encoding/xml: fix default namespace of tags

The struct XMLName sets the default namespace, but
that's not good enough for nested tags, because an
earlier tag can set the implicit parents of a subsequent
tag. This change makes sure that we always explicitly set the
namespace on a tag when possible.

See https://go-review.googlesource.com/#/c/5910/4/src/encoding/xml/marshal_test.go@628
for discussion.

Change-Id: If1afc536471c0be83e5dd80381b598476ea3f44d
Reviewed-on: https://go-review.googlesource.com/6927Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/encoding/xml/marshal_test.go

@@ -640,6 +640,8 @@ var marshalTests = []struct {
 			`<x xmlns="space1">` +
 			`<c>c1</c>` +
 			`<d>d1</d>` +
+			`</x>` +
+			`<x>` +
 			`<e>e1</e>` +
 			`</x>` +
 			`</top>`,

src/encoding/xml/typeinfo.go

@@ -194,6 +194,14 @@ func structFieldInfo(typ reflect.Type, f *reflect.StructField) (*fieldInfo, erro
 		return finfo, nil
 	}
 
+	if finfo.xmlns == "" && finfo.flags&fAttr == 0 {
+		// If it's an element no namespace specified, get the default
+		// from the XMLName of enclosing struct if possible.
+		if xmlname := lookupXMLName(typ); xmlname != nil {
+			finfo.xmlns = xmlname.xmlns
+		}
+	}
+
 	// Prepare field name and parents.
 	parents := strings.Split(tag, ">")
 	if parents[0] == "" {