encoding/gob: prevent panic from index out of range in Decoder.typeString

I believe this bug is introduced by CL 460543 which optimizes the allocations
by changing the type of `idToType` from map to slice, but didn't update the
access code in `Decoder.typeString` that is safe for map but not for slice.

Fixes #62117

Change-Id: I0f2e4cc2f34c54dada1f83458ba512a6fde6dcbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/520757
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/encoding/gob/codec_test.go b/src/encoding/gob/codec_test.go
index 11a38f5f58..ec56ad50b2 100644
--- a/src/encoding/gob/codec_test.go
+++ b/src/encoding/gob/codec_test.go
@@ -1616,3 +1616,15 @@ func TestLargeSlice(t *testing.T) {
 		testEncodeDecode(t, st, rt)
 	})
 }
+
+func TestLocalRemoteTypesMismatch(t *testing.T) {
+	// Test data is from https://go.dev/issue/62117.
+	testData := []byte{9, 127, 3, 1, 2, 255, 128, 0, 0, 0, 3, 255, 128, 0}
+
+	var v []*struct{}
+	buf := bytes.NewBuffer(testData)
+	err := NewDecoder(buf).Decode(&v)
+	if err == nil {
+		t.Error("Encode/Decode: expected error but got err == nil")
+	}
+}