gob: allow Decode(nil) and have it just discard the next value.

Fixes #1489.

R=rsc
CC=golang-dev
https://golang.org/cl/4187046

src/pkg/gob/decoder.go

@@ -153,9 +153,13 @@ func (dec *Decoder) decodeTypeSequence(isInterface bool) typeId {
 
 // Decode reads the next value from the connection and stores
 // it in the data represented by the empty interface value.
-// The value underlying e must be the correct type for the next
+// If e is nil, the value will be discarded. Otherwise,
+// the value underlying e must either be the correct type for the next
 // data item received, and must be a pointer.
 func (dec *Decoder) Decode(e interface{}) os.Error {
+	if e == nil {
+		return dec.DecodeValue(nil)
+	}
 	value := reflect.NewValue(e)
 	// If e represents a value as opposed to a pointer, the answer won't
 	// get back to the caller.  Make sure it's a pointer.
@@ -169,7 +173,8 @@ func (dec *Decoder) Decode(e interface{}) os.Error {
 // DecodeValue reads the next value from the connection and stores
 // it in the data represented by the reflection value.
 // The value must be the correct type for the next
-// data item received.
+// data item received, or it may be nil, which means the
+// value will be discarded.
 func (dec *Decoder) DecodeValue(value reflect.Value) os.Error {
 	// Make sure we're single-threaded through here.
 	dec.mutex.Lock()
@@ -179,7 +184,11 @@ func (dec *Decoder) DecodeValue(value reflect.Value) os.Error {
 	dec.err = nil
 	id := dec.decodeTypeSequence(false)
 	if id >= 0 {
-		dec.err = dec.decodeValue(id, value)
+		// A nil value means "ignore the data".  Since it's already read into
+		// the decoder's buffer, all we need to do is not bother to decode it.
+		if value != nil {
+			dec.err = dec.decodeValue(id, value)
+		}
 	}
 	return dec.err
 }

src/pkg/gob/encoder_test.go

@@ -384,6 +384,57 @@ func TestInterfaceIndirect(t *testing.T) {
 	}
 }
 
+func TestDecodeIntoEmptyStruct(t *testing.T) {
+	type Empty struct{}
+	empty := &Empty{}
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	err := enc.Encode(&struct{ A int }{23})
+	if err != nil {
+		t.Fatal("encode error:", err)
+	}
+	dec := NewDecoder(b)
+	err = dec.Decode(empty)
+	if err != nil {
+		t.Fatal("encode error:", err)
+	}
+}
+
+func TestStructDecodeIntoNil(t *testing.T) {
+	nonempty := &struct{ A int }{23}
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	err := enc.Encode(nonempty)
+	if err != nil {
+		t.Fatal("encode error:", err)
+	}
+	dec := NewDecoder(b)
+	err = dec.Decode(nil)
+	if err != nil {
+		t.Fatal("encode error:", err)
+	}
+	if b.Len() != 0 {
+		t.Fatalf("%d bytes remain after decode", b.Len())
+	}
+}
+
+func TestSingletonDecodeIntoNil(t *testing.T) {
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	err := enc.Encode("hello world")
+	if err != nil {
+		t.Fatal("encode error:", err)
+	}
+	dec := NewDecoder(b)
+	err = dec.Decode(nil)
+	if err != nil {
+		t.Fatal("encode error:", err)
+	}
+	if b.Len() != 0 {
+		t.Fatalf("%d bytes remain after decode", b.Len())
+	}
+}
+
 // Another bug from golang-nuts, involving nested interfaces.
 type Bug0Outer struct {
 	Bug0Field interface{}