Asset Transaction Payment (#488)

* use new pallet name based genesis config names

* use custom substrate and update polkadot

* add initial asset-tx-payment pallet

* update cargo.toml

* add (failing) tests

* dispatch Calls instead of using Pallet functions

* fix fee-refund split

* add test for transaction payment with tip

* update cargo.lock

* update cargo.lock

* remove mint workaround and use Mutable trait

* extract fee charging logic into OnChargeAssetTransaction trait

* use asset-tx-payment in statemint runtime

* make extrinsics public

* make extrinsics public

* use ChargeAssetIdOf type alias

* update deps

* move back to AssetIdOf

* remove extra rpc_http_threads

* use different substrate branch

* Update pallets/asset-tx-payment/src/payment.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Update pallets/asset-tx-payment/src/payment.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* remove overrides

* override substrate deps (again)

* increment spec_version and transaction_version (because we change transaction signing)

* remove direct dependency on pallet-balances from asset-tx-payment

* remove Assets pallet visibility workaround

* add docs and comments

* remove unused imports

* more docs

* add more debug asserts to document assumptions

* add test for tx payment from account with only assets

* add test for missing asset case

* extend test to cover non-sufficient assets

* add a test for Pays::No (refunded transaction)

* add type alias comments

* add more doc comments

* add asset-tx-payment to statemine and westmint

* improve formatting

* update license headers

* add default implementation of HandleCredit for ()

* update doc comments and format imports

* adjust Cargo.toml

* update cargo.lock

* cargo fmt

* cargo fmt

* cargo fmt

* cargo +nightly fmt

* add type alias for OnChargeTransaction

* cargo +nightly fmt

* convert ChargeAssetTxPayment from tuple struct to regular struct

* add more comments

* formatting

* adjust imports and comment

* cargo +nightly fmt

* reformat comment

* use ChargeTransactionPayment's own get_priority + update Substrate

* update Substrate and Polkadot

* cargo fmt

* cargo fmt

* add OperationalFeeMultiplier to asset tx payment tests

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* add doc links

* charge a minimum converted asset fee of 1 if the input fee is greater zero

* cargo +nightly fmt

* bump spec and transaction version

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

cumulus/Cargo.lock

@@ -5047,6 +5047,28 @@ dependencies = [
  "stable_deref_trait",
 ]
 
+[[package]]
+name = "pallet-asset-tx-payment"
+version = "0.1.0"
+dependencies = [
+ "frame-support",
+ "frame-system",
+ "pallet-assets",
+ "pallet-authorship",
+ "pallet-balances",
+ "pallet-transaction-payment",
+ "parity-scale-codec",
+ "scale-info",
+ "serde",
+ "serde_json",
+ "smallvec",
+ "sp-core",
+ "sp-io",
+ "sp-runtime",
+ "sp-std",
+ "sp-storage",
+]
+
 [[package]]
 name = "pallet-assets"
 version = "4.0.0-dev"
@@ -6114,6 +6136,8 @@ dependencies = [
  "frame-support",
  "frame-system",
  "node-primitives",
+ "pallet-asset-tx-payment",
+ "pallet-assets",
  "pallet-authorship",
  "pallet-balances",
  "pallet-collator-selection",
@@ -10724,6 +10748,7 @@ dependencies = [
  "hex-literal 0.3.3",
  "log",
  "node-primitives",
+ "pallet-asset-tx-payment",
  "pallet-assets",
  "pallet-aura",
  "pallet-authorship",
@@ -10789,6 +10814,7 @@ dependencies = [
  "hex-literal 0.3.3",
  "log",
  "node-primitives",
+ "pallet-asset-tx-payment",
  "pallet-assets",
  "pallet-aura",
  "pallet-authorship",
@@ -12225,6 +12251,7 @@ dependencies = [
  "hex-literal 0.3.3",
  "log",
  "node-primitives",
+ "pallet-asset-tx-payment",
  "pallet-assets",
  "pallet-aura",
  "pallet-authorship",

cumulus/Cargo.toml

@@ -7,6 +7,7 @@ members = [
 	"client/network",
 	"client/pov-recovery",
 	"client/service",
+	"pallets/asset-tx-payment",
 	"pallets/aura-ext",
 	"pallets/collator-selection",
 	"pallets/dmp-queue",

cumulus/pallets/asset-tx-payment/Cargo.toml

@@ -0,0 +1,51 @@
+[package]
+name = "pallet-asset-tx-payment"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "Apache-2.0"
+homepage = "https://substrate.dev"
+repository = "https://github.com/paritytech/cumulus/"
+description = "pallet to manage transaction payments in assets"
+readme = "README.md"
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
+
+[dependencies]
+# Substrate dependencies
+frame-support = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+frame-system = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+pallet-assets = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+pallet-authorship = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+pallet-transaction-payment = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-core = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-io = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-runtime = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-std = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+
+# Other dependencies
+codec = { package = "parity-scale-codec", version = "2.0.0", default-features = false, features = ["derive"] }
+scale-info = { version = "1.0", default-features = false, features = ["derive"] }
+serde = { version = "1.0.101", optional = true }
+
+[dev-dependencies]
+smallvec = "1.4.1"
+serde_json = "1.0.41"
+pallet-balances = { git = "https://github.com/paritytech/substrate", branch = "master" }
+sp-storage = { git = "https://github.com/paritytech/substrate", branch = "master" }
+
+[features]
+default = ["std"]
+std = [
+	"serde",
+	"codec/std",
+	"sp-std/std",
+	"sp-runtime/std",
+	"frame-support/std",
+	"frame-system/std",
+	"sp-io/std",
+	"sp-core/std",
+	"pallet-transaction-payment/std",
+]
+try-runtime = ["frame-support/try-runtime"]

