No tests for SSZ union type

[michaelsproul]

Ligthouse's SSZ impl was recently found to have a bug in its decoding of invalid Union types:

• SSZ Union Deserialization Vulnerable to Trailing Data for Certain Variants sigp/ethereum_ssz#64

TL;DR we would decode 0x00......: Union[T...] as None, ignoring any trailing bytes after the 0x00 selector.

Fortunately this does not cause a consensus failure as Union is currently unused in Ethereum consensus.

However, I still think it would be prudent for the spec to add some tests exercising the Union code paths. Writing new tests in ssz_generic might be the simplest.

[michaelsproul]

The same issue was reported by the same bug-hunter against remerkleable:

• SSZ Union Deserialization Vulnerable to Trailing Data for Certain Variants protolambda/remerkleable#28

(I have not validated this bug however)

[jtraglia]

Hey @michaelsproul. Great findings from UCSB! Yes, we should definitely add tests for this.

[michaelsproul]

I threw copilot at it and it seemed to do OK:

• Add Union type tests to ssz_generic covering trailing bytes after None selector michaelsproul/consensus-specs#1

But I am not well-versed in the test generators, so will leave it up to you. Obvs feel free to take anything/nothing from that PR.

[jtraglia]

I threw copilot at it and it seemed to do OK:

Oh cool. This does look pretty good. I'll leave it up to @leolara but yes, we might steal some of this 😄

[wemeetagain]

an alternative would be to remove Union entirely 🙈

[tersec]

an alternative would be to remove Union entirely 🙈

well

• Remove SSZ unions #3906

[michaelsproul]

RIP, Lighthouse uses them internally in our database types 💀

[leolara]

As it is unused by the CL, IMO these tests should not be handle here. In general, my opinion is that the SSZ vectors would be better if they are generated in the SSZ library, but we can have them here if they are used in CL.

[michaelsproul]

I would be fine with deleting the concept of unions from the spec. Lighthouse can continue using our non-standard extension for internal purposes, or migrate to a different serialization format for the handful of types where we use unions.