crypto/x509: don't include empty additional primes in PKCS#1 private key.

asn1 didn't have an omitempty tag, so the list of additional primes in
an RSA private key was serialised as an empty SEQUENCE, even for
version 1 structures. This tripped up external code that didn't handle
v2.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5729062

src/pkg/crypto/x509/pkcs1.go

@@ -24,7 +24,7 @@ type pkcs1PrivateKey struct {
 	Dq   *big.Int `asn1:"optional"`
 	Qinv *big.Int `asn1:"optional"`
 
-	AdditionalPrimes []pkcs1AdditionalRSAPrime `asn1:"optional"`
+	AdditionalPrimes []pkcs1AdditionalRSAPrime `asn1:"optional,omitempty"`
 }
 
 type pkcs1AdditionalRSAPrime struct {

src/pkg/encoding/asn1/asn1_test.go

@@ -321,7 +321,7 @@ var parseFieldParametersTestData []parseFieldParametersTest = []parseFieldParame
 	{"default:42", fieldParameters{defaultValue: newInt64(42)}},
 	{"tag:17", fieldParameters{tag: newInt(17)}},
 	{"optional,explicit,default:42,tag:17", fieldParameters{optional: true, explicit: true, defaultValue: newInt64(42), tag: newInt(17)}},
-	{"optional,explicit,default:42,tag:17,rubbish1", fieldParameters{true, true, false, newInt64(42), newInt(17), 0, false}},
+	{"optional,explicit,default:42,tag:17,rubbish1", fieldParameters{true, true, false, newInt64(42), newInt(17), 0, false, false}},
 	{"set", fieldParameters{set: true}},
 }
 

src/pkg/encoding/asn1/common.go

@@ -75,6 +75,7 @@ type fieldParameters struct {
 	tag          *int   // the EXPLICIT or IMPLICIT tag (maybe nil).
 	stringType   int    // the string tag to use when marshaling.
 	set          bool   // true iff this should be encoded as a SET
+	omitEmpty    bool   // true iff this should be omitted if empty when marshaling.
 
 	// Invariants:
 	//   if explicit is set, tag is non-nil.
@@ -116,6 +117,8 @@ func parseFieldParameters(str string) (ret fieldParameters) {
 			if ret.tag == nil {
 				ret.tag = new(int)
 			}
+		case part == "omitempty":
+			ret.omitEmpty = true
 		}
 	}
 	return

src/pkg/encoding/asn1/marshal.go

@@ -463,6 +463,10 @@ func marshalField(out *forkableWriter, v reflect.Value, params fieldParameters)
 		return marshalField(out, v.Elem(), params)
 	}
 
+	if v.Kind() == reflect.Slice && v.Len() == 0 && params.omitEmpty {
+		return
+	}
+
 	if params.optional && reflect.DeepEqual(v.Interface(), reflect.Zero(v.Type()).Interface()) {
 		return
 	}

src/pkg/encoding/asn1/marshal_test.go

@@ -54,6 +54,10 @@ type optionalRawValueTest struct {
 	A RawValue `asn1:"optional"`
 }
 
+type omitEmptyTest struct {
+	A []string `asn1:"omitempty"`
+}
+
 type testSET []int
 
 var PST = time.FixedZone("PST", -8*60*60)
@@ -116,6 +120,8 @@ var marshalTests = []marshalTest{
 	{rawContentsStruct{[]byte{0x30, 3, 1, 2, 3}, 64}, "3003010203"},
 	{RawValue{Tag: 1, Class: 2, IsCompound: false, Bytes: []byte{1, 2, 3}}, "8103010203"},
 	{testSET([]int{10}), "310302010a"},
+	{omitEmptyTest{[]string{}}, "3000"},
+	{omitEmptyTest{[]string{"1"}}, "30053003130131"},
 }
 
 func TestMarshal(t *testing.T) {