Support free gas networks when using London fee market

[siladu]

In order to support free gas private networks in a London/EIP1559 context, I propose that we support setting a zero baseFee.

If baseFeePerGas is overridden as 0 wei, then bypass the need for a transaction to have a gas price of >= 7 wei.

The genesis would look something like this:

{
  "config": {
    ...
    "londonBlock": 0,
    ...
  },
  "baseFeePerGas": "0x0",
  "nonce": "0x0",
  ...
}

One potential downside is that once baseFeePerGas is set to 0, there's no going back for that chain. You can still use --min-gas-price to enforce some gas, but the London base fee calculation will be forever opted out of as a consequence of the zero baseFee in the genesis block. This also means no ether would get burned since base fee will always be zero.

This is currently blocking users with free gas networks from upgrading past the Berlin fork.

Fixed Issue(s)

Fixes #3374
Fixes #3763

Documentation

• I thought about documentation and added the doc-change-required label to this PR if
  updates are required.

Changelog

• I thought about the changelog and included a changelog update if required.

[antonydenyer]

It's a fiddly one! We talked about previously over at #3397

Should we tie the minimum to what is set in the genesis?

[siladu]

It's a fiddly one! We talked about previously over at #3397

It is indeed! I saw that PR, was planning on asking for your review for this ;)

Pre-London, configuring a free gas network only involved setting --min-gas-price=0 on the nodes but post-London, with this fix it would involve the user setting baseFeePerGas to zero in the genesis as well, which is a bit annoying but documentable.

The reason I've gone with that for now is because zero baseFee is also required to pass this validation:

besu/ethereum/core/src/main/java/org/hyperledger/besu/ethereum/mainnet/MainnetTransactionValidator.java

Lines 134 to 141 in 645fdd0

	if (baseFee.isPresent()) {
	final Wei price = feeMarket.getTransactionPriceCalculator().price(transaction, baseFee);
	if (!transactionValidationParams.isAllowMaxFeeGasBelowBaseFee()
	&& price.compareTo(baseFee.orElseThrow()) < 0) {
	return ValidationResult.invalid(
	TransactionInvalidReason.INVALID_TRANSACTION_FORMAT,
	"gasPrice is less than the current BaseFee");
	}

Another option could be to default baseFee to zero and set it explicitly for public networks.

Was trying to get my head around how the fee gets burned and if there would be any implications for this configuration.

[siladu]

Keeping open for @garyschulte to look at.

[garyschulte]

0 or >=7 seems like a viable way to support both zero gas and "working" base fee networks 👍