encoding/xml: fix to allow xml declaration with EncodeToken

This changes allows the first token encoded to be a xml declaration. A ProcInst with target of xml. Any other ProcInst after that with a target of xml will fail

Fixes #7380.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/72410043

src/pkg/encoding/xml/marshal.go

@@ -184,10 +184,12 @@ var (
 // EncodeToken does not call Flush, because usually it is part of a larger operation
 // such as Encode or EncodeElement (or a custom Marshaler's MarshalXML invoked
 // during those), and those will call Flush when finished.
-//
 // Callers that create an Encoder and then invoke EncodeToken directly, without
 // using Encode or EncodeElement, need to call Flush when finished to ensure
 // that the XML is written to the underlying writer.
+//
+// EncodeToken allows writing a ProcInst with Target set to "xml" only as the first token
+// in the stream.
 func (enc *Encoder) EncodeToken(t Token) error {
 	p := &enc.p
 	switch t := t.(type) {
@@ -210,7 +212,12 @@ func (enc *Encoder) EncodeToken(t Token) error {
 		p.WriteString("-->")
 		return p.cachedWriteError()
 	case ProcInst:
-		if t.Target == "xml" || !isNameString(t.Target) {
+		// First token to be encoded which is also a ProcInst with target of xml
+		// is the xml declaration.  The only ProcInst where target of xml is allowed.
+		if t.Target == "xml" && p.Buffered() != 0 {
+			return fmt.Errorf("xml: EncodeToken of ProcInst xml target only valid for xml declaration, first token encoded")
+		}
+		if !isNameString(t.Target) {
 			return fmt.Errorf("xml: EncodeToken of ProcInst with invalid Target")
 		}
 		if bytes.Contains(t.Inst, endProcInst) {

src/pkg/encoding/xml/marshal_test.go

@@ -1203,7 +1203,7 @@ var encodeTokenTests = []struct {
 	{Comment("foo"), "<!--foo-->", true},
 	{Comment("foo-->"), "", false},
 	{ProcInst{"Target", []byte("Instruction")}, "<?Target Instruction?>", true},
-	{ProcInst{"xml", []byte("Instruction")}, "", false},
+	{ProcInst{"", []byte("Instruction")}, "", false},
 	{ProcInst{"Target", []byte("Instruction?>")}, "", false},
 	{Directive("foo"), "<!foo>", true},
 	{Directive("foo>"), "", false},
@@ -1230,3 +1230,37 @@ func TestEncodeToken(t *testing.T) {
 		}
 	}
 }
+
+func TestProcInstEncodeToken(t *testing.T) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+
+	if err := enc.EncodeToken(ProcInst{"xml", []byte("Instruction")}); err != nil {
+		t.Fatalf("enc.EncodeToken: expected to be able to encode xml target ProcInst as first token, %s", err)
+	}
+
+	if err := enc.EncodeToken(ProcInst{"Target", []byte("Instruction")}); err != nil {
+		t.Fatalf("enc.EncodeToken: expected to be able to add non-xml target ProcInst")
+	}
+
+	if err := enc.EncodeToken(ProcInst{"xml", []byte("Instruction")}); err == nil {
+		t.Fatalf("enc.EncodeToken: expected to not be allowed to encode xml target ProcInst when not first token")
+	}
+}
+
+func TestDecodeEncode(t *testing.T) {
+	var in, out bytes.Buffer
+	in.WriteString(`<?xml version="1.0" encoding="UTF-8"?>
+<?Target Instruction?>
+<root>
+</root>	
+`)
+	dec := NewDecoder(&in)
+	enc := NewEncoder(&out)
+	for tok, err := dec.Token(); err == nil; tok, err = dec.Token() {
+		err = enc.EncodeToken(tok)
+		if err != nil {
+			t.Fatalf("enc.EncodeToken: Unable to encode token (%#v), %d", tok, err)
+		}
+	}
+}