cumulus/pallets/asset-tx-payment/src/lib.rs

@@ -0,0 +1,295 @@
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! # Asset Transaction Payment Pallet
+//!
+//! This pallet allows runtimes that include it to pay for transactions in assets other than the
+//! main token of the chain.
+//!
+//! ## Overview
+//! It does this by extending transactions to include an optional `AssetId` that specifies the asset
+//! to be used for payment (defaulting to the native token on `None`). It expects an
+//! [`OnChargeAssetTransaction`] implementation analogously to [`pallet-transaction-payment`]. The
+//! included [`FungiblesAdapter`] (implementing [`OnChargeAssetTransaction`]) determines the fee
+//! amount by converting the fee calculated by [`pallet-transaction-payment`] into the desired
+//! asset.
+//!
+//! ## Integration
+//! This pallet wraps FRAME's transaction payment pallet and functions as a replacement. This means
+//! you should include both pallets in your `construct_runtime` macro, but only include this
+//! pallet's [`SignedExtension`] ([`ChargeAssetTxPayment`]).
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use sp_std::prelude::*;
+
+use codec::{Decode, Encode};
+use frame_support::{
+	dispatch::DispatchResult,
+	traits::{
+		tokens::{
+			fungibles::{Balanced, CreditOf, Inspect},
+			WithdrawConsequence,
+		},
+		IsType,
+	},
+	weights::{DispatchInfo, PostDispatchInfo},
+	DefaultNoBound,
+};
+use pallet_transaction_payment::OnChargeTransaction;
+use scale_info::TypeInfo;
+use sp_runtime::{
+	traits::{DispatchInfoOf, Dispatchable, PostDispatchInfoOf, SignedExtension, Zero},
+	transaction_validity::{
+		InvalidTransaction, TransactionValidity, TransactionValidityError, ValidTransaction,
+	},
+	FixedPointOperand,
+};
+
+#[cfg(test)]
+mod tests;
+
+mod payment;
+pub use payment::*;
+
+// Type aliases used for interaction with `OnChargeTransaction`.
+pub(crate) type OnChargeTransactionOf<T> =
+	<T as pallet_transaction_payment::Config>::OnChargeTransaction;
+// Balance type alias.
+pub(crate) type BalanceOf<T> = <OnChargeTransactionOf<T> as OnChargeTransaction<T>>::Balance;
+// Liquity info type alias.
+pub(crate) type LiquidityInfoOf<T> =
+	<OnChargeTransactionOf<T> as OnChargeTransaction<T>>::LiquidityInfo;
+
+// Type alias used for interaction with fungibles (assets).
+// Balance type alias.
+pub(crate) type AssetBalanceOf<T> =
+	<<T as Config>::Fungibles as Inspect<<T as frame_system::Config>::AccountId>>::Balance;
+/// Asset id type alias.
+pub(crate) type AssetIdOf<T> =
+	<<T as Config>::Fungibles as Inspect<<T as frame_system::Config>::AccountId>>::AssetId;
+
+// Type aliases used for interaction with `OnChargeAssetTransaction`.
+// Balance type alias.
+pub(crate) type ChargeAssetBalanceOf<T> =
+	<<T as Config>::OnChargeAssetTransaction as OnChargeAssetTransaction<T>>::Balance;
+// Asset id type alias.
+pub(crate) type ChargeAssetIdOf<T> =
+	<<T as Config>::OnChargeAssetTransaction as OnChargeAssetTransaction<T>>::AssetId;
+// Liquity info type alias.
+pub(crate) type ChargeAssetLiquidityOf<T> =
+	<<T as Config>::OnChargeAssetTransaction as OnChargeAssetTransaction<T>>::LiquidityInfo;
+
+/// Used to pass the initial payment info from pre- to post-dispatch.
+#[derive(Encode, Decode, DefaultNoBound, TypeInfo)]
+pub enum InitialPayment<T: Config> {
+	/// No initial fee was payed.
+	Nothing,
+	/// The initial fee was payed in the native currency.
+	Native(LiquidityInfoOf<T>),
+	/// The initial fee was payed in an asset.
+	Asset(CreditOf<T::AccountId, T::Fungibles>),
+}
+
+pub use pallet::*;
+
+#[frame_support::pallet]
+pub mod pallet {
+	use super::*;
+
+	use frame_support::pallet_prelude::*;
+	use frame_system::pallet_prelude::*;
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config + pallet_transaction_payment::Config {
+		/// The fungibles instance used to pay for transactions in assets.
+		type Fungibles: Balanced<Self::AccountId>;
+		/// The actual transaction charging logic that charges the fees.
+		type OnChargeAssetTransaction: OnChargeAssetTransaction<Self>;
+	}
+
+	#[pallet::pallet]
+	#[pallet::generate_store(pub(super) trait Store)]
+	pub struct Pallet<T>(_);
+
+	#[pallet::hooks]
+	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}
+
+	#[pallet::call]
+	impl<T: Config> Pallet<T> {}
+}
+
+/// Require the transactor pay for themselves and maybe include a tip to gain additional priority
+/// in the queue. Allows paying via both `Currency` as well as `fungibles::Balanced`.
+///
+/// Wraps the transaction logic in [`pallet_transaction_payment`] and extends it with assets.
+/// An asset id of `None` falls back to the underlying transaction payment via the native currency.
+#[derive(Encode, Decode, Clone, Eq, PartialEq, TypeInfo)]
+#[scale_info(skip_type_params(T))]
+pub struct ChargeAssetTxPayment<T: Config> {
+	#[codec(compact)]
+	tip: BalanceOf<T>,
+	asset_id: Option<ChargeAssetIdOf<T>>,
+}
+
+impl<T: Config> ChargeAssetTxPayment<T>
+where
+	T::Call: Dispatchable<Info = DispatchInfo, PostInfo = PostDispatchInfo>,
+	AssetBalanceOf<T>: Send + Sync + FixedPointOperand,
+	BalanceOf<T>: Send + Sync + FixedPointOperand + IsType<ChargeAssetBalanceOf<T>>,
+	ChargeAssetIdOf<T>: Send + Sync,
+	CreditOf<T::AccountId, T::Fungibles>: IsType<ChargeAssetLiquidityOf<T>>,
+{
+	/// utility constructor. Used only in client/factory code.
+	pub fn from(tip: BalanceOf<T>, asset_id: Option<ChargeAssetIdOf<T>>) -> Self {
+		Self { tip, asset_id }
+	}
+
+	/// Fee withdrawal logic that dispatches to either `OnChargeAssetTransaction` or `OnChargeTransaction`.
+	fn withdraw_fee(
+		&self,
+		who: &T::AccountId,
+		call: &T::Call,
+		info: &DispatchInfoOf<T::Call>,
+		len: usize,
+	) -> Result<(BalanceOf<T>, InitialPayment<T>), TransactionValidityError> {
+		let fee = pallet_transaction_payment::Pallet::<T>::compute_fee(len as u32, info, self.tip);
+		debug_assert!(self.tip <= fee, "tip should be included in the computed fee");
+		if fee.is_zero() {
+			Ok((fee, InitialPayment::Nothing))
+		} else if let Some(asset_id) = self.asset_id {
+			T::OnChargeAssetTransaction::withdraw_fee(
+				who,
+				call,
+				info,
+				asset_id,
+				fee.into(),
+				self.tip.into(),
+			)
+			.map(|i| (fee, InitialPayment::Asset(i.into())))
+		} else {
+			<OnChargeTransactionOf<T> as OnChargeTransaction<T>>::withdraw_fee(
+				who, call, info, fee, self.tip,
+			)
+			.map(|i| (fee, InitialPayment::Native(i)))
+			.map_err(|_| -> TransactionValidityError { InvalidTransaction::Payment.into() })
+		}
+	}
+}
+
+impl<T: Config> sp_std::fmt::Debug for ChargeAssetTxPayment<T> {
+	#[cfg(feature = "std")]
+	fn fmt(&self, f: &mut sp_std::fmt::Formatter) -> sp_std::fmt::Result {
+		write!(f, "ChargeAssetTxPayment<{:?}, {:?}>", self.tip, self.asset_id.encode())
+	}
+	#[cfg(not(feature = "std"))]
+	fn fmt(&self, _: &mut sp_std::fmt::Formatter) -> sp_std::fmt::Result {
+		Ok(())
+	}
+}
+
+impl<T: Config> SignedExtension for ChargeAssetTxPayment<T>
+where
+	T::Call: Dispatchable<Info = DispatchInfo, PostInfo = PostDispatchInfo>,
+	AssetBalanceOf<T>: Send + Sync + FixedPointOperand,
+	BalanceOf<T>: Send + Sync + From<u64> + FixedPointOperand + IsType<ChargeAssetBalanceOf<T>>,
+	ChargeAssetIdOf<T>: Send + Sync,
+	CreditOf<T::AccountId, T::Fungibles>: IsType<ChargeAssetLiquidityOf<T>>,
+{
+	const IDENTIFIER: &'static str = "ChargeAssetTxPayment";
+	type AccountId = T::AccountId;
+	type Call = T::Call;
+	type AdditionalSigned = ();
+	type Pre = (
+		// tip
+		BalanceOf<T>,
+		// who paid the fee
+		Self::AccountId,
+		// imbalance resulting from withdrawing the fee
+		InitialPayment<T>,
+	);
+
+	fn additional_signed(&self) -> sp_std::result::Result<(), TransactionValidityError> {
+		Ok(())
+	}
+
+	fn validate(
+		&self,
+		who: &Self::AccountId,
+		call: &Self::Call,
+		info: &DispatchInfoOf<Self::Call>,
+		len: usize,
+	) -> TransactionValidity {
+		use pallet_transaction_payment::ChargeTransactionPayment;
+		let (fee, _) = self.withdraw_fee(who, call, info, len)?;
+		let priority = ChargeTransactionPayment::<T>::get_priority(info, len, self.tip, fee);
+		Ok(ValidTransaction { priority, ..Default::default() })
+	}
+
+	fn pre_dispatch(
+		self,
+		who: &Self::AccountId,
+		call: &Self::Call,
+		info: &DispatchInfoOf<Self::Call>,
+		len: usize,
+	) -> Result<Self::Pre, TransactionValidityError> {
+		let (_fee, initial_payment) = self.withdraw_fee(who, call, info, len)?;
+		Ok((self.tip, who.clone(), initial_payment))
+	}
+
+	fn post_dispatch(
+		pre: Self::Pre,
+		info: &DispatchInfoOf<Self::Call>,
+		post_info: &PostDispatchInfoOf<Self::Call>,
+		len: usize,
+		_result: &DispatchResult,
+	) -> Result<(), TransactionValidityError> {
+		let (tip, who, initial_payment) = pre;
+		let actual_fee = pallet_transaction_payment::Pallet::<T>::compute_actual_fee(
+			len as u32, info, post_info, tip,
+		);
+		match initial_payment {
+			InitialPayment::Native(already_withdrawn) => {
+				<OnChargeTransactionOf<T> as OnChargeTransaction<T>>::correct_and_deposit_fee(
+					&who,
+					info,
+					post_info,
+					actual_fee,
+					tip,
+					already_withdrawn,
+				)?;
+			},
+			InitialPayment::Asset(already_withdrawn) => {
+				T::OnChargeAssetTransaction::correct_and_deposit_fee(
+					&who,
+					info,
+					post_info,
+					actual_fee.into(),
+					tip.into(),
+					already_withdrawn.into(),
+				)?;
+			},
+			InitialPayment::Nothing => {
+				debug_assert!(
+					actual_fee.is_zero(),
+					"actual fee should be zero if initial fee was zero."
+				);
+				debug_assert!(tip.is_zero(), "tip should be zero if initial fee was zero.");
+			},
+		}
+
+		Ok(())
+	}
+}

