encoding/xml: fix xmlns= behavior

When an xmlns="..." attribute was explicitly generated,
it was being ignored because the name space on the
attribute was assumed to have been explicitly set (to the empty
name space) and it's not possible to have an element in the
empty name space when there is a non-empty name space set.

We fix this by recording when a default name space has been
explicitly set and setting the name space of the element to that
so printer.defineNS can do its work correctly.

We do not attempt to add our own xmlns="..." attribute
when one is explicitly set.

We also add tests for EncodeElement, as that's the only way
to attain coverage of some of the changed behaviour.
Some other test coverage is also increased, although
more work remains to be done in this area.

This change was jointly developed with Martin Hilton (mhilton on github).

Fixes #11431.

Change-Id: I7b85e06eea5b18b2c15ec16dcbd92a8e1d6a9a4e
Reviewed-on: https://go-review.googlesource.com/11635
Reviewed-by: Russ Cox <[email protected]>

Author: rogpeppe

src/encoding/xml/marshal.go

@@ -578,12 +578,14 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 	// 3. type name
 	var start StartElement
 
-	// Historic behaviour: elements use the default name space
-	// they are contained in by default.
-	start.Name.Space = p.defaultNS
+	// explicitNS records whether the element's name
+	// space has been explicitly set (for example an
+	// and XMLName field).
+	explicitNS := false
 
 	if startTemplate != nil {
 		start.Name = startTemplate.Name
+		explicitNS = true
 		start.Attr = append(start.Attr, startTemplate.Attr...)
 	} else if tinfo.xmlname != nil {
 		xmlname := tinfo.xmlname
@@ -592,11 +594,13 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 		} else if v, ok := xmlname.value(val).Interface().(Name); ok && v.Local != "" {
 			start.Name = v
 		}
+		explicitNS = true
 	}
 	if start.Name.Local == "" && finfo != nil {
 		start.Name.Local = finfo.name
 		if finfo.xmlns != "" {
 			start.Name.Space = finfo.xmlns
+			explicitNS = true
 		}
 	}
 	if start.Name.Local == "" {
@@ -606,91 +610,39 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 		}
 		start.Name.Local = name
 	}
-	// Historic behaviour: an element that's in a namespace sets
-	// the default namespace for all elements contained within it.
-	start.setDefaultNamespace()
+
+	// defaultNS records the default name space as set by a xmlns="..."
+	// attribute. We don't set p.defaultNS because we want to let
+	// the attribute writing code (in p.defineNS) be solely responsible
+	// for maintaining that.
+	defaultNS := p.defaultNS
 
 	// Attributes
 	for i := range tinfo.fields {
 		finfo := &tinfo.fields[i]
 		if finfo.flags&fAttr == 0 {
 			continue
 		}
-		fv := finfo.value(val)
-		name := Name{Space: finfo.xmlns, Local: finfo.name}
-
-		if finfo.flags&fOmitEmpty != 0 && isEmptyValue(fv) {
-			continue
-		}
-
-		if fv.Kind() == reflect.Interface && fv.IsNil() {
-			continue
-		}
-
-		if fv.CanInterface() && fv.Type().Implements(marshalerAttrType) {
-			attr, err := fv.Interface().(MarshalerAttr).MarshalXMLAttr(name)
-			if err != nil {
-				return err
-			}
-			if attr.Name.Local != "" {
-				start.Attr = append(start.Attr, attr)
-			}
-			continue
-		}
-
-		if fv.CanAddr() {
-			pv := fv.Addr()
-			if pv.CanInterface() && pv.Type().Implements(marshalerAttrType) {
-				attr, err := pv.Interface().(MarshalerAttr).MarshalXMLAttr(name)
-				if err != nil {
-					return err
-				}
-				if attr.Name.Local != "" {
-					start.Attr = append(start.Attr, attr)
-				}
-				continue
-			}
-		}
-
-		if fv.CanInterface() && fv.Type().Implements(textMarshalerType) {
-			text, err := fv.Interface().(encoding.TextMarshaler).MarshalText()
-			if err != nil {
-				return err
-			}
-			start.Attr = append(start.Attr, Attr{name, string(text)})
-			continue
-		}
-
-		if fv.CanAddr() {
-			pv := fv.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
-			}
-		}
-
-		// Dereference or skip nil pointer, interface values.
-		switch fv.Kind() {
-		case reflect.Ptr, reflect.Interface:
-			if fv.IsNil() {
-				continue
-			}
-			fv = fv.Elem()
-		}
-
-		s, b, err := p.marshalSimple(fv.Type(), fv)
+		attr, add, err := p.fieldAttr(finfo, val)
 		if err != nil {
 			return err
 		}
-		if b != nil {
-			s = string(b)
+		if !add {
+			continue
+		}
+		start.Attr = append(start.Attr, attr)
+		if attr.Name.Space == "" && attr.Name.Local == "xmlns" {
+			defaultNS = attr.Value
 		}
-		start.Attr = append(start.Attr, Attr{name, s})
 	}
+	if !explicitNS {
+		// Historic behavior: elements use the default name space
+		// they are contained in by default.
+		start.Name.Space = defaultNS
+	}
+	// Historic behaviour: an element that's in a namespace sets
+	// the default namespace for all elements contained within it.
+	start.setDefaultNamespace()
 
