encoding/xml: make sure Encoder.Encode reports Write errors.

Fixes #4112.

R=remyoudompheng, daniel.morsing, dave, rsc
CC=golang-dev
https://golang.org/cl/7085053

src/pkg/encoding/xml/marshal.go

@@ -193,7 +193,9 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo) error {
 	if xmlns != "" {
 		p.WriteString(` xmlns="`)
 		// TODO: EscapeString, to avoid the allocation.
-		Escape(p, []byte(xmlns))
+		if err := EscapeText(p, []byte(xmlns)); err != nil {
+			return err
+		}
 		p.WriteByte('"')
 	}
 
@@ -252,19 +254,22 @@ func (p *printer) marshalSimple(typ reflect.Type, val reflect.Value) error {
 		p.WriteString(strconv.FormatFloat(val.Float(), 'g', -1, val.Type().Bits()))
 	case reflect.String:
 		// TODO: Add EscapeString.
-		Escape(p, []byte(val.String()))
+		EscapeText(p, []byte(val.String()))
 	case reflect.Bool:
 		p.WriteString(strconv.FormatBool(val.Bool()))
 	case reflect.Array:
 		// will be [...]byte
-		bytes := make([]byte, val.Len())
-		for i := range bytes {
-			bytes[i] = val.Index(i).Interface().(byte)
+		var bytes []byte
+		if val.CanAddr() {
+			bytes = val.Slice(0, val.Len()).Bytes()
+		} else {
+			bytes = make([]byte, val.Len())
+			reflect.Copy(reflect.ValueOf(bytes), val)
 		}
-		Escape(p, bytes)
+		EscapeText(p, bytes)
 	case reflect.Slice:
 		// will be []byte
-		Escape(p, val.Bytes())
+		EscapeText(p, val.Bytes())
 	default:
 		return &UnsupportedTypeError{typ}
 	}
@@ -298,10 +303,14 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 			case reflect.Bool:
 				Escape(p, strconv.AppendBool(scratch[:0], vf.Bool()))
 			case reflect.String:
-				Escape(p, []byte(vf.String()))
+				if err := EscapeText(p, []byte(vf.String())); err != nil {
+					return err
+				}
 			case reflect.Slice:
 				if elem, ok := vf.Interface().([]byte); ok {
-					Escape(p, elem)
+					if err := EscapeText(p, elem); err != nil {
+						return err
+					}
 				}
 			case reflect.Struct:
 				if vf.Type() == timeType {

src/pkg/encoding/xml/marshal_test.go

@@ -965,6 +965,16 @@ func TestMarshalWriteErrors(t *testing.T) {
 	}
 }
 
+func TestMarshalWriteIOErrors(t *testing.T) {
+	enc := NewEncoder(errWriter{})
+
+	expectErr := "unwritable"
+	err := enc.Encode(&Passenger{})
+	if err == nil || err.Error() != expectErr {
+		t.Errorf("EscapeTest = [error] %v, want %v", err, expectErr)
+	}
+}
+
 func BenchmarkMarshal(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Marshal(atomValue)

src/pkg/encoding/xml/xml.go

@@ -1720,9 +1720,9 @@ var (
 	esc_cr   = []byte("
")
 )
 
-// Escape writes to w the properly escaped XML equivalent
+// EscapeText writes to w the properly escaped XML equivalent
 // of the plain text data s.
-func Escape(w io.Writer, s []byte) {
+func EscapeText(w io.Writer, s []byte) error {
 	var esc []byte
 	last := 0
 	for i, c := range s {
@@ -1746,11 +1746,25 @@ func Escape(w io.Writer, s []byte) {
 		default:
 			continue
 		}
-		w.Write(s[last:i])
-		w.Write(esc)
+		if _, err := w.Write(s[last:i]); err != nil {
+			return err
+		}
+		if _, err := w.Write(esc); err != nil {
+			return err
+		}
 		last = i + 1
 	}
-	w.Write(s[last:])
+	if _, err := w.Write(s[last:]); err != nil {
+		return err
+	}
+	return nil
+}
+
+// Escape is like EscapeText but omits the error return value.
+// It is provided for backwards compatibility with Go 1.0.
+// Code targeting Go 1.1 or later should use EscapeText.
+func Escape(w io.Writer, s []byte) {
+	EscapeText(w, s)
 }
 
 // procInstEncoding parses the `encoding="..."` or `encoding='...'`

src/pkg/encoding/xml/xml_test.go

@@ -689,3 +689,17 @@ func TestDirectivesWithComments(t *testing.T) {
 		}
 	}
 }
+
+// Writer whose Write method always returns an error.
+type errWriter struct{}
+
+func (errWriter) Write(p []byte) (n int, err error) { return 0, fmt.Errorf("unwritable") }
+
+func TestEscapeTextIOErrors(t *testing.T) {
+	expectErr := "unwritable"
+	err := EscapeText(errWriter{}, []byte{'A'})
+
+	if err == nil || err.Error() != expectErr {
+		t.Errorf("EscapeTest = [error] %v, want %v", err, expectErr)
+	}
+}