bug: Marshal can produce malformed CBOR when marshaling a cbor.Marshaler that produces malformed CBOR

[benluddy]

What version of fxamacker/cbor are you using?

v2.5.0

Does this issue reproduce with the latest release?

Yes.

What OS and CPU architecture are you using (go env)?

go env Output

$ go env

What did you do?

Marshaled RawMessage([]byte{0xa6}) without error, then called Wellformed on the output.

https://go.dev/play/p/n_OxskfI3oA

What did you expect to see?

I expected the call to Marshal to return a non-nil error refusing to output malformed CBOR.

What did you see instead?

The Marshaler (in this case a RawMessage) is trusted to produce well-formed CBOR, and as a result the output from Marshal is not well-formed.

[fxamacker]

@benluddy Thanks for finding and reporting this bug! 👍

I'll open a PR to fix this by rejecting malformed CBOR data from Marshaler.