cumulus/pallets/asset-tx-payment/src/payment.rs

@@ -0,0 +1,168 @@
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+///! Traits and default implementation for paying transaction fees in assets.
+use super::*;
+use crate::Config;
+
+use codec::FullCodec;
+use frame_support::{
+	traits::{
+		fungibles::{Balanced, CreditOf, Inspect},
+		tokens::BalanceConversion,
+	},
+	unsigned::TransactionValidityError,
+};
+use scale_info::TypeInfo;
+use sp_runtime::{
+	traits::{
+		AtLeast32BitUnsigned, DispatchInfoOf, MaybeSerializeDeserialize, One, PostDispatchInfoOf,
+	},
+	transaction_validity::InvalidTransaction,
+};
+use sp_std::{fmt::Debug, marker::PhantomData};
+
+/// Handle withdrawing, refunding and depositing of transaction fees.
+pub trait OnChargeAssetTransaction<T: Config> {
+	/// The underlying integer type in which fees are calculated.
+	type Balance: AtLeast32BitUnsigned
+		+ FullCodec
+		+ Copy
+		+ MaybeSerializeDeserialize
+		+ Debug
+		+ Default
+		+ TypeInfo;
+	/// The type used to identify the assets used for transaction payment.
+	type AssetId: FullCodec + Copy + MaybeSerializeDeserialize + Debug + Default + Eq + TypeInfo;
+	/// The type used to store the intermediate values between pre- and post-dispatch.
+	type LiquidityInfo;
+
+	/// Before the transaction is executed the payment of the transaction fees needs to be secured.
+	///
+	/// Note: The `fee` already includes the `tip`.
+	fn withdraw_fee(
+		who: &T::AccountId,
+		call: &T::Call,
+		dispatch_info: &DispatchInfoOf<T::Call>,
+		asset_id: Self::AssetId,
+		fee: Self::Balance,
+		tip: Self::Balance,
+	) -> Result<Self::LiquidityInfo, TransactionValidityError>;
+
+	/// After the transaction was executed the actual fee can be calculated.
+	/// This function should refund any overpaid fees and optionally deposit
+	/// the corrected amount.
+	///
+	/// Note: The `fee` already includes the `tip`.
+	fn correct_and_deposit_fee(
+		who: &T::AccountId,
+		dispatch_info: &DispatchInfoOf<T::Call>,
+		post_info: &PostDispatchInfoOf<T::Call>,
+		corrected_fee: Self::Balance,
+		tip: Self::Balance,
+		already_withdrawn: Self::LiquidityInfo,
+	) -> Result<(), TransactionValidityError>;
+}
+
+/// Allows specifying what to do with the withdrawn asset fees.
+pub trait HandleCredit<AccountId, B: Balanced<AccountId>> {
+	/// Implement to determine what to do with the withdrawn asset fees.
+	/// Default for `CreditOf` from the assets pallet is to burn and
+	/// decrease total issuance.
+	fn handle_credit(credit: CreditOf<AccountId, B>);
+}
+
+/// Default implementation that just drops the credit according to the `OnDrop` in the underlying
+/// imbalance type.
+impl<A, B: Balanced<A>> HandleCredit<A, B> for () {
+	fn handle_credit(_credit: CreditOf<A, B>) {}
+}
+
+/// Implements the asset transaction for a balance to asset converter (implementing
+/// [`BalanceConversion`]) and a credit handler (implementing [`HandleCredit`]).
+///
+/// The credit handler is given the complete fee in terms of the asset used for the transaction.
+pub struct FungiblesAdapter<CON, HC>(PhantomData<(CON, HC)>);
+
+/// Default implementation for a runtime instantiating this pallet, a balance to asset converter and
+/// a credit handler.
+impl<T, CON, HC> OnChargeAssetTransaction<T> for FungiblesAdapter<CON, HC>
+where
+	T: Config,
+	CON: BalanceConversion<BalanceOf<T>, AssetIdOf<T>, AssetBalanceOf<T>>,
+	HC: HandleCredit<T::AccountId, T::Fungibles>,
+	AssetIdOf<T>: FullCodec + Copy + MaybeSerializeDeserialize + Debug + Default + Eq + TypeInfo,
+{
+	type Balance = BalanceOf<T>;
+	type AssetId = AssetIdOf<T>;
+	type LiquidityInfo = CreditOf<T::AccountId, T::Fungibles>;
+
+	/// Withdraw the predicted fee from the transaction origin.
+	///
+	/// Note: The `fee` already includes the `tip`.
+	fn withdraw_fee(
+		who: &T::AccountId,
+		_call: &T::Call,
+		_info: &DispatchInfoOf<T::Call>,
+		asset_id: Self::AssetId,
+		fee: Self::Balance,
+		_tip: Self::Balance,
+	) -> Result<Self::LiquidityInfo, TransactionValidityError> {
+		// We don't know the precision of the underlying asset. Because the converted fee could be
+		// less than one (e.g. 0.5) but gets rounded down by integer division we introduce a minimum
+		// fee.
+		let min_converted_fee = if fee.is_zero() { Zero::zero() } else { One::one() };
+		let converted_fee = CON::to_asset_balance(fee, asset_id)
+			.map_err(|_| TransactionValidityError::from(InvalidTransaction::Payment))?
+			.max(min_converted_fee);
+		let can_withdraw = <T::Fungibles as Inspect<T::AccountId>>::can_withdraw(
+			asset_id.into(),
+			who,
+			converted_fee,
+		);
+		if !matches!(can_withdraw, WithdrawConsequence::Success) {
+			return Err(InvalidTransaction::Payment.into())
+		}
+		<T::Fungibles as Balanced<T::AccountId>>::withdraw(asset_id.into(), who, converted_fee)
+			.map_err(|_| TransactionValidityError::from(InvalidTransaction::Payment))
+	}
+
+	/// Hand the fee and the tip over to the `[HandleCredit]` implementation.
+	/// Since the predicted fee might have been too high, parts of the fee may be refunded.
+	///
+	/// Note: The `corrected_fee` already includes the `tip`.
+	fn correct_and_deposit_fee(
+		who: &T::AccountId,
+		_dispatch_info: &DispatchInfoOf<T::Call>,
+		_post_info: &PostDispatchInfoOf<T::Call>,
+		corrected_fee: Self::Balance,
+		_tip: Self::Balance,
+		paid: Self::LiquidityInfo,
+	) -> Result<(), TransactionValidityError> {
+		let min_converted_fee = if corrected_fee.is_zero() { Zero::zero() } else { One::one() };
+		// Convert the corrected fee into the asset used for payment.
+		let converted_fee = CON::to_asset_balance(corrected_fee, paid.asset().into())
+			.map_err(|_| -> TransactionValidityError { InvalidTransaction::Payment.into() })?
+			.max(min_converted_fee);
+		// Calculate how much refund we should return.
+		let (final_fee, refund) = paid.split(converted_fee);
+		// Refund to the account that paid the fees. If this fails, the account might have dropped
+		// below the existential balance. In that case we don't refund anything.
+		let _ = <T::Fungibles as Balanced<T::AccountId>>::resolve(who, refund);
+		// Handle the final fee, e.g. by transferring to the block author or burning.
+		HC::handle_credit(final_fee);
+		Ok(())
+	}
+}

