encoding/asn1: support set tag when unmarshaling.

This change also addresses some places where the comments were lacking.

Fixes #7087.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/56700043

src/pkg/encoding/asn1/asn1.go

@@ -634,6 +634,10 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 		universalTag = tagGeneralizedTime
 	}
 
+	if params.set {
+		universalTag = tagSet
+	}
+
 	expectedClass := classUniversal
 	expectedTag := universalTag
 
@@ -854,13 +858,20 @@ func setDefaultValue(v reflect.Value, params fieldParameters) (ok bool) {
 //
 // The following tags on struct fields have special meaning to Unmarshal:
 //
-//	optional		marks the field as ASN.1 OPTIONAL
-//	[explicit] tag:x	specifies the ASN.1 tag number; implies ASN.1 CONTEXT SPECIFIC
-//	default:x		sets the default value for optional integer fields
+//	application	specifies that a APPLICATION tag is used
+//	default:x	sets the default value for optional integer fields
+//	explicit	specifies that an additional, explicit tag wraps the implicit one
+//	optional	marks the field as ASN.1 OPTIONAL
+//	set		causes a SET, rather than a SEQUENCE type to be expected
+//	tag:x		specifies the ASN.1 tag number; implies ASN.1 CONTEXT SPECIFIC
 //
 // If the type of the first field of a structure is RawContent then the raw
 // ASN1 contents of the struct will be stored in it.
 //
+// If the type name of a slice element ends with "SET" then it's treated as if
+// the "set" tag was set on it. This can be used with nested slices where a
+// struct tag cannot be given.
+//
 // Other ASN.1 types are not supported; if it encounters them,
 // Unmarshal returns a parse error.
 func Unmarshal(b []byte, val interface{}) (rest []byte, err error) {

src/pkg/encoding/asn1/asn1_test.go

@@ -397,6 +397,10 @@ type TestBigInt struct {
 	X *big.Int
 }
 
+type TestSet struct {
+	Ints []int `asn1:"set"`
+}
+
 var unmarshalTestData = []struct {
 	in  []byte
 	out interface{}
@@ -416,6 +420,7 @@ var unmarshalTestData = []struct {
 	{[]byte{0x01, 0x01, 0xff}, newBool(true)},
 	{[]byte{0x30, 0x0b, 0x13, 0x03, 0x66, 0x6f, 0x6f, 0x02, 0x01, 0x22, 0x02, 0x01, 0x33}, &TestElementsAfterString{"foo", 0x22, 0x33}},
 	{[]byte{0x30, 0x05, 0x02, 0x03, 0x12, 0x34, 0x56}, &TestBigInt{big.NewInt(0x123456)}},
+	{[]byte{0x30, 0x0b, 0x31, 0x09, 0x02, 0x01, 0x01, 0x02, 0x01, 0x02, 0x02, 0x01, 0x03}, &TestSet{Ints: []int{1, 2, 3}}},
 }
 
 func TestUnmarshal(t *testing.T) {

src/pkg/encoding/asn1/marshal.go

@@ -568,6 +568,14 @@ func marshalField(out *forkableWriter, v reflect.Value, params fieldParameters)
 }
 
 // Marshal returns the ASN.1 encoding of val.
+//
+// In addition to the struct tags recognised by Unmarshal, the following can be
+// used:
+//
+//	ia5:		causes strings to be marshaled as ASN.1, IA5 strings
+//	omitempty:	causes empty slices to be skipped
+//	printable:	causes strings to be marshaled as ASN.1, PrintableString strings.
+//	utf8:		causes strings to be marshaled as ASN.1, UTF8 strings
 func Marshal(val interface{}) ([]byte, error) {
 	var out bytes.Buffer
 	v := reflect.ValueOf(val)