encoding/gob: fix hang from skipping large slices

Change-Id: I4e59b5b1702e08d7c6191d0a70fb0a555f3340c8
Reviewed-on: https://go-review.googlesource.com/9061
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rob Pike <r@golang.org>

src/encoding/gob/decode.go

@@ -575,6 +575,9 @@ func (dec *Decoder) decodeMap(mtyp reflect.Type, state *decoderState, value refl
 func (dec *Decoder) ignoreArrayHelper(state *decoderState, elemOp decOp, length int) {
 	instr := &decInstr{elemOp, 0, nil, errors.New("no error")}
 	for i := 0; i < length; i++ {
+		if state.b.Len() == 0 {
+			errorf("decoding array or slice: length exceeds input size (%d elements)", length)
+		}
 		elemOp(instr, state, noValue)
 	}
 }

src/encoding/gob/encoder_test.go

@@ -953,6 +953,8 @@ var badDataTests = []badDataTest{
 	{"130a00fb5dad0bf8ff020263e70002fa28020202a89859", "slice length too large", nil},
 	{"0f1000fb285d003316020735ff023a65c5", "interface encoding", nil},
 	{"03fffb0616fffc00f902ff02ff03bf005d02885802a311a8120228022c028ee7", "GobDecoder", nil},
+	// Issue 10491.
+	{"10fe010f020102fe01100001fe010e000016fe010d030102fe010e00010101015801fe01100000000bfe011000f85555555555555555", "length exceeds input size", nil},
 }
 
 // TestBadData tests that various problems caused by malformed input