Add notary service

[doubiliu]

close #2646
This is the second part of the code for the notary service

• Need to reset the package path for neo-core

[ZhangTao1596]

Move to NotaryService constructor, use

neoSystem.ActorSystem.EventStream.Subscribe(Self, typeof(Blockchain.PersistCompleted));
neoSystem.ActorSystem.EventStream.Subscribe(Self, typeof(Blockchain.RelayResult));

[AnnaShaleva]

You use a simple queue to store and relay NotaryRequest payloads. This scheme allows some malicious clients to easily spam the network with useless (and cheap) NotaryRequest payloads. To prevent such behaviour we need to range payloads using all of the transaction mempool rules with some additional logic.

Please, take a look at reference, we use separate notaryRequestPool to store NotaryRequest payloads. In this mempool the payloads are being ranged by fallback's fees and/or attributes. To accept the incoming NotaryRequest payload we need to ensure that payer (which is the second fallback's signer) has enough funds deposited to Notary contract to pay for all fallback transactions that are currently in the notaryRequestPool. We also need to ensure that incoming NotaryRequest's fallback tx doesn't conflict with mempooled NotaryRequests. And several other checks need to be done (very similar to simple tx processing).

To support the described scheme we extended our transaction mempool in two ways:

1. It accepts payerIndex in the constructor (see the code). For standard completed transactions payerIndex is 0 (sender of transaction should pay the fees). For incompleted fallback transactions of incoming NotaryRequests payerIndex is 1 (fallback's first signer is Notary contract, and the second signer is the sender of NotaryRequest, so he should deposit funds to Notary).
2. It uses two different balance checkers (aka mempool.Feer, see the code and the interface itself). For standard completed transactions Feer returns sender's GAS balance is enough to pay the fees. For incompleted fallback transactions Feer returns Notary deposit balance of the NotaryRequest sender (see fee.GetUtilityTokenBalance(payer)).

So we need to refactor the Queue<NotaryRequest> payloadCache logic with regard to this comment (maybe replace it with modified Mempool).

[ZhangTao1596]

See doubiliu#2

[AnnaShaleva]

Somehow we need to write tests for this code or for separate parts of it. We need to be sure that a set of different notary requests can be properly handled, signatures can be properly extracted and transactions can be properly completed.

[AnnaShaleva]

Fallback transactions should be compared like any standard mempooled transactions, so we need to consider HighPriority attribute.

[AnnaShaleva]

Imagine the situation when user has 1 GAS deposited to notary contract and one notary request in the pool that costs 0.8 GAS. If the user sends more prioritised notary request (i.e. request with 0.9 GAS fee), then the old request should be kicked off. This check doesn't perform this. So probably, this check should be done after CheckConflicts and take into account requests that should be removed from the pool.

[AnnaShaleva]

Check conflicts logic should be exactly the same as the standard mempool has in neo-project/neo#2661.

[AnnaShaleva]

Request shouldn't be mark as sent until main transaction appears in the subsequent block. Otherwise resending may be required.

[AnnaShaleva]

Not sure, do you have a way to avoid re-finalisation of the same fallback? fb is not removed from the r.FallbackTxs list after finalisation.

[AnnaShaleva]

Instead of
(tx.Witnesses[i].InvocationScript[0] != (byte)OpCode.PUSHDATA1 && tx.Witnesses[i].InvocationScript[1] != 64)
should be
(tx.Witnesses[i].InvocationScript[0] != (byte)OpCode.PUSHDATA1 || tx.Witnesses[i].InvocationScript[1] != 64)

[AnnaShaleva]

Why is that? Standard signature witness has the single public key binded to it, and the key can be retrieved from verification script, so don't use pubs here.

[AnnaShaleva]

This should be done after the whole set of main transaction witnesses is processed (outside for loop).

[AnnaShaleva]

The same note about isSent, this field should be updated only after main tx persisting.

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