cumulus/pallets/asset-tx-payment/src/tests.rs

@@ -0,0 +1,636 @@
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use super::*;
+use crate as pallet_asset_tx_payment;
+
+use frame_support::{
+	assert_ok,
+	pallet_prelude::*,
+	parameter_types,
+	traits::{fungibles::Mutate, FindAuthor},
+	weights::{
+		DispatchClass, DispatchInfo, PostDispatchInfo, Weight, WeightToFeeCoefficient,
+		WeightToFeeCoefficients, WeightToFeePolynomial,
+	},
+	ConsensusEngineId,
+};
+use frame_system as system;
+use frame_system::EnsureRoot;
+use pallet_balances::Call as BalancesCall;
+use pallet_transaction_payment::CurrencyAdapter;
+use smallvec::smallvec;
+use sp_core::H256;
+use sp_runtime::{
+	testing::Header,
+	traits::{BlakeTwo256, ConvertInto, IdentityLookup, StaticLookup},
+	Perbill,
+};
+use std::cell::RefCell;
+
+type UncheckedExtrinsic = frame_system::mocking::MockUncheckedExtrinsic<Runtime>;
+type Block = frame_system::mocking::MockBlock<Runtime>;
+type Balance = u64;
+type AccountId = u64;
+
+frame_support::construct_runtime!(
+	pub enum Runtime where
+		Block = Block,
+		NodeBlock = Block,
+		UncheckedExtrinsic = UncheckedExtrinsic,
+	{
+		System: system::{Pallet, Call, Config, Storage, Event<T>},
+		Balances: pallet_balances::{Pallet, Call, Storage, Config<T>, Event<T>},
+		TransactionPayment: pallet_transaction_payment::{Pallet, Storage},
+		Assets: pallet_assets::{Pallet, Call, Storage, Event<T>},
+		Authorship: pallet_authorship::{Pallet, Call, Storage},
+		AssetTxPayment: pallet_asset_tx_payment::{Pallet},
+	}
+);
+
+const CALL: &<Runtime as frame_system::Config>::Call =
+	&Call::Balances(BalancesCall::transfer { dest: 2, value: 69 });
+
+thread_local! {
+	static EXTRINSIC_BASE_WEIGHT: RefCell<u64> = RefCell::new(0);
+}
+
+pub struct BlockWeights;
+impl Get<frame_system::limits::BlockWeights> for BlockWeights {
+	fn get() -> frame_system::limits::BlockWeights {
+		frame_system::limits::BlockWeights::builder()
+			.base_block(0)
+			.for_class(DispatchClass::all(), |weights| {
+				weights.base_extrinsic = EXTRINSIC_BASE_WEIGHT.with(|v| *v.borrow()).into();
+			})
+			.for_class(DispatchClass::non_mandatory(), |weights| {
+				weights.max_total = 1024.into();
+			})
+			.build_or_panic()
+	}
+}
+
+parameter_types! {
+	pub const BlockHashCount: u64 = 250;
+	pub static TransactionByteFee: u64 = 1;
+	pub static WeightToFee: u64 = 1;
+}
+
+impl frame_system::Config for Runtime {
+	type BaseCallFilter = frame_support::traits::Everything;
+	type BlockWeights = BlockWeights;
+	type BlockLength = ();
+	type DbWeight = ();
+	type Origin = Origin;
+	type Index = u64;
+	type BlockNumber = u64;
+	type Call = Call;
+	type Hash = H256;
+	type Hashing = BlakeTwo256;
+	type AccountId = AccountId;
+	type Lookup = IdentityLookup<Self::AccountId>;
+	type Header = Header;
+	type Event = Event;
+	type BlockHashCount = BlockHashCount;
+	type Version = ();
+	type PalletInfo = PalletInfo;
+	type AccountData = pallet_balances::AccountData<u64>;
+	type OnNewAccount = ();
+	type OnKilledAccount = ();
+	type SystemWeightInfo = ();
+	type SS58Prefix = ();
+	type OnSetCode = ();
+}
+
+parameter_types! {
+	pub const ExistentialDeposit: u64 = 10;
+	pub const MaxReserves: u32 = 50;
+}
+
+impl pallet_balances::Config for Runtime {
+	type Balance = Balance;
+	type Event = Event;
+	type DustRemoval = ();
+	type ExistentialDeposit = ExistentialDeposit;
+	type AccountStore = System;
+	type MaxLocks = ();
+	type WeightInfo = ();
+	type MaxReserves = MaxReserves;
+	type ReserveIdentifier = [u8; 8];
+}
+
+impl WeightToFeePolynomial for WeightToFee {
+	type Balance = u64;
+
+	fn polynomial() -> WeightToFeeCoefficients<Self::Balance> {
+		smallvec![WeightToFeeCoefficient {
+			degree: 1,
+			coeff_frac: Perbill::zero(),
+			coeff_integer: WEIGHT_TO_FEE.with(|v| *v.borrow()),
+			negative: false,
+		}]
+	}
+}
+
+parameter_types! {
+	pub const OperationalFeeMultiplier: u8 = 5;
+}
+
+impl pallet_transaction_payment::Config for Runtime {
+	type OnChargeTransaction = CurrencyAdapter<Balances, ()>;
+	type TransactionByteFee = TransactionByteFee;
+	type WeightToFee = WeightToFee;
+	type FeeMultiplierUpdate = ();
+	type OperationalFeeMultiplier = OperationalFeeMultiplier;
+}
+
+parameter_types! {
+	pub const AssetDeposit: u64 = 2;
+	pub const MetadataDeposit: u64 = 0;
+	pub const StringLimit: u32 = 20;
+}
+
+impl pallet_assets::Config for Runtime {
+	type Event = Event;
+	type Balance = Balance;
+	type AssetId = u32;
+	type Currency = Balances;
+	type ForceOrigin = EnsureRoot<AccountId>;
+	type AssetDeposit = AssetDeposit;
+	type MetadataDepositBase = MetadataDeposit;
+	type MetadataDepositPerByte = MetadataDeposit;
+	type ApprovalDeposit = MetadataDeposit;
+	type StringLimit = StringLimit;
+	type Freezer = ();
+	type Extra = ();
+	type WeightInfo = ();
+}
+
+pub struct HardcodedAuthor;
+const BLOCK_AUTHOR: AccountId = 1234;
+impl FindAuthor<AccountId> for HardcodedAuthor {
+	fn find_author<'a, I>(_: I) -> Option<AccountId>
+	where
+		I: 'a + IntoIterator<Item = (ConsensusEngineId, &'a [u8])>,
+	{
+		Some(BLOCK_AUTHOR)
+	}
+}
+
+impl pallet_authorship::Config for Runtime {
+	type FindAuthor = HardcodedAuthor;
+	type UncleGenerations = ();
+	type FilterUncle = ();
+	type EventHandler = ();
+}
+
+pub struct CreditToBlockAuthor;
+impl HandleCredit<AccountId, Assets> for CreditToBlockAuthor {
+	fn handle_credit(credit: CreditOf<AccountId, Assets>) {
+		let author = pallet_authorship::Pallet::<Runtime>::author();
+		// TODO: what to do in case paying the author fails (e.g. because `fee < min_balance`)
+		// default: drop the result which will trigger the `OnDrop` of the imbalance.
+		let _ = <Assets as Balanced<AccountId>>::resolve(&author, credit);
+	}
+}
+
+impl Config for Runtime {
+	type Fungibles = Assets;
+	type OnChargeAssetTransaction = FungiblesAdapter<
+		pallet_assets::BalanceToAssetBalance<Balances, Runtime, ConvertInto>,
+		CreditToBlockAuthor,
+	>;
+}
+
+pub struct ExtBuilder {
+	balance_factor: u64,
+	base_weight: u64,
+	byte_fee: u64,
+	weight_to_fee: u64,
+}
+
+impl Default for ExtBuilder {
+	fn default() -> Self {
+		Self { balance_factor: 1, base_weight: 0, byte_fee: 1, weight_to_fee: 1 }
+	}
+}
+
+impl ExtBuilder {
+	pub fn base_weight(mut self, base_weight: u64) -> Self {
+		self.base_weight = base_weight;
+		self
+	}
+	pub fn balance_factor(mut self, factor: u64) -> Self {
+		self.balance_factor = factor;
+		self
+	}
+	fn set_constants(&self) {
+		EXTRINSIC_BASE_WEIGHT.with(|v| *v.borrow_mut() = self.base_weight);
+		TRANSACTION_BYTE_FEE.with(|v| *v.borrow_mut() = self.byte_fee);
+		WEIGHT_TO_FEE.with(|v| *v.borrow_mut() = self.weight_to_fee);
+	}
+	pub fn build(self) -> sp_io::TestExternalities {
+		self.set_constants();
+		let mut t = frame_system::GenesisConfig::default().build_storage::<Runtime>().unwrap();
+		pallet_balances::GenesisConfig::<Runtime> {
+			balances: if self.balance_factor > 0 {
+				vec![
+					(1, 10 * self.balance_factor),
+					(2, 20 * self.balance_factor),
+					(3, 30 * self.balance_factor),
+					(4, 40 * self.balance_factor),
+					(5, 50 * self.balance_factor),
+					(6, 60 * self.balance_factor),
+				]
+			} else {
+				vec![]
+			},
+		}
+		.assimilate_storage(&mut t)
+		.unwrap();
+		t.into()
+	}
+}
+
+/// create a transaction info struct from weight. Handy to avoid building the whole struct.
+pub fn info_from_weight(w: Weight) -> DispatchInfo {
+	// pays_fee: Pays::Yes -- class: DispatchClass::Normal
+	DispatchInfo { weight: w, ..Default::default() }
+}
+
+fn post_info_from_weight(w: Weight) -> PostDispatchInfo {
+	PostDispatchInfo { actual_weight: Some(w), pays_fee: Default::default() }
+}
+
+fn info_from_pays(p: Pays) -> DispatchInfo {
+	DispatchInfo { pays_fee: p, ..Default::default() }
+}
+
+fn post_info_from_pays(p: Pays) -> PostDispatchInfo {
+	PostDispatchInfo { actual_weight: None, pays_fee: p }
+}
+
+fn default_post_info() -> PostDispatchInfo {
+	PostDispatchInfo { actual_weight: None, pays_fee: Default::default() }
+}
+
+#[test]
+fn transaction_payment_in_native_possible() {
+	let balance_factor = 100;
+	ExtBuilder::default()
+		.balance_factor(balance_factor)
+		.base_weight(5)
+		.build()
+		.execute_with(|| {
+			let len = 10;
+			let pre = ChargeAssetTxPayment::<Runtime>::from(0, None)
+				.pre_dispatch(&1, CALL, &info_from_weight(5), len)
+				.unwrap();
+			let initial_balance = 10 * balance_factor;
+			assert_eq!(Balances::free_balance(1), initial_balance - 5 - 5 - 10);
+
+			assert_ok!(ChargeAssetTxPayment::<Runtime>::post_dispatch(
+				pre,
+				&info_from_weight(5),
+				&default_post_info(),
+				len,
+				&Ok(())
+			));
+			assert_eq!(Balances::free_balance(1), initial_balance - 5 - 5 - 10);
+
+			let pre = ChargeAssetTxPayment::<Runtime>::from(5 /* tipped */, None)
+				.pre_dispatch(&2, CALL, &info_from_weight(100), len)
+				.unwrap();
+			let initial_balance_for_2 = 20 * balance_factor;
+			assert_eq!(Balances::free_balance(2), initial_balance_for_2 - 5 - 10 - 100 - 5);
+
+			assert_ok!(ChargeAssetTxPayment::<Runtime>::post_dispatch(
+				pre,
+				&info_from_weight(100),
+				&post_info_from_weight(50),
+				len,
+				&Ok(())
+			));
+			assert_eq!(Balances::free_balance(2), initial_balance_for_2 - 5 - 10 - 50 - 5);
+		});
+}
+
+#[test]
+fn transaction_payment_in_asset_possible() {
+	let base_weight = 5;
+	let balance_factor = 100;
+	ExtBuilder::default()
+		.balance_factor(balance_factor)
+		.base_weight(base_weight)
+		.build()
+		.execute_with(|| {
+			// create the asset
+			let asset_id = 1;
+			let min_balance = 2;
+			assert_ok!(Assets::force_create(
+				Origin::root(),
