Introduce intrinsic fees for transaction validation beyond script & byte costs

[Voxelot]

closes: #461

Increases the granularity of how the base fee for a transaction is computed. These base fee components are referred to as intrinsic costs, which are intended to cover the overhead of operations such as:

• read, insert, and removal of transaction outputs
• witness recovery with secp256k1
• vm initialization for predicates & scripts
• contract bytecode bmt (binary merkle tree) root calculation
• storage slot smt (sparse merkle tree) insertions

[xgreenx]

The maxWitness - witnessSize(tx) also affects the maximum gas=)

[bvrooman]

I'm a bit confused by how max gas is now calculated.

I see in the trait definition of max_gas() is:

fn max_gas(&self, gas_costs: &GasCosts, fee: &FeeParameters) -> Word {
    let remaining_allowed_witness_gas = self
        .witness_limit()
        .saturating_sub(self.witnesses().size_dynamic() as u64)
        .saturating_mul(fee.gas_per_byte);

    self.min_gas(gas_costs, fee)
        .saturating_add(remaining_allowed_witness_gas)
}

This means:

$MaxGas = MinGas + (WitnessBytesLimit - WitnessBytes) * GasPerByte$

In this case, there's no mention of gas limit.

In some checked_transaction tests (e.g. fee_multiple_signed_inputs):

...
let max_fee = fee.max_fee();
let expected_max_fee = expected_min_fee + gas_limit * gas_price;
assert_eq!(max_fee, expected_max_fee);

This means:

$MaxGas = MinGas + GasLimit$

In this case, there's no mention of witnesses.

Is there a connection between gas limit and witnesses now? I.e., $GasLimit = (WitnessBytesLimit - WitnessBytes) * GasPerByte$
?

[xgreenx]

You are looking into max_gas implementation for Create transaction. The Script transaction implements this method in another place.

The final formula looks like:

$MaxGas = MinGas + (WitnessBytesLimit - ActualWitnessBytes) * GasPerByte + GasLimit$

[xgreenx]

gas_balance sound strange=) Maybe max_gas would be better?

[bvrooman]

This function (if I understand correctly) is meant to calculate how much the user owes (AKA the balance) after transaction execution. I can update the comment to reflect this (because currently it is misleading).

In contrast, max_gas is the maximum gas that the user is willing to spend.

Edit: It looks like this is meant to be the max_gas, and we are later converting max_gas to a balance (reserved_fee_balance). It sounds like we reserve the max gas. We don't actually specify the formula for the actual cost here (which is fine).