2
 				break Input

src/encoding/xml/xml_test.go

@@ -626,6 +626,30 @@ type item struct {
 	FieldA string
 }
 
+func TestIssue68387(t *testing.T) {
+	data := `<item b=']]>'/>`
+	dec := NewDecoder(strings.NewReader(data))
+	var tok1, tok2, tok3 Token
+	var err error
+	if tok1, err = dec.RawToken(); err != nil {
+		t.Fatalf("RawToken() failed: %v", err)
+	}
+	if tok2, err = dec.RawToken(); err != nil {
+		t.Fatalf("RawToken() failed: %v", err)
+	}
+	if tok3, err = dec.RawToken(); err != io.EOF || tok3 != nil {
+		t.Fatalf("Missed EOF")
+	}
+	s := StartElement{Name{"", "item"}, []Attr{Attr{Name{"","b"}, "]]>"}}}
+	if !reflect.DeepEqual(tok1.(StartElement), s) {
+		t.Error("Wrong start element")
+	}
+	e := EndElement{Name{"","item"}}
+	if tok2.(EndElement) != e {
+		t.Error("Wrong end element")
+	}
+}
+
 func TestIssue569(t *testing.T) {
 	data := `<item><FieldA>abcd</FieldA></item>`
 	var i item