encoding/xml: add wildcard support for collecting all attributes

- Like ",any" for elements, add ",any,attr" for attributes to allow
  a mop-up field that gets any otherwise unmapped attributes.
- Map attributes to fields of type slice by extending the slice,
  just like for elements.
- Allow storing an attribute into an xml.Attr directly, to provide
  a way to record the name.

Combined, these three independent features allow

	AllAttrs []Attr `xml:",any,attr"`

to collect all attributes not otherwise spoken for in a particular struct.

Tests based on CL 16292 by Charles Weill.

Fixes #3633.

Change-Id: I2d75817f17ca8752d7df188080a407836af92611
Reviewed-on: https://go-review.googlesource.com/30946
Reviewed-by: Quentin Smith <quentin@golang.org>

1 files changed, 60 insertions, 21 deletions

diff --git a/src/encoding/xml/read.go b/src/encoding/xml/read.go
index 53c15a2840..ba62366560 100644
--- a/src/encoding/xml/read.go
+++ b/src/encoding/xml/read.go
@@ -52,6 +52,11 @@ import (
 //      the explicit name in a struct field tag of the form "name,attr",
 //      Unmarshal records the attribute value in that field.
 //
+//   * If the XML element has an attribute not handled by the previous
+//      rule and the struct has a field with an associated tag containing
+//      ",any,attr", Unmarshal records the attribute value in the first
+//      such field.
+//
 //   * If the XML element contains character data, that data is
 //      accumulated in the first struct field that has tag ",chardata".
 //      The struct field may have type []byte or string.
@@ -94,8 +99,12 @@ import (
 // Unmarshal maps an attribute value to a string or []byte by saving
 // the value in the string or slice.
 //
-// Unmarshal maps an XML element to a slice by extending the length of
-// the slice and mapping the element to the newly created value.
+// Unmarshal maps an attribute value to an Attr by saving the attribute,
+// including its name, in the Attr.
+//
+// Unmarshal maps an XML element or attribute value to a slice by
+// extending the length of the slice and mapping the element or attribute
+// to the newly created value.
 //
 // Unmarshal maps an XML element or attribute value to a bool by
 // setting it to the boolean value represented by the string.
@@ -256,10 +265,31 @@ func (p *Decoder) unmarshalAttr(val reflect.Value, attr Attr) error {
 			return pv.Interface().(encoding.TextUnmarshaler).UnmarshalText([]byte(attr.Value))
 		}
 	}
+
+	if val.Type().Kind() == reflect.Slice && val.Type().Elem().Kind() != reflect.Uint8 {
+		// Slice of element values.
+		// Grow slice.
+		n := val.Len()
+		val.Set(reflect.Append(val, reflect.Zero(val.Type().Elem())))
+
+		// Recur to read element into slice.
+		if err := p.unmarshalAttr(val.Index(n), attr); err != nil {
+			val.SetLen(n)
+			return err
+		}
+		return nil
+	}
+
+	if val.Type() == attrType {
+		val.Set(reflect.ValueOf(attr))
+		return nil
+	}
+
 	return copyValue(val, []byte(attr.Value))
 }
 
 var (
+	attrType            = reflect.TypeOf(Attr{})
 	unmarshalerType     = reflect.TypeOf((*Unmarshaler)(nil)).Elem()
 	unmarshalerAttrType = reflect.TypeOf((*UnmarshalerAttr)(nil)).Elem()
 	textUnmarshalerType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
@@ -356,16 +386,7 @@ func (p *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 		// Slice of element values.
 		// Grow slice.
 		n := v.Len()
-		if n >= v.Cap() {
-			ncap := 2 * n
-			if ncap < 4 {
-				ncap = 4
-			}
-			new := reflect.MakeSlice(typ, n, ncap)
-			reflect.Copy(new, v)
-			v.Set(new)
-		}
-		v.SetLen(n + 1)
+		v.Set(reflect.Append(val, reflect.Zero(v.Type().Elem())))
 
 		// Recur to read element into slice.
 		if err := p.unmarshal(v.Index(n), start); err != nil {
@@ -412,22 +433,40 @@ func (p *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 		}
 
 		// Assign attributes.
-		// Also, determine whether we need to save character data or comments.
-		for i := range tinfo.fields {
-			finfo := &tinfo.fields[i]
-			switch finfo.flags & fMode {
-			case fAttr:
-				strv := finfo.value(sv)
-				// Look for attribute.
-				for _, a := range start.Attr {
+		for _, a := range start.Attr {
+			handled := false
+			any := -1
+			for i := range tinfo.fields {
+				finfo := &tinfo.fields[i]
+				switch finfo.flags & fMode {
+				case fAttr:
+					strv := finfo.value(sv)
 					if a.Name.Local == finfo.name && (finfo.xmlns == "" || finfo.xmlns == a.Name.Space) {
 						if err := p.unmarshalAttr(strv, a); err != nil {
 							return err
 						}
-						break
+						handled = true
+					}
+
+				case fAny | fAttr:
+					if any == -1 {
+						any = i
 					}
 				}
+			}
+			if !handled && any >= 0 {
+				finfo := &tinfo.fields[any]
+				strv := finfo.value(sv)
+				if err := p.unmarshalAttr(strv, a); err != nil {
+					return err
+				}
+			}
+		}
 
+		// Determine whether we need to save character data or comments.
+		for i := range tinfo.fields {
+			finfo := &tinfo.fields[i]
+			switch finfo.flags & fMode {
 			case fCDATA, fCharData:
 				if !saveData.IsValid() {
 					saveData = finfo.value(sv)