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>

1 files changed, 8 insertions, 8 deletions

diff --git a/src/encoding/xml/read.go b/src/encoding/xml/read.go
index 10a60eed1a..ef5df3f7f6 100644
--- a/src/encoding/xml/read.go
+++ b/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