chore[arbitrary]: make arbitrary and proptest an optional dependency

[skaunov]

Resolve #188

It's WIP yet - though I'd be happy if you advice on the linting issues I had to add to commit.

Also I'm looking into property testing and wonder if there's another task to get hands on if you'd suggest one.

[Sword-Smith]

I have two ideas:

• You can ensure that the same peer never requests the same block more than twice. And if they do, they can be punished for that. peer_loop would be where that functionality would live.
• You can work on style: Block::is_valid_internal #293, change its return type to result type, and start adding negative tests of all failure modes.

[skaunov]

Thanks, it's great to have a choice! I'll take a look. Meanwhile this one can be merged I guess.

[skaunov]

I started to look into the tests of peer_loop -- will take some time as I didn't even start the proptest book from any end. What is considered to request twice? A time frame, or a consecutive request?

• You can ensure that the same peer never requests the same block more than twice. And if they do, they can be punished for that. peer_loop would be where that functionality would live.

[Sword-Smith]

I started to look into the tests of peer_loop -- will take some time as I didn't even start the proptest from any end. What is considered to request twice? A time frame, or a consecutive request?

• You can ensure that the same peer never requests the same block more than twice. And if they do, they can be punished for that. peer_loop would be where that functionality would live.

There are a few ways that peers can request blocks. E.g. by PeerMessage::BlockRequestByHeight and PeerMessage::BlockRequestByHash. If successful, this will result in the sharing of a block through a call like.

peer.send(PeerMessage::Block(Box::new(b.try_into().unwrap())))
    .await?;

Every time a block is shared with a peer, it could be recorded in a field of type HashMap<Digest, usize> in the MutablePeerState structure, where this hashmap counts how many times each block was shared. If a block is shared with a specific peer more than twice, we could start punishing that peer.