fully validate a block before publishing

[realbigsean]

Issue Addressed

Which issue # does this PR address?

Proposed Changes

Please list or describe the changes introduced by this PR.

Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.

[paulhauner]

Just a quick fly-by review on my phone. I'm still not fully across the ObservedProposers concept though.

[realbigsean]

The ObservedProposals cache is meant to track slashable proposals and stop the node from publishing if the block would be slashable. This stops a slashable block's transactions from being revealed because otherwise you could double sign and broadcast your block locally first, view the revealed transactions and steal them or whatever in the next block

[michaelsproul]

I think we could replace observed_block_producers by the observed_proposers cache, i.e. add the block hash to the previous cache. We could preserve the prior behaviour of rejecting any duplicate proposal seen on gossip by checking which hashes are present in the cache. The intent of the gossip rule is to block slashable blocks and the same-hash-duplicate case would only be reachable via the observed_block_producers/proposers very rarely because we check for duplicates immediately prior:

lighthouse/beacon_node/beacon_chain/src/block_verification.rs

Lines 724 to 730 in a53830f

	if chain
	.canonical_head
	.fork_choice_read_lock()
	.contains_block(&block_root)
	{
	return Err(BlockError::BlockIsAlreadyKnown);
	}

[paulhauner]

I've had another look over this and I'm feeling good about it. Not necessarily for merging it into unstable, but it looks good for a hot-fix for the relays. Nice work!

I think returning an error on the HTTP API if a conflicting/slashable block has been observed (like you have implemented) is good. This should prevent the relays from revealing their payloads if the proposer has started some sort of nefarious double-proposal work already.

[michaelsproul]

Closing this now that #4293 is resolved.