encoding/xml: do not ignore error return from copyValue

The error return from copyValue was ignored causing some XML attribute
parsing to swallow an error.

Additionally, type MyMarshalerAttrTest had no UnmarshalXMLAttr method
causing marshalTests not to be symmetrical and the test suite to fail
for test case 101.

Fixes #16158

Change-Id: Icebc505295a2c656ca4b42ba37bb0957dd7260c6
Reviewed-on: https://go-review.googlesource.com/27455Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/xml/marshal_test.go

@@ -317,6 +317,10 @@ func (m *MyMarshalerAttrTest) MarshalXMLAttr(name Name) (Attr, error) {
 	return Attr{name, "hello world"}, nil
 }
 
+func (m *MyMarshalerAttrTest) UnmarshalXMLAttr(attr Attr) error {
+	return nil
+}
+
 type MarshalerStruct struct {
 	Foo MyMarshalerAttrTest `xml:",attr"`
 }
@@ -1823,3 +1827,14 @@ func TestSimpleUseOfEncodeToken(t *testing.T) {
 		t.Errorf("enc.EncodeToken: expected %q; got %q", want, buf.String())
 	}
 }
+
+// Issue 16158. Decoder.unmarshalAttr ignores the return value of copyValue.
+func TestIssue16158(t *testing.T) {
+	const data = `<foo b="HELLOWORLD"></foo>`
+	err := Unmarshal([]byte(data), &struct {
+		B byte `xml:"b,attr,omitempty"`
+	}{})
+	if err == nil {
+		t.Errorf("Unmarshal: expected error, got nil")
+	}
+}

src/encoding/xml/read.go

@@ -232,7 +232,6 @@ func (p *Decoder) unmarshalAttr(val reflect.Value, attr Attr) error {
 		}
 		val = val.Elem()
 	}
-
 	if val.CanInterface() && val.Type().Implements(unmarshalerAttrType) {
 		// This is an unmarshaler with a non-pointer receiver,
 		// so it's likely to be incorrect, but we do what we're told.
@@ -257,9 +256,7 @@ func (p *Decoder) unmarshalAttr(val reflect.Value, attr Attr) error {
 			return pv.Interface().(encoding.TextUnmarshaler).UnmarshalText([]byte(attr.Value))
 		}
 	}
-
-	copyValue(val, []byte(attr.Value))
-	return nil
+	return copyValue(val, []byte(attr.Value))
 }
 
 var (