InvalidTxExcpetion - Fee too low

[lontivero]

It seems Wasabi in some situations calculates a transaction fee that is below the MinFeePolicy calculated by NBitcoin.

@NicolasDorier suggests Wasabi could use TransactionBuilder::SendEstimatedFee method to calculate the fee instead of using its own fee calculation algorithm.

[NicolasDorier]

To be precise Wasabi may generate transactions below the minrelayfee because of rounding error. Using the SendEstimatedFee would fix it.

[benthecarman]

In the build transaction function we just do

Transaction tx = builder
		.SetChange(changeScriptPubKey)
		.SendFees(fee)
		.BuildTransaction(sign);

if we change it to

Transaction tx = builder
		.SetChange(changeScriptPubKey)
		.SendEstimatedFees(feePerBytes)
		.BuildTransaction(sign);

It looks like it'll only affect the case where subtractFeeFromAmountIndex is set and if it is a rounding error then it'll only be a 1 satoshi amount change.

Since it is only called from the send tab and we do not set subtractFeeFromAmountIndex it should be fine to just switch from SendFees() to SendEstimatedFees() with no functional difference.

[nopara73]

The 2s/b sanity limit was removed with the manual fee estimation PR and it got merged, even though the regtests brought out this error. In fact I fixed it a while ago in this PR it just didn't get merged yet: https://github.com/zkSNACKs/WalletWasabi/pull/2047/files#diff-0a80ca357eb532ff3ed358fb4c8caae1R1156-R1161

[nopara73]

Merged.

[NicolasDorier]

@nopara73 please use SendEstimatedFees of NBitcoin. I spend lot's of time to get it right, manually calculating it is very hard.

@benthecarman NBitcoin has a SubractFee() which allow to substract the fees from the destination already.

[nopara73]

@NicolasDorier You trust your code more and I trust my code more 😄 Although I'm pretty sure the regtests would catch if there are issues with your code, so I'm not against a refactoring.