Validate miner transaction fees

[oxarbitrage]

Motivation

We need to validate the last (i think) consensus rule for the coinbase subsidy which is directly related with the miner collected fees.

Close #1162

Specifications

Consensus rule: The total value in zatoshi of transparent outputs from a coinbase transaction, minus vbalanceSapling, minus vbalanceOrchard, MUST NOT be greater than the value in zatoshi of block subsidy plus the transaction fees paid by transactions in this block.

https://zips.z.cash/protocol/protocol.pdf#txnencodingandconsensus

Solution

This is the reasoning i used, i may be wrong.

The rule is:

valueBalanceTransparent - valueBalanceSapling - valueBalanceOrchard <= blockSubsidy + fees

Lets call valueBalanceTransparent - valueBalanceSapling - valueBalanceOrchard just valueBalance, then we have:

valueBalance - blockSubsidy <= fees

valueBalance - blockSubsidy - fees <= 0

valueBalance - blockSubsidy should be at least 0(outputs should at least cover block subsidy), so the fee should never be negative to hold the inequality.

Review

Anyone who already worked in consensus rules before should be able to review.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

[jvff]

Looks good to me overall. I added some comments, but they are all optional.

[oxarbitrage]

@teor2345 can you take a look at this one when you get some time? thanks :)

[teor2345]

I think we need to validate the transaction fees.

Can we try validating the rule exactly as written?
If we change the equation, it's hard to get the overflow checks and field checks exactly right.

This is a merge blocker.

[teor2345]

Sorry for the long response, I'll try to summarise it here.

The consensus rule says that we need to verify the transaction fees:

Consensus rule: The total value in zatoshi of transparent outputs from a coinbase transaction, minus vbalanceSapling, minus vbalanceOrchard, MUST NOT be greater than the value in zatoshi of block subsidy plus the transaction fees paid by transactions in this block.

You can get the transaction fees from the transaction verifier response:

zebra/zebra-consensus/src/block.rs

Lines 216 to 218 in 1d14032

	let response = result
	.map_err(Into::into)
	.map_err(VerifyBlockError::Transaction)?;

Here's the Response method that returns the transaction fee:

zebra/zebra-consensus/src/transaction.rs

Lines 221 to 224 in 9b17bbc

	/// The miner fee for the transaction in this response.
	///
	/// Coinbase transactions do not have a miner fee.
	pub fn miner_fee(&self) -> Option<Amount<NonNegative>> {

[teor2345]

I opened PR #3093 to sum the miner fees from each transaction.

[teor2345]

Looks good, I'm just going to delete a TODO, then merge.

[teor2345]

Looks good, thanks for all the tests!