encoding/xml: track, use default tag ns in Encoder

src/pkg/encoding/xml/marshal.go

Index: src/pkg/encoding/xml/marshal.go
===================================================================
--- a/src/pkg/encoding/xml/marshal.go
+++ b/src/pkg/encoding/xml/marshal.go
@@ -258,6 +258,7 @@
 	putNewline bool
 	attrNS     map[string]string // map prefix -> name space
 	attrPrefix map[string]string // map name space -> prefix
+	defaultNS  string            // current default name space
 	prefixes   []string
 	tags       []Name
 }
@@ -322,6 +323,10 @@
 
 // deleteAttrPrefix removes an attribute name space prefix.
 func (p *printer) deleteAttrPrefix(prefix string) {
+	if strings.HasPrefix(prefix, "=ns=") {

[anacrolix, 2014/05/13 23:00:23]

Is this some kind of special value?

+		p.defaultNS = prefix[len("=ns="):]
+		return
+	}
 	delete(p.attrPrefix, p.attrNS[prefix])
 	delete(p.attrNS, prefix)
 }
@@ -341,6 +346,21 @@
 	}
 }
 
+func (p *printer) pushDefaultNS(ns string) {
+	p.prefixes = append(p.prefixes, "=ns="+ns)
+}
+
+func (p *printer) fieldName(f *fieldInfo) Name {
+	if f.name == "" {
+		return Name{}
+	}
+	space := f.xmlns
+	if f.xmlnsMissing && f.flags&fAttr == 0 {
+		space = p.defaultNS
+	}
+	return Name{space, f.name}
+}
+
 var (
 	marshalerType     = reflect.TypeOf((*Marshaler)(nil)).Elem()
 	marshalerAttrType = reflect.TypeOf((*MarshalerAttr)(nil)).Elem()
@@ -376,23 +396,23 @@
 
 	// Check for marshaler.
 	if val.CanInterface() && typ.Implements(marshalerType) {
-		return p.marshalInterface(val.Interface().(Marshaler), defaultStart(typ, finfo, startTemplate))
+		return p.marshalInterface(val.Interface().(Marshaler), p.defaultStart(typ, finfo, startTemplate))
 	}
 	if val.CanAddr() {
 		pv := val.Addr()
 		if pv.CanInterface() && pv.Type().Implements(marshalerType) {
-			return p.marshalInterface(pv.Interface().(Marshaler), defaultStart(pv.Type(), finfo, startTemplate))
+			return p.marshalInterface(pv.Interface().(Marshaler), p.defaultStart(pv.Type(), finfo, startTemplate))
 		}
 	}
 
 	// Check for text marshaler.
 	if val.CanInterface() && typ.Implements(textMarshalerType) {
-		return p.marshalTextInterface(val.Interface().(encoding.TextMarshaler), defaultStart(typ, finfo, startTemplate))
+		return p.marshalTextInterface(val.Interface().(encoding.TextMarshaler), p.defaultStart(typ, finfo, startTemplate))
 	}
 	if val.CanAddr() {
 		pv := val.Addr()
 		if pv.CanInterface() && pv.Type().Implements(textMarshalerType) {
-			return p.marshalTextInterface(pv.Interface().(encoding.TextMarshaler), defaultStart(pv.Type(), finfo, startTemplate))
+			return p.marshalTextInterface(pv.Interface().(encoding.TextMarshaler), p.defaultStart(pv.Type(), finfo, startTemplate))
 		}
 	}
 
@@ -425,19 +445,20 @@
 	} else if tinfo.xmlname != nil {
 		xmlname := tinfo.xmlname
 		if xmlname.name != "" {
-			start.Name.Space, start.Name.Local = xmlname.xmlns, xmlname.name
+			start.Name = p.fieldName(xmlname)
 		} else if v, ok := xmlname.value(val).Interface().(Name); ok && v.Local != "" {
 			start.Name = v
 		}
 	}
 	if start.Name.Local == "" && finfo != nil {
-		start.Name.Space, start.Name.Local = finfo.xmlns, finfo.name
+		start.Name = p.fieldName(finfo)
 	}
 	if start.Name.Local == "" {
 		name := typ.Name()
 		if name == "" {
 			return &UnsupportedTypeError{typ}
 		}
+		start.Name.Space = p.defaultNS
 		start.Name.Local = name
 	}
 
@@ -448,7 +469,7 @@
 			continue
 		}
 		fv := finfo.value(val)
-		name := Name{Space: finfo.xmlns, Local: finfo.name}
+		name := p.fieldName(finfo)
 
 		if finfo.flags&fOmitEmpty != 0 && isEmptyValue(fv) {
 			continue
@@ -552,7 +573,7 @@
 
 // defaultStart returns the default start element to use,
 // given the reflect type, field info, and start template.
-func defaultStart(typ reflect.Type, finfo *fieldInfo, startTemplate *StartElement) StartElement {
+func (p *printer) defaultStart(typ reflect.Type, finfo *fieldInfo, startTemplate *StartElement) StartElement {
 	var start StartElement
 	// Precedence for the XML element name is as above,
 	// except that we do not look inside structs for the first field.
@@ -560,13 +581,14 @@
 		start.Name = startTemplate.Name
 		start.Attr = append(start.Attr, startTemplate.Attr...)
 	} else if finfo != nil && finfo.name != "" {
-		start.Name.Local = finfo.name
-		start.Name.Space = finfo.xmlns
+		start.Name = p.fieldName(finfo)
 	} else if typ.Name() != "" {
+		start.Name.Space = p.defaultNS
 		start.Name.Local = typ.Name()
 	} else {
 		// Must be a pointer to a named type,
 		// since it has the Marshaler methods.
+		start.Name.Space = p.defaultNS
 		start.Name.Local = typ.Elem().Name()
 	}
 	return start
@@ -618,8 +640,10 @@
 	p.WriteByte('<')
 	p.WriteString(start.Name.Local)
 
-	if start.Name.Space != "" {
+	if start.Name.Space != p.defaultNS {
 		p.WriteString(` xmlns="`)
+		p.pushDefaultNS(p.defaultNS)
+		p.defaultNS = start.Name.Space
 		p.EscapeString(start.Name.Space)
 		p.WriteByte('"')
 	}
@@ -823,7 +847,7 @@
 			}
 			if len(finfo.parents) > len(s.stack) {
 				if vf.Kind() != reflect.Ptr && vf.Kind() != reflect.Interface || !vf.IsNil() {
-					if err := s.push(finfo.parents[len(s.stack):]); err != nil {
+					if err := s.push(p.defaultNS, finfo.parents[len(s.stack):]); err != nil {
 						return err
 					}
 				}
@@ -899,9 +923,9 @@
 }
 
 // push adds parent elements to the stack and writes open tags.
-func (s *parentStack) push(parents []string) error {
+func (s *parentStack) push(defaultNS string, parents []string) error {
 	for i := 0; i < len(parents); i++ {
-		if err := s.p.writeStart(&StartElement{Name: Name{Local: parents[i]}}); err != nil {
+		if err := s.p.writeStart(&StartElement{Name: Name{Space: defaultNS, Local: parents[i]}}); err != nil {
 			return err
 		}
 	}