encoding/gob: ensure "duplicate type received" decoder errors surface up

Previously re-using a decoder with a new stream resulted in a confusing
"extra data in buffer" error message.

Change-Id: Ia4c4c3a2d4b63c59e37e53faa61a500d5ff6e5f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/297949
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

src/encoding/gob/decoder.go

@@ -152,6 +152,9 @@ func (dec *Decoder) decodeTypeSequence(isInterface bool) typeId {
 		}
 		// Type definition for (-id) follows.
 		dec.recvType(-id)
+		if dec.err != nil {
+			break
+		}
 		// When decoding an interface, after a type there may be a
 		// DelimitedValue still in the buffer. Skip its count.
 		// (Alternatively, the buffer is empty and the byte count

src/encoding/gob/encoder_test.go

@@ -1127,3 +1127,28 @@ func TestBadData(t *testing.T) {
 		}
 	}
 }
+
+func TestDecodeErrorMultipleTypes(t *testing.T) {
+	type Test struct {
+		A string
+		B int
+	}
+	var b bytes.Buffer
+	NewEncoder(&b).Encode(Test{"one", 1})
+
+	var result, result2 Test
+	dec := NewDecoder(&b)
+	err := dec.Decode(&result)
+	if err != nil {
+		t.Errorf("decode: unexpected error %v", err)
+	}
+
+	b.Reset()
+	NewEncoder(&b).Encode(Test{"two", 2})
+	err = dec.Decode(&result2)
+	if err == nil {
+		t.Errorf("decode: expected duplicate type error, got nil")
+	} else if !strings.Contains(err.Error(), "duplicate type") {
+		t.Errorf("decode: expected duplicate type error, got %s", err.Error())
+	}
+}