Ethereum Transact Xcm

[tgmichel]

This PR introduces a new transact_xcm dispatchable.

Co-authored by @girazoki

Relates to

@nanocryk 's paritytech/polkadot#5657

Motivation

• Enable Transact Xcm to pallet-ethereum.
• Store Xcm Ethereum transactions as part of the Ethereum block.

Origin and account

The origin parachain A can send a Transact Xcm to Frontier-enabled parachain B using a MultiLocation origin extended with a DescendOrigin instruction. The resultant Hash for this MultiLocation is the sender's controlled account in parachain B. This way, the sender's controlled account in parachain B is as-if it was the only possible derivation from the sender's controlled account in parachain A.

• A new XcmEthereumTransaction variant is added to RawOrigin and we ensure this to be the origin when allowing calls to the new dispatchable.
• Derived account is privatekey-less and thus only meant to be used as the origin through Xcm.

Check and validate

Multiple Xcm fragments are part of the same inherent extrinsic, and current UncheckedExtrinsic::check and CheckedExtrinsic::validate implementations - which are part of transaction pool submission and validation - will not be executed.

Ethereum Xcm transactions are instead validated directly within the dispatchable body using the same logic as the rest of the existing code paths.

Xcm payload to transaction conversion

transact_xcm input contains all the data necessary to perform a conversion to ethereum::TransactionV2 type.

pub struct EthereumXcmTransactionV1 {
	/// Gas limit to be consumed by EVM execution.
	pub gas_limit: U256,
	/// Fee configuration of choice.
	pub fee_payment: EthereumXcmFee,
	/// Either a Call (the callee, account or contract address) or Create (currently unsupported).
	pub action: TransactionAction,
	/// Value to be transfered.
	pub value: U256,
	/// Input data for a contract call.
	pub input: Vec<u8>,
	/// Map of addresses to be pre-paid to warm storage.
	pub access_list: Option<Vec<(H160, Vec<H256>)>>,
}

EthereumXcmFee is a custom enum meant to provide two options to build the message in the origin chain:

• ManualEthereumXcmFee: fee is know in advance, and can be used to build Legacy, EIP2930 and EIP1559 transaction types.
• AutoEthereumXcmFee: fee is not known in advance, and allows to choose from Low, Medium and High pre-configured gas fees. We introduce this option because cross-chain messages will be processed in a future unknown block, and users might want to execute the transaction using the (potentially congestion-based) on-chain fee rate.

The EthereumXcmTransaction is converted to a TransactionV2, populating signature fields with bogus data. We can do this because we assume:
1. recover_signer is not needed - which is part of promoting an UncheckedExtrinsic to a CheckedExtrinsic.
2. We already know the parachain A sender controls the derived account in parachain B.
3. Current chain id is used.
4. The next nonce for the derived account will be used to execute the transaction.

Restrictions

Currently we deliberately disabled TransactionAction::Create. The reasons are:

• Input data for contract creation likely exceeds allowed Xcm size.
• We don't see a practical use case for it anyways.

[tgmichel]

Closing by now. #741 does some minor refactor so external code can interface with pallet-ethereum.