encoding/xml: only initialize nil struct fields when decoding

fieldInfo.value used to initialize nil anonymous struct fields if they
were encountered. This behavior is wanted when decoding, but not when
encoding. When encoding, the value should never be modified, and these
nil fields should be skipped entirely.

To fix the bug, add a bool argument to the function which tells the
code whether we are encoding or decoding.

Finally, add a couple of tests to cover the edge cases pointed out in
the original issue.

Fixes #27240.

Change-Id: Ic97ae4bfe5f2062c8518e03d1dec07c3875e18f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/196809
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/encoding/xml/marshal.go

@@ -482,10 +482,13 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 		xmlname := tinfo.xmlname
 		if xmlname.name != "" {
 			start.Name.Space, start.Name.Local = xmlname.xmlns, xmlname.name
-		} else if v, ok := xmlname.value(val).Interface().(Name); ok && v.Local != "" {
+		} else {
+			fv := xmlname.value(val, dontInitNilPointers)
+			if v, ok := fv.Interface().(Name); ok && v.Local != "" {
 				start.Name = v
 			}
 		}
+	}
 	if start.Name.Local == "" && finfo != nil {
 		start.Name.Space, start.Name.Local = finfo.xmlns, finfo.name
 	}
@@ -503,7 +506,7 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 		if finfo.flags&fAttr == 0 {
 			continue
 		}
-		fv := finfo.value(val)
+		fv := finfo.value(val, dontInitNilPointers)
 
 		if finfo.flags&fOmitEmpty != 0 && isEmptyValue(fv) {
 			continue
@@ -806,7 +809,12 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 		if finfo.flags&fAttr != 0 {
 			continue
 		}
-		vf := finfo.value(val)
+		vf := finfo.value(val, dontInitNilPointers)
+		if !vf.IsValid() {
+			// The field is behind an anonymous struct field that's
+			// nil. Skip it.
+			continue
+		}
 
 		switch finfo.flags & fMode {
 		case fCDATA, fCharData:

src/encoding/xml/marshal_test.go

@@ -309,6 +309,11 @@ type ChardataEmptyTest struct {
 	Contents *string `xml:",chardata"`
 }
 
+type PointerAnonFields struct {
+	*MyInt
+	*NamedType
+}
+
 type MyMarshalerTest struct {
 }
 
@@ -889,6 +894,18 @@ var marshalTests = []struct {
 			`</EmbedA>`,
 	},
 
+	// Anonymous struct pointer field which is nil
+	{
+		Value:     &EmbedB{},
+		ExpectXML: `<EmbedB><FieldB></FieldB></EmbedB>`,
+	},
+
+	// Other kinds of nil anonymous fields
+	{
+		Value:     &PointerAnonFields{},
+		ExpectXML: `<PointerAnonFields></PointerAnonFields>`,
+	},
+
 	// Test that name casing matters
 	{
 		Value:     &NameCasing{Xy: "mixed", XY: "upper", XyA: "mixedA", XYA: "upperA"},

src/encoding/xml/read.go

@@ -435,7 +435,7 @@ func (d *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 				}
 				return UnmarshalError(e)
 			}
-			fv := finfo.value(sv)
+			fv := finfo.value(sv, initNilPointers)
 			if _, ok := fv.Interface().(Name); ok {
 				fv.Set(reflect.ValueOf(start.Name))
 			}
@@ -449,7 +449,7 @@ func (d *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 				finfo := &tinfo.fields[i]
 				switch finfo.flags & fMode {
 				case fAttr:
-					strv := finfo.value(sv)
+					strv := finfo.value(sv, initNilPointers)
 					if a.Name.Local == finfo.name && (finfo.xmlns == "" || finfo.xmlns == a.Name.Space) {
 						if err := d.unmarshalAttr(strv, a); err != nil {
 							return err
@@ -465,7 +465,7 @@ func (d *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 			}
 			if !handled && any >= 0 {
 				finfo := &tinfo.fields[any]
-				strv := finfo.value(sv)
+				strv := finfo.value(sv, initNilPointers)
 				if err := d.unmarshalAttr(strv, a); err != nil {
 					return err
 				}
@@ -478,22 +478,22 @@ func (d *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 			switch finfo.flags & fMode {
 			case fCDATA, fCharData:
 				if !saveData.IsValid() {
-					saveData = finfo.value(sv)
+					saveData = finfo.value(sv, initNilPointers)
 				}
 
 			case fComment:
 				if !saveComment.IsValid() {
-					saveComment = finfo.value(sv)
+					saveComment = finfo.value(sv, initNilPointers)
 				}
 
 			case fAny, fAny | fElement:
 				if !saveAny.IsValid() {
-					saveAny = finfo.value(sv)
+					saveAny = finfo.value(sv, initNilPointers)
 				}
 
 			case fInnerXML:
 				if !saveXML.IsValid() {
-					saveXML = finfo.value(sv)
+					saveXML = finfo.value(sv, initNilPointers)
 					if d.saved == nil {
 						saveXMLIndex = 0
 						d.saved = new(bytes.Buffer)
@@ -687,7 +687,7 @@ Loop:
 		}
 		if len(finfo.parents) == len(parents) && finfo.name == start.Name.Local {
 			// It's a perfect match, unmarshal the field.
-			return true, d.unmarshal(finfo.value(sv), start)
+			return true, d.unmarshal(finfo.value(sv, initNilPointers), 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/encoding/xml/typeinfo.go

@@ -344,15 +344,25 @@ 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)
 }
 
+const (
+	initNilPointers     = true
+	dontInitNilPointers = false
+)
+
 // 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 {
+// It's equivalent to v.FieldByIndex(finfo.idx), but when passed
+// initNilPointers, it initializes and dereferences pointers as necessary.
+// When passed dontInitNilPointers and a nil pointer is reached, the function
+// returns a zero reflect.Value.
+func (finfo *fieldInfo) value(v reflect.Value, shouldInitNilPointers bool) 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() {
+					if !shouldInitNilPointers {
+						return reflect.Value{}
+					}
 					v.Set(reflect.New(v.Type().Elem()))
 				}
 				v = v.Elem()