encoding/gob: fix check for short input in slice decode

R=golang-dev, dsymonds, r, nigeltao
CC=golang-dev
https://golang.org/cl/6374059

src/pkg/encoding/gob/decode.go

@@ -562,6 +562,9 @@ func (dec *Decoder) ignoreSingle(engine *decEngine) {
 func (dec *Decoder) decodeArrayHelper(state *decoderState, p uintptr, elemOp decOp, elemWid uintptr, length, elemIndir int, ovfl error) {
 	instr := &decInstr{elemOp, 0, elemIndir, 0, ovfl}
 	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding array or slice: length exceeds input size (%d elements)", length)
+		}
 		up := unsafe.Pointer(p)
 		if elemIndir > 1 {
 			up = decIndirect(up, elemIndir)
@@ -652,9 +655,6 @@ func (dec *Decoder) ignoreMap(state *decoderState, keyOp, elemOp decOp) {
 // Slices are encoded as an unsigned length followed by the elements.
 func (dec *Decoder) decodeSlice(atyp reflect.Type, state *decoderState, p uintptr, elemOp decOp, elemWid uintptr, indir, elemIndir int, ovfl error) {
 	nr := state.decodeUint()
-	if nr > uint64(state.b.Len()) {
-		errorf("length of slice exceeds input size (%d elements)", nr)
-	}
 	n := int(nr)
 	if indir > 0 {
 		up := unsafe.Pointer(p)

src/pkg/encoding/gob/encoder_test.go

@@ -813,3 +813,32 @@ func TestMutipleEncodingsOfBadType(t *testing.T) {
 		t.Errorf("expected error about no exported fields; got %v", err)
 	}
 }
+
+// There was an error check comparing the length of the input with the
+// length of the slice being decoded. It was wrong because the next
+// thing in the input might be a type definition, which would lead to
+// an incorrect length check.  This test reproduces the corner case.
+
+type Z struct {
+}
+
+func Test29ElementSlice(t *testing.T) {
+	Register(Z{})
+	src := make([]interface{}, 100) // Size needs to be bigger than size of type definition.
+	for i := range src {
+		src[i] = Z{}
+	}
+	buf := new(bytes.Buffer)
+	err := NewEncoder(buf).Encode(src)
+	if err != nil {
+		t.Fatalf("encode: %v", err)
+		return
+	}
+
+	var dst []interface{}
+	err = NewDecoder(buf).Decode(&dst)
+	if err != nil {
+		t.Errorf("decode: %v", err)
+		return
+	}
+}