+				asset_id,
+				42,   /* owner */
+				true, /* is_sufficient */
+				min_balance
+			));
+
+			// mint into the caller account
+			let caller = 1;
+			let beneficiary = <Runtime as system::Config>::Lookup::unlookup(caller);
+			let balance = 100;
+			assert_ok!(Assets::mint_into(asset_id, &beneficiary, balance));
+			assert_eq!(Assets::balance(asset_id, caller), balance);
+			let weight = 5;
+			let len = 10;
+			// we convert the from weight to fee based on the ratio between asset min balance and
+			// existential deposit
+			let fee = (base_weight + weight + len as u64) * min_balance / ExistentialDeposit::get();
+			let pre = ChargeAssetTxPayment::<Runtime>::from(0, Some(asset_id))
+				.pre_dispatch(&caller, CALL, &info_from_weight(weight), len)
+				.unwrap();
+			// assert that native balance is not used
+			assert_eq!(Balances::free_balance(caller), 10 * balance_factor);
+			// check that fee was charged in the given asset
+			assert_eq!(Assets::balance(asset_id, caller), balance - fee);
+			assert_eq!(Assets::balance(asset_id, BLOCK_AUTHOR), 0);
+
+			assert_ok!(ChargeAssetTxPayment::<Runtime>::post_dispatch(
+				pre,
+				&info_from_weight(weight),
+				&default_post_info(),
+				len,
+				&Ok(())
+			));
+			assert_eq!(Assets::balance(asset_id, caller), balance - fee);
+			// check that the block author gets rewarded
+			assert_eq!(Assets::balance(asset_id, BLOCK_AUTHOR), fee);
+		});
+}
+
+#[test]
+fn transaction_payment_without_fee() {
+	let base_weight = 5;
+	let balance_factor = 100;
+	ExtBuilder::default()
+		.balance_factor(balance_factor)
+		.base_weight(base_weight)
+		.build()
+		.execute_with(|| {
+			// create the asset
+			let asset_id = 1;
+			let min_balance = 2;
+			assert_ok!(Assets::force_create(
+				Origin::root(),
+				asset_id,
+				42,   /* owner */
+				true, /* is_sufficient */
+				min_balance
+			));
+
+			// mint into the caller account
+			let caller = 1;
+			let beneficiary = <Runtime as system::Config>::Lookup::unlookup(caller);
+			let balance = 100;
+			assert_ok!(Assets::mint_into(asset_id, &beneficiary, balance));
+			assert_eq!(Assets::balance(asset_id, caller), balance);
+			let weight = 5;
+			let len = 10;
+			// we convert the from weight to fee based on the ratio between asset min balance and
+			// existential deposit
+			let fee = (base_weight + weight + len as u64) * min_balance / ExistentialDeposit::get();
+			let pre = ChargeAssetTxPayment::<Runtime>::from(0, Some(asset_id))
+				.pre_dispatch(&caller, CALL, &info_from_weight(weight), len)
+				.unwrap();
+			// assert that native balance is not used
+			assert_eq!(Balances::free_balance(caller), 10 * balance_factor);
+			// check that fee was charged in the given asset
+			assert_eq!(Assets::balance(asset_id, caller), balance - fee);
+			assert_eq!(Assets::balance(asset_id, BLOCK_AUTHOR), 0);
+
+			assert_ok!(ChargeAssetTxPayment::<Runtime>::post_dispatch(
+				pre,
+				&info_from_weight(weight),
+				&post_info_from_pays(Pays::No),
+				len,
+				&Ok(())
+			));
+			// caller should be refunded
+			assert_eq!(Assets::balance(asset_id, caller), balance);
+			// check that the block author did not get rewarded
+			assert_eq!(Assets::balance(asset_id, BLOCK_AUTHOR), 0);
+		});
+}
+
+#[test]
+fn asset_transaction_payment_with_tip_and_refund() {
+	let base_weight = 5;
+	ExtBuilder::default()
+		.balance_factor(100)
+		.base_weight(base_weight)
+		.build()
+		.execute_with(|| {
+			// create the asset
+			let asset_id = 1;
+			let min_balance = 2;
+			assert_ok!(Assets::force_create(
+				Origin::root(),
+				asset_id,
+				42,   /* owner */
+				true, /* is_sufficient */
+				min_balance
+			));
+
+			// mint into the caller account
+			let caller = 2;
+			let beneficiary = <Runtime as system::Config>::Lookup::unlookup(caller);
+			let balance = 1000;
+			assert_ok!(Assets::mint_into(asset_id, &beneficiary, balance));
+			assert_eq!(Assets::balance(asset_id, caller), balance);
+			let weight = 100;
+			let tip = 5;
+			let len = 10;
+			// we convert the from weight to fee based on the ratio between asset min balance and
+			// existential deposit
+			let fee_with_tip =
+				(base_weight + weight + len as u64 + tip) * min_balance / ExistentialDeposit::get();
+			let pre = ChargeAssetTxPayment::<Runtime>::from(tip, Some(asset_id))
+				.pre_dispatch(&caller, CALL, &info_from_weight(weight), len)
+				.unwrap();
+			assert_eq!(Assets::balance(asset_id, caller), balance - fee_with_tip);
+
+			let final_weight = 50;
+			assert_ok!(ChargeAssetTxPayment::<Runtime>::post_dispatch(
+				pre,
+				&info_from_weight(weight),
+				&post_info_from_weight(final_weight),
+				len,
+				&Ok(())
+			));
+			let final_fee =
+				fee_with_tip - (weight - final_weight) * min_balance / ExistentialDeposit::get();
+			assert_eq!(Assets::balance(asset_id, caller), balance - (final_fee));
+			assert_eq!(Assets::balance(asset_id, BLOCK_AUTHOR), final_fee);
+		});
+}
+
+#[test]
+fn payment_from_account_with_only_assets() {
+	let base_weight = 5;
+	ExtBuilder::default()
+		.balance_factor(100)
+		.base_weight(base_weight)
+		.build()
+		.execute_with(|| {
+			// create the asset
+			let asset_id = 1;
+			let min_balance = 2;
+			assert_ok!(Assets::force_create(
+				Origin::root(),
+				asset_id,
+				42,   /* owner */
+				true, /* is_sufficient */
+				min_balance
+			));
+
+			// mint into the caller account
+			let caller = 333;
+			let beneficiary = <Runtime as system::Config>::Lookup::unlookup(caller);
+			let balance = 100;
+			assert_ok!(Assets::mint_into(asset_id, &beneficiary, balance));
+			assert_eq!(Assets::balance(asset_id, caller), balance);
+			// assert that native balance is not necessary
+			assert_eq!(Balances::free_balance(caller), 0);
+			let weight = 5;
+			let len = 10;
+			// we convert the from weight to fee based on the ratio between asset min balance and
+			// existential deposit
+			let fee = (base_weight + weight + len as u64) * min_balance / ExistentialDeposit::get();
+			let pre = ChargeAssetTxPayment::<Runtime>::from(0, Some(asset_id))
+				.pre_dispatch(&caller, CALL, &info_from_weight(weight), len)
+				.unwrap();
+			assert_eq!(Balances::free_balance(caller), 0);
+			// check that fee was charged in the given asset
+			assert_eq!(Assets::balance(asset_id, caller), balance - fee);
+
+			assert_ok!(ChargeAssetTxPayment::<Runtime>::post_dispatch(
+				pre,
+				&info_from_weight(weight),
+				&default_post_info(),
+				len,
+				&Ok(())
+			));
+			assert_eq!(Assets::balance(asset_id, caller), balance - fee);
+			assert_eq!(Balances::free_balance(caller), 0);
+		});
+}
+
+#[test]
+fn payment_only_with_existing_sufficient_asset() {
+	let base_weight = 5;
+	ExtBuilder::default()
+		.balance_factor(100)
+		.base_weight(base_weight)
+		.build()
+		.execute_with(|| {
+			let asset_id = 1;
+			let caller = 1;
+			let weight = 5;
+			let len = 10;
+			// pre_dispatch fails for non-existent asset
+			assert!(ChargeAssetTxPayment::<Runtime>::from(0, Some(asset_id))
+				.pre_dispatch(&caller, CALL, &info_from_weight(weight), len)
+				.is_err());
+
+			// create the non-sufficient asset
+			let min_balance = 2;
+			assert_ok!(Assets::force_create(
+				Origin::root(),
+				asset_id,
+				42,    /* owner */
+				false, /* is_sufficient */
+				min_balance
+			));
+			// pre_dispatch fails for non-sufficient asset
+			assert!(ChargeAssetTxPayment::<Runtime>::from(0, Some(asset_id))
+				.pre_dispatch(&caller, CALL, &info_from_weight(weight), len)
+				.is_err());
+		});
+}
+
+#[test]
+fn converted_fee_is_never_zero_if_input_fee_is_not() {
+	let base_weight = 1;
+	ExtBuilder::default()
+		.balance_factor(100)
+		.base_weight(base_weight)
+		.build()
+		.execute_with(|| {
+			// create the asset
+			let asset_id = 1;
+			let min_balance = 1;
+			assert_ok!(Assets::force_create(
+				Origin::root(),
+				asset_id,
+				42,   /* owner */
+				true, /* is_sufficient */
+				min_balance
+			));
+
+			// mint into the caller account
+			let caller = 333;
+			let beneficiary = <Runtime as system::Config>::Lookup::unlookup(caller);
+			let balance = 100;
+			assert_ok!(Assets::mint_into(asset_id, &beneficiary, balance));
+			assert_eq!(Assets::balance(asset_id, caller), balance);
+			let weight = 1;
+			let len = 1;
+			// we convert the from weight to fee based on the ratio between asset min balance and
+			// existential deposit
+			let fee = (base_weight + weight + len as u64) * min_balance / ExistentialDeposit::get();
+			// naive fee calculation would round down to zero
+			assert_eq!(fee, 0);
+			{
+				let pre = ChargeAssetTxPayment::<Runtime>::from(0, Some(asset_id))
+					.pre_dispatch(&caller, CALL, &info_from_pays(Pays::No), len)
+					.unwrap();
+				// `Pays::No` still implies no fees
+				assert_eq!(Assets::balance(asset_id, caller), balance);
+
+				assert_ok!(ChargeAssetTxPayment::<Runtime>::post_dispatch(
+					pre,
+					&info_from_pays(Pays::No),
+					&post_info_from_pays(Pays::No),
+					len,
+					&Ok(())
+				));
+				assert_eq!(Assets::balance(asset_id, caller), balance);
+			}
+			let pre = ChargeAssetTxPayment::<Runtime>::from(0, Some(asset_id))
+				.pre_dispatch(&caller, CALL, &info_from_weight(weight), len)
+				.unwrap();
+			// check that at least one coin was charged in the given asset
+			assert_eq!(Assets::balance(asset_id, caller), balance - 1);
+
+			assert_ok!(ChargeAssetTxPayment::<Runtime>::post_dispatch(
+				pre,
+				&info_from_weight(weight),
+				&default_post_info(),
+				len,
+				&Ok(())
+			));
+			assert_eq!(Assets::balance(asset_id, caller), balance - 1);
+		});
+}

