encoding/xml: prevent omitempty from omitting non-nil pointers to empty values

There was an inconsistency between the (json encoding + documentation)
and the xml encoding implementation. Pointer to an empty value was
not being serialized (i.e simply ignored). Which had the effect of making
impossible to have a struct with a string field for which we wanted to
serialize the value ""

Fixes #5452

Change-Id: Id858701801158409be01e962d2cda843424bd22a
Reviewed-on: https://go-review.googlesource.com/15684Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/xml/marshal.go

@@ -760,14 +760,6 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 		}
 		vf := finfo.value(val)
 
-		// Dereference or skip nil pointer, interface values.
-		switch vf.Kind() {
-		case reflect.Ptr, reflect.Interface:
-			if !vf.IsNil() {
-				vf = vf.Elem()
-			}
-		}
-
 		switch finfo.flags & fMode {
 		case fCDATA, fCharData:
 			emit := EscapeText
@@ -800,6 +792,16 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 					continue
 				}
 			}
+			// Drill into interfaces and pointers.
+			// This can turn into an infinite loop given a cyclic chain,
+			// but it matches the Go 1 behavior.
+			for vf.Kind() == reflect.Interface || vf.Kind() == reflect.Ptr {
+				if vf.IsNil() {
+					return nil
+				}
+				vf = vf.Elem()
+			}
+
 			var scratch [64]byte
 			switch vf.Kind() {
 			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:

src/encoding/xml/marshal_test.go

@@ -207,6 +207,7 @@ type OmitAttrTest struct {
 	Bool  bool    `xml:",attr,omitempty"`
 	Str   string  `xml:",attr,omitempty"`
 	Bytes []byte  `xml:",attr,omitempty"`
+	PStr  *string `xml:",attr,omitempty"`
 }
 
 type OmitFieldTest struct {
@@ -217,6 +218,7 @@ type OmitFieldTest struct {
 	Bool  bool          `xml:",omitempty"`
 	Str   string        `xml:",omitempty"`
 	Bytes []byte        `xml:",omitempty"`
+	PStr  *string       `xml:",omitempty"`
 	Ptr   *PresenceTest `xml:",omitempty"`
 }
 
@@ -377,6 +379,7 @@ var (
 	nameAttr     = "Sarah"
 	ageAttr      = uint(12)
 	contentsAttr = "lorem ipsum"
+	empty = ""
 )
 
 // Unless explicitly stated as such (or *Plain), all of the
@@ -835,9 +838,10 @@ var marshalTests = []struct {
 			Bool:  true,
 			Str:   "str",
 			Bytes: []byte("byt"),
+			PStr:  &empty,
 		},
 		ExpectXML: `<OmitAttrTest Int="8" int="9" Float="23.5" Uint8="255"` +
-			` Bool="true" Str="str" Bytes="byt"></OmitAttrTest>`,
+			` Bool="true" Str="str" Bytes="byt" PStr=""></OmitAttrTest>`,
 	},
 	{
 		Value:     &OmitAttrTest{},
@@ -868,6 +872,7 @@ var marshalTests = []struct {
 			Bool:  true,
 			Str:   "str",
 			Bytes: []byte("byt"),
+			PStr:   &empty,
 			Ptr:   &PresenceTest{},
 		},
 		ExpectXML: `<OmitFieldTest>` +
@@ -878,6 +883,7 @@ var marshalTests = []struct {
 			`<Bool>true</Bool>` +
 			`<Str>str</Str>` +
 			`<Bytes>byt</Bytes>` +
+			`<PStr></PStr>` +
 			`<Ptr></Ptr>` +
 			`</OmitFieldTest>`,
 	},