encoding/xml: handle anonymous pointer fields

This CL makes

    type T struct { *U }

behave in a similar way to:

    type T struct { U }

Fixes #3108.

R=golang-dev, rsc, gustavo
CC=golang-dev
https://golang.org/cl/5694044

src/pkg/encoding/xml/marshal.go

@@ -57,8 +57,8 @@ const (
 //       if the field value is empty. The empty values are false, 0, any
 //       nil pointer or interface value, and any array, slice, map, or
 //       string of length zero.
-//     - a non-pointer anonymous struct field is handled as if the
-//       fields of its value were part of the outer struct.
+//     - an anonymous struct field is handled as if the fields of its
+//       value were part of the outer struct.
 //
 // If a field uses a tag "a>b>c", then the element c will be nested inside
 // parent elements a and b.  Fields that appear next to each other that name
@@ -164,7 +164,7 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo) error {
 		xmlname := tinfo.xmlname
 		if xmlname.name != "" {
 			xmlns, name = xmlname.xmlns, xmlname.name
-		} else if v, ok := val.FieldByIndex(xmlname.idx).Interface().(Name); ok && v.Local != "" {
+		} else if v, ok := xmlname.value(val).Interface().(Name); ok && v.Local != "" {
 			xmlns, name = v.Space, v.Local
 		}
 	}
@@ -195,7 +195,7 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo) error {
 		if finfo.flags&fAttr == 0 {
 			continue
 		}
-		fv := val.FieldByIndex(finfo.idx)
+		fv := finfo.value(val)
 		if finfo.flags&fOmitEmpty != 0 && isEmptyValue(fv) {
 			continue
 		}
@@ -276,7 +276,7 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 		if finfo.flags&(fAttr|fAny) != 0 {
 			continue
 		}
-		vf := val.FieldByIndex(finfo.idx)
+		vf := finfo.value(val)
 		switch finfo.flags & fMode {
 		case fCharData:
 			switch vf.Kind() {

src/pkg/encoding/xml/marshal_test.go

@@ -108,7 +108,7 @@ type EmbedA struct {
 
 type EmbedB struct {
 	FieldB string
-	EmbedC
+	*EmbedC
 }
 
 type EmbedC struct {
@@ -493,7 +493,7 @@ var marshalTests = []struct {
 			},
 			EmbedB: EmbedB{
 				FieldB: "A.B.B",
-				EmbedC: EmbedC{
+				EmbedC: &EmbedC{
 					FieldA1: "A.B.C.A1",
 					FieldA2: "A.B.C.A2",
 					FieldB:  "", // Shadowed by A.B.B

src/pkg/encoding/xml/read.go

@@ -81,8 +81,8 @@ import (
 //      of the above rules and the struct has a field with tag ",any",
 //      unmarshal maps the sub-element to that struct field.
 //
-//   * A non-pointer anonymous struct field is handled as if the
-//      fields of its value were part of the outer struct.
+//   * An anonymous struct field is handled as if the fields of its
+//      value were part of the outer struct.
 //
 //   * A struct field with tag "-" is never unmarshalled into.
 //
@@ -248,7 +248,7 @@ func (p *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 				}
 				return UnmarshalError(e)
 			}
-			fv := sv.FieldByIndex(finfo.idx)
+			fv := finfo.value(sv)
 			if _, ok := fv.Interface().(Name); ok {
 				fv.Set(reflect.ValueOf(start.Name))
 			}
@@ -260,7 +260,7 @@ func (p *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 			finfo := &tinfo.fields[i]
 			switch finfo.flags & fMode {
 			case fAttr:
-				strv := sv.FieldByIndex(finfo.idx)
+				strv := finfo.value(sv)
 				// Look for attribute.
 				for _, a := range start.Attr {
 					if a.Name.Local == finfo.name {
@@ -271,22 +271,22 @@ func (p *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 
 			case fCharData:
 				if !saveData.IsValid() {
-					saveData = sv.FieldByIndex(finfo.idx)
+					saveData = finfo.value(sv)
 				}
 
 			case fComment:
 				if !saveComment.IsValid() {
-					saveComment = sv.FieldByIndex(finfo.idx)
+					saveComment = finfo.value(sv)
 				}
 
 			case fAny:
 				if !saveAny.IsValid() {
-					saveAny = sv.FieldByIndex(finfo.idx)
+					saveAny = finfo.value(sv)
 				}
 
 			case fInnerXml:
 				if !saveXML.IsValid() {
-					saveXML = sv.FieldByIndex(finfo.idx)
+					saveXML = finfo.value(sv)
 					if p.saved == nil {
 						saveXMLIndex = 0
 						p.saved = new(bytes.Buffer)
@@ -461,7 +461,7 @@ Loop:
 		}
 		if len(finfo.parents) == len(parents) && finfo.name == start.Name.Local {
 			// It's a perfect match, unmarshal the field.
-			return true, p.unmarshal(sv.FieldByIndex(finfo.idx), start)
+			return true, p.unmarshal(finfo.value(sv), start)
 		}
 		if len(finfo.parents) > len(parents) && finfo.parents[len(parents)] == start.Name.Local {
 			// It's a prefix for the field. Break and recurse

src/pkg/encoding/xml/typeinfo.go

@@ -66,10 +66,14 @@ func getTypeInfo(typ reflect.Type) (*typeInfo, error) {
 
 			// For embedded structs, embed its fields.
 			if f.Anonymous {
-				if f.Type.Kind() != reflect.Struct {
+				t := f.Type
+				if t.Kind() == reflect.Ptr {
+					t = t.Elem()
+				}
+				if t.Kind() != reflect.Struct {
 					continue
 				}
-				inner, err := getTypeInfo(f.Type)
+				inner, err := getTypeInfo(t)
 				if err != nil {
 					return nil, err
 				}
@@ -327,3 +331,22 @@ type TagPathError struct {
 func (e *TagPathError) Error() string {
 	return fmt.Sprintf("%s field %q with tag %q conflicts with field %q with tag %q", e.Struct, e.Field1, e.Tag1, e.Field2, e.Tag2)
 }
+
+// value returns v's field value corresponding to finfo.
+// It's equivalent to v.FieldByIndex(finfo.idx), but initializes
+// and dereferences pointers as necessary.
+func (finfo *fieldInfo) value(v reflect.Value) reflect.Value {
+	for i, x := range finfo.idx {
+		if i > 0 {
+			t := v.Type()
+			if t.Kind() == reflect.Ptr && t.Elem().Kind() == reflect.Struct {
+				if v.IsNil() {
+					v.Set(reflect.New(v.Type().Elem()))
+				}
+				v = v.Elem()
+			}
+		}
+		v = v.Field(x)
+	}
+	return v
+}