cumulus/polkadot-parachains/parachains-common/Cargo.toml

@@ -20,6 +20,8 @@ sp-io = { git = 'https://github.com/paritytech/substrate', branch = "master", de
 frame-executive = { git = 'https://github.com/paritytech/substrate', branch = "master", default-features = false }
 frame-support = { git = 'https://github.com/paritytech/substrate', branch = "master", default-features = false }
 frame-system = { git = 'https://github.com/paritytech/substrate', branch = "master", default-features = false }
+pallet-assets = { git = 'https://github.com/paritytech/substrate', branch = "master", default-features = false }
+pallet-authorship = { git = 'https://github.com/paritytech/substrate', branch = "master", default-features = false }
 pallet-balances = { git = 'https://github.com/paritytech/substrate', branch = "master", default-features = false }
 sp-runtime = { git = 'https://github.com/paritytech/substrate', branch = "master", default-features = false }
 sp-core = { git = 'https://github.com/paritytech/substrate', branch = "master", default-features = false }
@@ -32,6 +34,7 @@ xcm = { git = 'https://github.com/paritytech/polkadot', branch = "master", defau
 xcm-executor = { git = 'https://github.com/paritytech/polkadot', branch = "master", default-features = false }
 
 # Local dependencies
+pallet-asset-tx-payment = { path = '../../pallets/asset-tx-payment', default-features = false }
 pallet-collator-selection = { path = '../../pallets/collator-selection', default-features = false }
 
 [dev-dependencies]
@@ -52,7 +55,10 @@ std = [
 	'frame-support/std',
 	'frame-executive/std',
 	'frame-system/std',
+	'pallet-asset-tx-payment/std',
 	'pallet-collator-selection/std',
+	'pallet-assets/std',
+	'pallet-authorship/std',
 	'pallet-balances/std',
 	'node-primitives/std',
 	'polkadot-runtime-common/std',

cumulus/polkadot-parachains/parachains-common/src/impls.rs

@@ -16,23 +16,31 @@
 //! Auxillary struct/enums for parachain runtimes.
 //! Taken from polkadot/runtime/common (at a21cd64) and adapted for parachains.
 
