Check vsize credentials delta in ReissuanceAsync

[onvej-sl]

Fixes #7821.

[lontivero]

What a bug!

I have no permission to contribute to this branch. Could you please add this UT in WalletWasabi.Tests/UnitTests/WabiSabi/Backend/PostRequests/CredentialReissuanceTests.cs please?

using NBitcoin;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using WalletWasabi.Tests.Helpers;
using WalletWasabi.WabiSabi.Backend;
using WalletWasabi.WabiSabi.Backend.Models;
using WalletWasabi.WabiSabi.Backend.Rounds;
using Xunit;

namespace WalletWasabi.Tests.UnitTests.WabiSabi.Backend.PostRequests;

public class CredentialReissuanceTest
{
	[Fact]
	public async Task ReissueExactDeltaAmountAsync()
	{
		WabiSabiConfig cfg = new();
		var round = WabiSabiFactory.CreateRound(cfg);
		round.SetPhase(Phase.OutputRegistration);
		var alice = WabiSabiFactory.CreateAlice(round);
		round.Alices.Add(alice);
		using Arena arena = await ArenaBuilder.From(cfg).CreateAndStartAsync(round);

		// Step 1. Create credentials
		var (amClient, vsClient, amIssuer, vsIssuer, amZeroCredentials, vsZeroCredentials) = WabiSabiFactory.CreateWabiSabiClientsAndIssuers(round);

		var amountsToRequest = new[]
			{alice.CalculateRemainingAmountCredentials(round.FeeRate, round.CoordinationFeeRate).Satoshi};
		var (amCredentialRequest, amValid) = amClient.CreateRequest(
			amountsToRequest,
			amZeroCredentials, // FIXME doesn't make much sense
			CancellationToken.None);
		var startingVsizeCredentialAmount = 100L; // any number is okay here for this test
		var (vsCredentialRequest, weValid) = vsClient.CreateRequest(
			new[] { startingVsizeCredentialAmount },
			vsZeroCredentials, // FIXME doesn't make much sense
			CancellationToken.None);

		var amResp = amIssuer.HandleRequest(amCredentialRequest);
		var weResp = vsIssuer.HandleRequest(vsCredentialRequest);
		var amountCredentials = amClient.HandleResponse(amResp, amValid).ToArray();
		var vsizeCredentials = vsClient.HandleResponse(weResp, weValid).ToArray();

		// Step 2. Try to request an invalid reissuance
		var arenaClient = WabiSabiFactory.CreateArenaClient(arena);
		var ex = await Assert.ThrowsAsync<WabiSabiProtocolException>(async () =>
			await arenaClient.ReissueCredentialAsync(
				round.Id,
				amountsToRequest,
				vsizeCredentials.Select(x => 2 * x.Value), // we request the double than what we can
				amountCredentials,
				vsizeCredentials,
				CancellationToken.None
			));
		Assert.Equal(WabiSabiProtocolErrorCode.DeltaNotZero, ex.ErrorCode);
		await arena.StopAsync(CancellationToken.None);
	}
}

Btw, feel free to modify it if you want.

[molnard]

Look a reasonable change to me. @lontivero you have the final call on this.

[lontivero]

I don't think we can merge this too soon. The first that is missing for me here is to understand where we are dropping the excess of vsize credentials. This is something we should remember but honestly I don't.

The rest are just minor things: a few UTs (at least two) and implement the validation on the client side too as is the case of other validations. Finally testing.