Introduce EVMFungibleAdapter (polkadot-evm#1282) (#22)

* new EVMFungibleAdapter struct

* Update comment

Co-authored-by: Ahmad Kaouk <56095276+ahmadkaouk@users.noreply.github.com>

frame/evm/src/lib.rs

@@ -77,11 +77,12 @@ use frame_support::{
 	dispatch::{DispatchResultWithPostInfo, Pays, PostDispatchInfo},
 	storage::child::KillStorageResult,
 	traits::{
+		fungible::{Balanced, Credit, Debt},
 		tokens::{
 			currency::Currency,
 			fungible::Inspect,
 			imbalance::{Imbalance, OnUnbalanced, SignedImbalance},
-			ExistenceRequirement, Fortitude, Preservation, WithdrawReasons,
+			ExistenceRequirement, Fortitude, Precision, Preservation, WithdrawReasons,
 		},
 		FindAuthor, Get, Time,
 	},
@@ -1037,6 +1038,79 @@ where
 		}
 	}
 }
+/// Implements transaction payment for a pallet implementing the [`fungible`]
+/// trait (eg. pallet_balances) using an unbalance handler (implementing
+/// [`OnUnbalanced`]).
+///
+/// Equivalent of `EVMCurrencyAdapter` but for fungible traits. Similar to `FungibleAdapter` of
+/// `pallet_transaction_payment`
+pub struct EVMFungibleAdapter<F, OU>(sp_std::marker::PhantomData<(F, OU)>);
+
+impl<T, F, OU> OnChargeEVMTransaction<T> for EVMFungibleAdapter<F, OU>
+where
+	T: Config,
+	F: Balanced<T::AccountId>,
+	OU: OnUnbalanced<Credit<T::AccountId, F>>,
+	U256: UniqueSaturatedInto<<F as Inspect<<T as frame_system::Config>::AccountId>>::Balance>,
+{
+	// Kept type as Option to satisfy bound of Default
+	type LiquidityInfo = Option<Credit<T::AccountId, F>>;
+
+	fn withdraw_fee(who: &H160, fee: U256) -> Result<Self::LiquidityInfo, Error<T>> {
+		if fee.is_zero() {
+			return Ok(None);
+		}
+		let account_id = T::AddressMapping::into_account_id(*who);
+		let imbalance = F::withdraw(
+			&account_id,
+			fee.unique_saturated_into(),
+			Precision::Exact,
+			Preservation::Preserve,
+			Fortitude::Polite,
+		)
+		.map_err(|_| Error::<T>::BalanceLow)?;
+		Ok(Some(imbalance))
+	}
+
+	fn correct_and_deposit_fee(
+		who: &H160,
+		corrected_fee: U256,
+		base_fee: U256,
+		already_withdrawn: Self::LiquidityInfo,
+	) -> Self::LiquidityInfo {
+		if let Some(paid) = already_withdrawn {
+			let account_id = T::AddressMapping::into_account_id(*who);
+
+			// Calculate how much refund we should return
+			let refund_amount = paid
+				.peek()
+				.saturating_sub(corrected_fee.unique_saturated_into());
+			// refund to the account that paid the fees.
+			let refund_imbalance = F::deposit(&account_id, refund_amount, Precision::BestEffort)
+				.unwrap_or_else(|_| Debt::<T::AccountId, F>::zero());
+
+			// merge the imbalance caused by paying the fees and refunding parts of it again.
+			let adjusted_paid = paid
+				.offset(refund_imbalance)
+				.same()
+				.unwrap_or_else(|_| Credit::<T::AccountId, F>::zero());
+
+			let (base_fee, tip) = adjusted_paid.split(base_fee.unique_saturated_into());
+			// Handle base fee. Can be either burned, rationed, etc ...
+			OU::on_unbalanced(base_fee);
+			return Some(tip);
+		}
+		None
+	}
+
+	fn pay_priority_fee(tip: Self::LiquidityInfo) {
+		// Default Ethereum behaviour: issue the tip to the block author.
+		if let Some(tip) = tip {
+			let account_id = T::AddressMapping::into_account_id(<Pallet<T>>::find_author());
+			let _ = F::deposit(&account_id, tip.peek(), Precision::BestEffort);
+		}
+	}
+}
 
 /// Implementation for () does not specify what to do with imbalance
 impl<T> OnChargeEVMTransaction<T> for ()