encoding/xml: fix incorrect indirect code in chardata, comment, innerxml fields

The new tests in this CL have been checked against Go 1.7 as well
and all pass in Go 1.7, with the one exception noted in a comment
(an intentional change to omitempty already present before this CL).

CL 15684 made the intentional change to omitempty.
This CL fixes bugs introduced along the way.

Most of these are corner cases that are arguably not that important,
but they've always worked all the way back to Go 1, and someone
cared enough to file #19063. The most significant problem found
while adding tests is that in the case of a nil *string field with
`xml:",chardata"`, the existing code silently stops processing not just
that field but the entire remainder of the struct.
Even if #19063 were not worth fixing, this chardata bug would be.

Fixes #19063.

Change-Id: I318cf8f9945e1a4615982d9904e109fde577ebf9
Reviewed-on: https://go-review.googlesource.com/36954
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/xml/marshal.go

@@ -775,6 +775,20 @@ func (p *printer) marshalSimple(typ reflect.Type, val reflect.Value) (string, []
 
 var ddBytes = []byte("--")
 
+// indirect drills into interfaces and pointers, returning the pointed-at value.
+// If it encounters a nil interface or pointer, indirect returns that nil value.
+// This can turn into an infinite loop given a cyclic chain,
+// but it matches the Go 1 behavior.
+func indirect(vf reflect.Value) reflect.Value {
+	for vf.Kind() == reflect.Interface || vf.Kind() == reflect.Ptr {
+		if vf.IsNil() {
+			return vf
+		}
+		vf = vf.Elem()
+	}
+	return vf
+}
+
 func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 	s := parentStack{p: p}
 	for i := range tinfo.fields {
@@ -816,17 +830,9 @@ 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
+			vf = indirect(vf)
 			switch vf.Kind() {
 			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 				if err := emit(p, strconv.AppendInt(scratch[:0], vf.Int(), 10)); err != nil {
@@ -861,6 +867,7 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 			if err := s.trim(finfo.parents); err != nil {
 				return err
 			}
+			vf = indirect(vf)
 			k := vf.Kind()
 			if !(k == reflect.String || k == reflect.Slice && vf.Type().Elem().Kind() == reflect.Uint8) {
 				return fmt.Errorf("xml: bad type for comment field of %s", val.Type())
@@ -901,6 +908,7 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 			continue
 
 		case fInnerXml:
+			vf = indirect(vf)
 			iface := vf.Interface()
 			switch raw := iface.(type) {
 			case []byte: