encoding/xml: split attribute marshaling into its own method

No functional changes here. Just makes next CL easier to read.

Change-Id: Icf7b2281b4da6cb59ff4edff05943b2ee288576a
Reviewed-on: https://go-review.googlesource.com/30945
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/xml/marshal.go

@@ -494,7 +494,6 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 			continue
 		}
 		fv := finfo.value(val)
-		name := Name{Space: finfo.xmlns, Local: finfo.name}
 
 		if finfo.flags&fOmitEmpty != 0 && isEmptyValue(fv) {
 			continue
@@ -504,19 +503,54 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 			continue
 		}
 
-		if fv.CanInterface() && fv.Type().Implements(marshalerAttrType) {
-			attr, err := fv.Interface().(MarshalerAttr).MarshalXMLAttr(name)
+		name := Name{Space: finfo.xmlns, Local: finfo.name}
+		if err := p.marshalAttr(&start, name, fv); err != nil {
+			return err
+		}
+	}
+
+	if err := p.writeStart(&start); err != nil {
+		return err
+	}
+
+	if val.Kind() == reflect.Struct {
+		err = p.marshalStruct(tinfo, val)
+	} else {
+		s, b, err1 := p.marshalSimple(typ, val)
+		if err1 != nil {
+			err = err1
+		} else if b != nil {
+			EscapeText(p, b)
+		} else {
+			p.EscapeString(s)
+		}
+	}
+	if err != nil {
+		return err
+	}
+
+	if err := p.writeEnd(start.Name); err != nil {
+		return err
+	}
+
+	return p.cachedWriteError()
+}
+
+// marshalAttr marshals an attribute with the given name and value, adding to start.Attr.
+func (p *printer) marshalAttr(start *StartElement, name Name, val reflect.Value) error {
+	if val.CanInterface() && val.Type().Implements(marshalerAttrType) {
+		attr, err := val.Interface().(MarshalerAttr).MarshalXMLAttr(name)
 		if err != nil {
 			return err
 		}
 		if attr.Name.Local != "" {
 			start.Attr = append(start.Attr, attr)
 		}
-			continue
+		return nil
 	}
 
-		if fv.CanAddr() {
-			pv := fv.Addr()
+	if val.CanAddr() {
+		pv := val.Addr()
 		if pv.CanInterface() && pv.Type().Implements(marshalerAttrType) {
 			attr, err := pv.Interface().(MarshalerAttr).MarshalXMLAttr(name)
 			if err != nil {
@@ -525,41 +559,41 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 			if attr.Name.Local != "" {
 				start.Attr = append(start.Attr, attr)
 			}
-				continue
+			return nil
 		}
 	}
 
-		if fv.CanInterface() && fv.Type().Implements(textMarshalerType) {
-			text, err := fv.Interface().(encoding.TextMarshaler).MarshalText()
+	if val.CanInterface() && val.Type().Implements(textMarshalerType) {
+		text, err := val.Interface().(encoding.TextMarshaler).MarshalText()
 		if err != nil {
 			return err
 		}
 		start.Attr = append(start.Attr, Attr{name, string(text)})
-			continue
+		return nil
 	}
 
-		if fv.CanAddr() {
-			pv := fv.Addr()
+	if val.CanAddr() {
+		pv := val.Addr()
 		if pv.CanInterface() && pv.Type().Implements(textMarshalerType) {
 			text, err := pv.Interface().(encoding.TextMarshaler).MarshalText()
 			if err != nil {
 				return err
 			}
 			start.Attr = append(start.Attr, Attr{name, string(text)})
-				continue
+			return nil
 		}
 	}
 
 	// Dereference or skip nil pointer, interface values.
-		switch fv.Kind() {
+	switch val.Kind() {
 	case reflect.Ptr, reflect.Interface:
-			if fv.IsNil() {
-				continue
+		if val.IsNil() {
+			return nil
 		}
-			fv = fv.Elem()
+		val = val.Elem()
 	}
 
-		s, b, err := p.marshalSimple(fv.Type(), fv)
+	s, b, err := p.marshalSimple(val.Type(), val)
 	if err != nil {
 		return err
 	}
@@ -567,33 +601,7 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 		s = string(b)
 	}
 	start.Attr = append(start.Attr, Attr{name, s})
-	}
-
-	if err := p.writeStart(&start); err != nil {
-		return err
-	}
-
-	if val.Kind() == reflect.Struct {
-		err = p.marshalStruct(tinfo, val)
-	} else {
-		s, b, err1 := p.marshalSimple(typ, val)
-		if err1 != nil {
-			err = err1
-		} else if b != nil {
-			EscapeText(p, b)
-		} else {
-			p.EscapeString(s)
-		}
-	}
-	if err != nil {
-		return err
-	}
-
-	if err := p.writeEnd(start.Name); err != nil {
-		return err
-	}
-
-	return p.cachedWriteError()
+	return nil
 }
 
 // defaultStart returns the default start element to use,