 	if err := p.writeStart(&start); err != nil {
 		return err
@@ -719,6 +671,64 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 	return p.cachedWriteError()
 }
 
+// fieldAttr returns the attribute of the given field and
+// whether it should actually be added as an attribute;
+// val holds the value containing the field.
+func (p *printer) fieldAttr(finfo *fieldInfo, val reflect.Value) (Attr, bool, error) {
+	fv := finfo.value(val)
+	name := Name{Space: finfo.xmlns, Local: finfo.name}
+	if finfo.flags&fOmitEmpty != 0 && isEmptyValue(fv) {
+		return Attr{}, false, nil
+	}
+	if fv.Kind() == reflect.Interface && fv.IsNil() {
+		return Attr{}, false, nil
+	}
+	if fv.CanInterface() && fv.Type().Implements(marshalerAttrType) {
+		attr, err := fv.Interface().(MarshalerAttr).MarshalXMLAttr(name)
+		return attr, attr.Name.Local != "", err
+	}
+	if fv.CanAddr() {
+		pv := fv.Addr()
+		if pv.CanInterface() && pv.Type().Implements(marshalerAttrType) {
+			attr, err := pv.Interface().(MarshalerAttr).MarshalXMLAttr(name)
+			return attr, attr.Name.Local != "", err
+		}
+	}
+	if fv.CanInterface() && fv.Type().Implements(textMarshalerType) {
+		text, err := fv.Interface().(encoding.TextMarshaler).MarshalText()
+		if err != nil {
+			return Attr{}, false, err
+		}
+		return Attr{name, string(text)}, true, nil
+	}
+	if fv.CanAddr() {
+		pv := fv.Addr()
+		if pv.CanInterface() && pv.Type().Implements(textMarshalerType) {
+			text, err := pv.Interface().(encoding.TextMarshaler).MarshalText()
+			if err != nil {
+				return Attr{}, false, err
+			}
+			return Attr{name, string(text)}, true, nil
+		}
+	}
+	// Dereference or skip nil pointer, interface values.
+	switch fv.Kind() {
+	case reflect.Ptr, reflect.Interface:
+		if fv.IsNil() {
+			return Attr{}, false, nil
+		}
+		fv = fv.Elem()
+	}
+	s, b, err := p.marshalSimple(fv.Type(), fv)
+	if err != nil {
+		return Attr{}, false, err
+	}
+	if b != nil {
+		s = string(b)
+	}
+	return Attr{name, s}, true, nil
+}
+
 // defaultStart returns the default start element to use,
 // given the reflect type, field info, and start template.
 func (p *printer) defaultStart(typ reflect.Type, finfo *fieldInfo, startTemplate *StartElement) StartElement {