encoding/asn1: add MarshalWithParams

Fixes #18873

Change-Id: Idb9750f739f91ebca34efcbc177254d412b4d90d
Reviewed-on: https://go-review.googlesource.com/44111
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/asn1/marshal.go

@@ -658,7 +658,13 @@ func makeField(v reflect.Value, params fieldParameters) (e encoder, err error) {
 //	utc:         causes time.Time to be marshaled as ASN.1, UTCTime values
 //	generalized: causes time.Time to be marshaled as ASN.1, GeneralizedTime values
 func Marshal(val interface{}) ([]byte, error) {
-	e, err := makeField(reflect.ValueOf(val), fieldParameters{})
+	return MarshalWithParams(val, "")
+}
+
+// MarshalWithParams allows field parameters to be specified for the
+// top-level element. The form of the params is the same as the field tags.
+func MarshalWithParams(val interface{}, params string) ([]byte, error) {
+	e, err := makeField(reflect.ValueOf(val), parseFieldParameters(params))
 	if err != nil {
 		return nil, err
 	}

src/encoding/asn1/marshal_test.go

@@ -180,6 +180,31 @@ func TestMarshal(t *testing.T) {
 	}
 }
 
+type marshalWithParamsTest struct {
+	in     interface{}
+	params string
+	out    string // hex encoded
+}
+
+var marshalWithParamsTests = []marshalWithParamsTest{
+	{intStruct{10}, "set", "310302010a"},
+	{intStruct{10}, "application", "600302010a"},
+}
+
+func TestMarshalWithParams(t *testing.T) {
+	for i, test := range marshalWithParamsTests {
+		data, err := MarshalWithParams(test.in, test.params)
+		if err != nil {
+			t.Errorf("#%d failed: %s", i, err)
+		}
+		out, _ := hex.DecodeString(test.out)
+		if !bytes.Equal(out, data) {
+			t.Errorf("#%d got: %x want %x\n\t%q\n\t%q", i, data, out, data, out)
+
+		}
+	}
+}
+
 type marshalErrTest struct {
 	in  interface{}
 	err string