-use frame_support::traits::{fungibles, Contains, Currency, Get, Imbalance, OnUnbalanced};
+use frame_support::traits::{
+	fungibles::{self, Balanced, CreditOf},
+	Contains, Currency, Get, Imbalance, OnUnbalanced,
+};
+use pallet_asset_tx_payment::HandleCredit;
 use sp_runtime::traits::Zero;
 use sp_std::marker::PhantomData;
 use xcm::latest::{AssetId, Fungibility::Fungible, MultiAsset, MultiLocation};
 use xcm_executor::traits::FilterAssetLocation;
 
+/// Type alias to conveniently refer to the `Currency::NegativeImbalance` associated type.
 pub type NegativeImbalance<T> = <pallet_balances::Pallet<T> as Currency<
 	<T as frame_system::Config>::AccountId,
 >>::NegativeImbalance;
 
-/// Logic for the author to get a portion of fees.
+/// Type alias to conveniently refer to `frame_system`'s `Config::AccountId`.
-pub struct ToStakingPot<R>(sp_std::marker::PhantomData<R>);
+pub type AccountIdOf<R> = <R as frame_system::Config>::AccountId;
+
+/// Implementation of `OnUnbalanced` that deposits the fees into a staking pot for later payout.
+pub struct ToStakingPot<R>(PhantomData<R>);
 impl<R> OnUnbalanced<NegativeImbalance<R>> for ToStakingPot<R>
 where
 	R: pallet_balances::Config + pallet_collator_selection::Config,
-	<R as frame_system::Config>::AccountId: From<polkadot_primitives::v1::AccountId>,
+	AccountIdOf<R>:
-	<R as frame_system::Config>::AccountId: Into<polkadot_primitives::v1::AccountId>,
+		From<polkadot_primitives::v1::AccountId> + Into<polkadot_primitives::v1::AccountId>,
 	<R as frame_system::Config>::Event: From<pallet_balances::Event<R>>,
 {
 	fn on_nonzero_unbalanced(amount: NegativeImbalance<R>) {
@@ -46,13 +54,14 @@ where
 	}
 }
 
-/// Merge the fees into one item and pass them on to the staking pot.
+/// Implementation of `OnUnbalanced` that deals with the fees by combining tip and fee and passing
-pub struct DealWithFees<R>(sp_std::marker::PhantomData<R>);
+/// the result on to `ToStakingPot`.
+pub struct DealWithFees<R>(PhantomData<R>);
 impl<R> OnUnbalanced<NegativeImbalance<R>> for DealWithFees<R>
 where
 	R: pallet_balances::Config + pallet_collator_selection::Config,
-	<R as frame_system::Config>::AccountId: From<polkadot_primitives::v1::AccountId>,
+	AccountIdOf<R>:
-	<R as frame_system::Config>::AccountId: Into<polkadot_primitives::v1::AccountId>,
+		From<polkadot_primitives::v1::AccountId> + Into<polkadot_primitives::v1::AccountId>,
 	<R as frame_system::Config>::Event: From<pallet_balances::Event<R>>,
 {
 	fn on_unbalanceds<B>(mut fees_then_tips: impl Iterator<Item = NegativeImbalance<R>>) {
@@ -65,6 +74,22 @@ where
 	}
 }
 
+/// A `HandleCredit` implementation that naively transfers the fees to the block author.
+/// Will drop and burn the assets in case the transfer fails.
+pub struct AssetsToBlockAuthor<R>(PhantomData<R>);
+impl<R> HandleCredit<AccountIdOf<R>, pallet_assets::Pallet<R>> for AssetsToBlockAuthor<R>
+where
+	R: pallet_authorship::Config + pallet_assets::Config,
+	AccountIdOf<R>:
+		From<polkadot_primitives::v1::AccountId> + Into<polkadot_primitives::v1::AccountId>,
+{
+	fn handle_credit(credit: CreditOf<AccountIdOf<R>, pallet_assets::Pallet<R>>) {
+		let author = pallet_authorship::Pallet::<R>::author();
+		// In case of error: Will drop the result triggering the `OnDrop` of the imbalance.
+		let _ = pallet_assets::Pallet::<R>::resolve(&author, credit);
+	}
+}
+
 /// Allow checking in assets that have issuance > 0.
 pub struct NonZeroIssuance<AccountId, Assets>(PhantomData<(AccountId, Assets)>);
 impl<AccountId, Assets> Contains<<Assets as fungibles::Inspect<AccountId>>::AssetId>

cumulus/polkadot-parachains/statemine/Cargo.toml

@@ -62,6 +62,7 @@ cumulus-pallet-xcmp-queue = { path = "../../pallets/xcmp-queue", default-feature
 cumulus-pallet-xcm = { path = "../../pallets/xcm", default-features = false }
 cumulus-pallet-session-benchmarking = {path = "../../pallets/session-benchmarking", default-features = false, version = "3.0.0"}
 cumulus-ping = { path = "../pallets/ping", default-features = false }
+pallet-asset-tx-payment = { path = "../../pallets/asset-tx-payment", default-features = false }
 pallet-collator-selection = { path = "../../pallets/collator-selection", default-features = false }
 parachains-common = { path = "../parachains-common", default-features = false }
 
@@ -136,6 +137,7 @@ std = [
 	"pallet-utility/std",
 	"parachain-info/std",
 	"cumulus-pallet-aura-ext/std",
+	"pallet-asset-tx-payment/std",
 	"pallet-collator-selection/std",
 	"cumulus-pallet-dmp-queue/std",
 	"cumulus-pallet-parachain-system/std",

cumulus/polkadot-parachains/statemine/src/lib.rs

@@ -29,7 +29,7 @@ use sp_api::impl_runtime_apis;
 use sp_core::{crypto::KeyTypeId, OpaqueMetadata};
 use sp_runtime::{
 	create_runtime_str, generic, impl_opaque_keys,
-	traits::{AccountIdLookup, BlakeTwo256, Block as BlockT},
+	traits::{AccountIdLookup, BlakeTwo256, Block as BlockT, ConvertInto},
 	transaction_validity::{TransactionSource, TransactionValidity},
 	ApplyExtrinsicResult,
 };
@@ -56,7 +56,7 @@ use frame_system::{
 };
 pub use parachains_common as common;
 use parachains_common::{
-	impls::{DealWithFees, NonZeroIssuance},
+	impls::{AssetsToBlockAuthor, DealWithFees, NonZeroIssuance},
 	opaque, AccountId, AssetId, AuraId, Balance, BlockNumber, Hash, Header, Index, Signature,
 	AVERAGE_ON_INITIALIZE_RATIO, HOURS, MAXIMUM_BLOCK_WEIGHT, NORMAL_DISPATCH_RATIO, SLOT_DURATION,
 };
@@ -90,10 +90,10 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("statemine"),
 	impl_name: create_runtime_str!("statemine"),
 	authoring_version: 1,
-	spec_version: 4,
+	spec_version: 5,
 	impl_version: 0,
 	apis: RUNTIME_API_VERSIONS,
-	transaction_version: 2,
+	transaction_version: 3,
 };
 
 /// The version information used to identify this runtime when compiled natively.
@@ -693,6 +693,14 @@ impl pallet_collator_selection::Config for Runtime {
 	type WeightInfo = weights::pallet_collator_selection::WeightInfo<Runtime>;
 }
 
+impl pallet_asset_tx_payment::Config for Runtime {
+	type Fungibles = Assets;
+	type OnChargeAssetTransaction = pallet_asset_tx_payment::FungiblesAdapter<
+		pallet_assets::BalanceToAssetBalance<Balances, Runtime, ConvertInto>,
+		AssetsToBlockAuthor<Runtime>,
+	>;
+}
+
 // Create the runtime by composing the FRAME pallets that were previously configured.
 construct_runtime!(
 	pub enum Runtime where
@@ -712,6 +720,7 @@ construct_runtime!(
 		// Monetary stuff.
 		Balances: pallet_balances::{Pallet, Call, Storage, Config<T>, Event<T>} = 10,
 		TransactionPayment: pallet_transaction_payment::{Pallet, Storage} = 11,
+		AssetTxPayment: pallet_asset_tx_payment::{Pallet} = 12,
 
 		// Collator support. the order of these 4 are important and shall not change.
 		Authorship: pallet_authorship::{Pallet, Call, Storage} = 20,
@@ -753,7 +762,7 @@ pub type SignedExtra = (
 	frame_system::CheckEra<Runtime>,
 	frame_system::CheckNonce<Runtime>,
 	frame_system::CheckWeight<Runtime>,
-	pallet_transaction_payment::ChargeTransactionPayment<Runtime>,
+	pallet_asset_tx_payment::ChargeAssetTxPayment<Runtime>,
 );
 /// Unchecked extrinsic type as expected by this runtime.
 pub type UncheckedExtrinsic = generic::UncheckedExtrinsic<Address, Call, Signature, SignedExtra>;

cumulus/polkadot-parachains/statemint/Cargo.toml

@@ -62,6 +62,7 @@ cumulus-pallet-xcmp-queue = { path = "../../pallets/xcmp-queue", default-feature
 cumulus-pallet-xcm = { path = "../../pallets/xcm", default-features = false }
 cumulus-pallet-session-benchmarking = { path = "../../pallets/session-benchmarking", default-features = false, version = "3.0.0" }
 cumulus-ping = { path = "../pallets/ping", default-features = false }
+pallet-asset-tx-payment = { path = "../../pallets/asset-tx-payment", default-features = false }
 pallet-collator-selection = { path = "../../pallets/collator-selection", default-features = false }
 parachains-common = { path = "../parachains-common", default-features = false }
 
