proto: treat bad wire types as unknown fields (#511)

Previously, an error was returned during unmarshal when a wiretype
was encountered that did not match the expected wiretype.

In order to match the behavior of the C++ and Python implementations,
we no longer return an error and instead store the bad wire fragment
as an unknown field (or skip them if unknown field preservation is disabled).

The generator still produces code that references ErrInternalBadWireType
for unmarshal logic for oneof fields.
However, the current proto package does not use the generated unmarshalers
for oneofs, so their existence has no bearing on unmarshal semantics.

Cleaning up the generator to stop producing these is future work.

proto/all_test.go

@@ -1131,21 +1131,25 @@ func TestBigRepeated(t *testing.T) {
 	}
 }
 
-// Verify we give a useful message when decoding to the wrong structure type.
-func TestTypeMismatch(t *testing.T) {
-	pb1 := initGoTest(true)
+func TestBadWireTypeUnknown(t *testing.T) {
+	var b []byte
+	fmt.Sscanf("0a01780d00000000080b101612036161611521000000202c220362626225370000002203636363214200000000000000584d5a036464645900000000000056405d63000000", "%x", &b)
 
-	// Marshal
-	o := old()
-	o.Marshal(pb1)
+	m := new(MyMessage)
+	if err := Unmarshal(b, m); err != nil {
+		t.Errorf("unexpected Unmarshal error: %v", err)
+	}
 
-	// Now Unmarshal it to the wrong type.
-	pb2 := initGoTestField()
-	err := o.Unmarshal(pb2)
-	if err == nil {
-		t.Error("expected error, got no error")
-	} else if !strings.Contains(err.Error(), "bad wiretype") {
-		t.Error("expected bad wiretype error, got", err)
+	var unknown []byte
+	fmt.Sscanf("0a01780d0000000010161521000000202c2537000000214200000000000000584d5a036464645d63000000", "%x", &unknown)
+	if !bytes.Equal(m.XXX_unrecognized, unknown) {
+		t.Errorf("unknown bytes mismatch:\ngot  %x\nwant %x", m.XXX_unrecognized, unknown)
+	}
+	DiscardUnknown(m)
+
+	want := &MyMessage{Count: Int32(11), Name: String("aaa"), Pet: []string{"bbb", "ccc"}, Bigfloat: Float64(88)}
+	if !Equal(m, want) {
+		t.Errorf("message mismatch:\ngot  %v\nwant %v", m, want)
 	}
 }