@@ -136,6 +137,7 @@ std = [
 	"pallet-utility/std",
 	"parachain-info/std",
 	"cumulus-pallet-aura-ext/std",
+	"pallet-asset-tx-payment/std",
 	"pallet-collator-selection/std",
 	"cumulus-pallet-dmp-queue/std",
 	"cumulus-pallet-parachain-system/std",

cumulus/polkadot-parachains/statemint/src/lib.rs

@@ -29,7 +29,7 @@ use sp_api::impl_runtime_apis;
 use sp_core::{crypto::KeyTypeId, OpaqueMetadata};
 use sp_runtime::{
 	create_runtime_str, generic, impl_opaque_keys,
-	traits::{AccountIdLookup, BlakeTwo256, Block as BlockT},
+	traits::{AccountIdLookup, BlakeTwo256, Block as BlockT, ConvertInto},
 	transaction_validity::{TransactionSource, TransactionValidity},
 	ApplyExtrinsicResult,
 };
@@ -56,7 +56,7 @@ use frame_system::{
 };
 pub use parachains_common as common;
 use parachains_common::{
-	impls::{DealWithFees, NonZeroIssuance},
+	impls::{AssetsToBlockAuthor, DealWithFees, NonZeroIssuance},
 	opaque, AccountId, AssetId, AuraId, Balance, BlockNumber, Hash, Header, Index, Signature,
 	AVERAGE_ON_INITIALIZE_RATIO, HOURS, MAXIMUM_BLOCK_WEIGHT, NORMAL_DISPATCH_RATIO, SLOT_DURATION,
 };
@@ -90,10 +90,10 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("statemint"),
 	impl_name: create_runtime_str!("statemint"),
 	authoring_version: 1,
-	spec_version: 1,
+	spec_version: 2,
 	impl_version: 1,
 	apis: RUNTIME_API_VERSIONS,
-	transaction_version: 1,
+	transaction_version: 2,
 };
 
 /// The version information used to identify this runtime when compiled natively.
@@ -656,6 +656,14 @@ impl pallet_collator_selection::Config for Runtime {
 	type WeightInfo = weights::pallet_collator_selection::WeightInfo<Runtime>;
 }
 
+impl pallet_asset_tx_payment::Config for Runtime {
+	type Fungibles = Assets;
+	type OnChargeAssetTransaction = pallet_asset_tx_payment::FungiblesAdapter<
+		pallet_assets::BalanceToAssetBalance<Balances, Runtime, ConvertInto>,
+		AssetsToBlockAuthor<Runtime>,
+	>;
+}
+
 parameter_types! {
 	pub const ClassDeposit: Balance = UNITS; // 1 UNIT deposit to create asset class
 	pub const InstanceDeposit: Balance = UNITS / 100; // 1/100 UNIT deposit to create asset instance
@@ -703,6 +711,7 @@ construct_runtime!(
 		// Monetary stuff.
 		Balances: pallet_balances::{Pallet, Call, Storage, Config<T>, Event<T>} = 10,
 		TransactionPayment: pallet_transaction_payment::{Pallet, Storage} = 11,
+		AssetTxPayment: pallet_asset_tx_payment::{Pallet} = 12,
 
 		// Collator support. the order of these 4 are important and shall not change.
 		Authorship: pallet_authorship::{Pallet, Call, Storage} = 20,
@@ -744,7 +753,7 @@ pub type SignedExtra = (
 	frame_system::CheckEra<Runtime>,
 	frame_system::CheckNonce<Runtime>,
 	frame_system::CheckWeight<Runtime>,
-	pallet_transaction_payment::ChargeTransactionPayment<Runtime>,
+	pallet_asset_tx_payment::ChargeAssetTxPayment<Runtime>,
 );
 /// Unchecked extrinsic type as expected by this runtime.
 pub type UncheckedExtrinsic = generic::UncheckedExtrinsic<Address, Call, Signature, SignedExtra>;

cumulus/polkadot-parachains/westmint/Cargo.toml

@@ -62,6 +62,7 @@ cumulus-pallet-xcmp-queue = { path = "../../pallets/xcmp-queue", default-feature
 cumulus-pallet-xcm = { path = "../../pallets/xcm", default-features = false }
 cumulus-pallet-session-benchmarking = {path = "../../pallets/session-benchmarking", default-features = false, version = "3.0.0"}
 cumulus-ping = { path = "../pallets/ping", default-features = false }
+pallet-asset-tx-payment = { path = "../../pallets/asset-tx-payment", default-features = false }
 pallet-collator-selection = { path = "../../pallets/collator-selection", default-features = false }
 parachains-common = { path = "../parachains-common", default-features = false }
 
@@ -136,6 +137,7 @@ std = [
 	"pallet-utility/std",
 	"parachain-info/std",
 	"cumulus-pallet-aura-ext/std",
+	"pallet-asset-tx-payment/std",
 	"pallet-collator-selection/std",
 	"cumulus-pallet-dmp-queue/std",
 	"cumulus-pallet-parachain-system/std",

cumulus/polkadot-parachains/westmint/src/lib.rs

@@ -29,7 +29,7 @@ use sp_api::impl_runtime_apis;
 use sp_core::{crypto::KeyTypeId, OpaqueMetadata};
 use sp_runtime::{
 	create_runtime_str, generic, impl_opaque_keys,
-	traits::{AccountIdLookup, BlakeTwo256, Block as BlockT},
+	traits::{AccountIdLookup, BlakeTwo256, Block as BlockT, ConvertInto},
 	transaction_validity::{TransactionSource, TransactionValidity},
 	ApplyExtrinsicResult,
 };
@@ -56,7 +56,7 @@ use frame_system::{
 };
 pub use parachains_common as common;
 use parachains_common::{
-	impls::{DealWithFees, NonZeroIssuance},
+	impls::{AssetsToBlockAuthor, DealWithFees, NonZeroIssuance},
 	opaque, AccountId, AssetId, AuraId, Balance, BlockNumber, Hash, Header, Index, Signature,
 	AVERAGE_ON_INITIALIZE_RATIO, HOURS, MAXIMUM_BLOCK_WEIGHT, NORMAL_DISPATCH_RATIO, SLOT_DURATION,
 };
@@ -90,10 +90,10 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("westmint"),
 	impl_name: create_runtime_str!("westmint"),
 	authoring_version: 1,
-	spec_version: 4,
+	spec_version: 5,
 	impl_version: 0,
 	apis: RUNTIME_API_VERSIONS,
-	transaction_version: 2,
+	transaction_version: 3,
 };
 
 /// The version information used to identify this runtime when compiled natively.
@@ -648,6 +648,14 @@ impl pallet_collator_selection::Config for Runtime {
 	type WeightInfo = weights::pallet_collator_selection::WeightInfo<Runtime>;
 }
 
+impl pallet_asset_tx_payment::Config for Runtime {
+	type Fungibles = Assets;
+	type OnChargeAssetTransaction = pallet_asset_tx_payment::FungiblesAdapter<
+		pallet_assets::BalanceToAssetBalance<Balances, Runtime, ConvertInto>,
+		AssetsToBlockAuthor<Runtime>,
+	>;
+}
+
 parameter_types! {
 	pub const ClassDeposit: Balance = UNITS; // 1 UNIT deposit to create asset class
 	pub const InstanceDeposit: Balance = UNITS / 100; // 1/100 UNIT deposit to create asset instance
@@ -718,6 +726,9 @@ construct_runtime!(
 
 		// More things for the main stage
 		Uniques: pallet_uniques::{Pallet, Call, Storage, Event<T>},
+
+		// More Monetary stuff
+		AssetTxPayment: pallet_asset_tx_payment::{Pallet},
 	}
 );
 
@@ -737,7 +748,7 @@ pub type SignedExtra = (
 	frame_system::CheckEra<Runtime>,
 	frame_system::CheckNonce<Runtime>,
 	frame_system::CheckWeight<Runtime>,
-	pallet_transaction_payment::ChargeTransactionPayment<Runtime>,
+	pallet_asset_tx_payment::ChargeAssetTxPayment<Runtime>,
 );
 /// Unchecked extrinsic type as expected by this runtime.
 pub type UncheckedExtrinsic = generic::UncheckedExtrinsic<Address, Call, Signature, SignedExtra>;