Add a linear fee multiplier (#127) (#3790)

Bridging fees are calculated using a static ETH/DOT exchange rate that
can deviate significantly from the real-world exchange rate. We
therefore need to add a safety margin to the fee so that users almost
aways cover the cost of relaying.

# FAQ

> Why introduce a `multiplier` parameter instead of configuring an
exchange rate which already has a safety factor applied?

When converting from ETH to DOT, we need to _divide_ the multiplier by
the exchange rate, and to convert from DOT to ETH we need to _multiply_
the multiplier by the exchange rate.

> Other input parameters to the fee calculation can also deviate from
real-world values. These include substrate weights, gas prices, and so
on. Why does the multiplier introduced here not adjust those?

A single scalar multiplier won't be able to accommodate the different
volatilities efficiently. For example, gas prices are much more volatile
than exchange rates, and substrate weights hardly ever change.

So the pricing config relating to weights and gas prices should already
have some appropriate safety margin pre-applied.

# Detailed Changes:

* Added `multiplier` field to `PricingParameters`
* Outbound-queue fee is multiplied by `multiplier`
* This `multiplier` is synced to the Ethereum side
* Improved Runtime API for calculating outbound-queue fees. This API
makes it much easier to for configure parts of the system in preparation
for launch.
* Improve and clarify code documentation

Upstreamed from https://github.com/Snowfork/polkadot-sdk/pull/127

---------

Co-authored-by: Clara van Staden <claravanstaden64@gmail.com>
Co-authored-by: Adrian Catangiu <adrian@parity.io>

bridges/snowbridge/pallets/inbound-queue/src/mock.rs

@@ -173,7 +173,8 @@ parameter_types! {
 	pub Parameters: PricingParameters<u128> = PricingParameters {
 		exchange_rate: FixedU128::from_rational(1, 400),
 		fee_per_gas: gwei(20),
-		rewards: Rewards { local: DOT, remote: meth(1) }
+		rewards: Rewards { local: DOT, remote: meth(1) },
+		multiplier: FixedU128::from_rational(1, 1),
 	};
 }
 

bridges/snowbridge/pallets/outbound-queue/runtime-api/src/lib.rs

@@ -3,7 +3,10 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 
 use frame_support::traits::tokens::Balance as BalanceT;
-use snowbridge_core::outbound::Message;
+use snowbridge_core::{
+	outbound::{Command, Fee},
+	PricingParameters,
+};
 use snowbridge_outbound_queue_merkle_tree::MerkleProof;
 
 sp_api::decl_runtime_apis! {
@@ -11,10 +14,10 @@ sp_api::decl_runtime_apis! {
 	{
 		/// Generate a merkle proof for a committed message identified by `leaf_index`.
 		/// The merkle root is stored in the block header as a
-		/// `\[`sp_runtime::generic::DigestItem::Other`\]`
+		/// `sp_runtime::generic::DigestItem::Other`
 		fn prove_message(leaf_index: u64) -> Option<MerkleProof>;
 
-		/// Calculate the delivery fee for `message`
-		fn calculate_fee(message: Message) -> Option<Balance>;
+		/// Calculate the delivery fee for `command`
+		fn calculate_fee(command: Command, parameters: Option<PricingParameters<Balance>>) -> Fee<Balance>;
 	}
 }

bridges/snowbridge/pallets/outbound-queue/src/api.rs

@@ -4,8 +4,12 @@
 
 use crate::{Config, MessageLeaves};
 use frame_support::storage::StorageStreamIter;
-use snowbridge_core::outbound::{Message, SendMessage};
+use snowbridge_core::{
+	outbound::{Command, Fee, GasMeter},
+	PricingParameters,
+};
 use snowbridge_outbound_queue_merkle_tree::{merkle_proof, MerkleProof};
+use sp_core::Get;
 
 pub fn prove_message<T>(leaf_index: u64) -> Option<MerkleProof>
 where
@@ -19,12 +23,14 @@ where
 	Some(proof)
 }
 
-pub fn calculate_fee<T>(message: Message) -> Option<T::Balance>
+pub fn calculate_fee<T>(
+	command: Command,
+	parameters: Option<PricingParameters<T::Balance>>,
+) -> Fee<T::Balance>
 where
 	T: Config,
 {
-	match crate::Pallet::<T>::validate(&message) {
-		Ok((_, fees)) => Some(fees.total()),
-		_ => None,
-	}
+	let gas_used_at_most = T::GasMeter::maximum_gas_used_at_most(&command);
+	let parameters = parameters.unwrap_or(T::PricingParameters::get());
+	crate::Pallet::<T>::calculate_fee(gas_used_at_most, parameters)
 }

bridges/snowbridge/pallets/outbound-queue/src/lib.rs

@@ -47,24 +47,37 @@
 //! consume on Ethereum. Using this upper bound, a final fee can be calculated.
 //!
 //! The fee calculation also requires the following parameters:
-//! * ETH/DOT exchange rate
-//! * Ether fee per unit of gas
+//! * Average ETH/DOT exchange rate over some period
+//! * Max fee per unit of gas that bridge is willing to refund relayers for
 //!
 //! By design, it is expected that governance should manually update these
 //! parameters every few weeks using the `set_pricing_parameters` extrinsic in the
 //! system pallet.
 //!
+//! This is an interim measure. Once ETH/DOT liquidity pools are available in the Polkadot network,
+//! we'll use them as a source of pricing info, subject to certain safeguards.
+//!
 //! ## Fee Computation Function
 //!
 //! ```text
 //! LocalFee(Message) = WeightToFee(ProcessMessageWeight(Message))
-//! RemoteFee(Message) = MaxGasRequired(Message) * FeePerGas + Reward
-//! Fee(Message) = LocalFee(Message) + (RemoteFee(Message) / Ratio("ETH/DOT"))
+//! RemoteFee(Message) = MaxGasRequired(Message) * Params.MaxFeePerGas + Params.Reward
+//! RemoteFeeAdjusted(Message) = Params.Multiplier * (RemoteFee(Message) / Params.Ratio("ETH/DOT"))
+//! Fee(Message) = LocalFee(Message) + RemoteFeeAdjusted(Message)
 //! ```
 //!
-//! By design, the computed fee is always going to conservative, to cover worst-case
-//! costs of dispatch on Ethereum. In future iterations of the design, we will optimize
-//! this, or provide a mechanism to asynchronously refund a portion of collected fees.
+//! By design, the computed fee includes a safety factor (the `Multiplier`) to cover
+//! unfavourable fluctuations in the ETH/DOT exchange rate.
+//!
+//! ## Fee Settlement
+//!
+//! On the remote side, in the gateway contract, the relayer accrues
+//!
+//! ```text
+//! Min(GasPrice, Message.MaxFeePerGas) * GasUsed() + Message.Reward
+//! ```
+//! Or in plain english, relayers are refunded for gas consumption, using a
+//! price that is a minimum of the actual gas price, or `Message.MaxFeePerGas`.
 //!
 //! # Extrinsics
 //!
@@ -106,7 +119,7 @@ pub use snowbridge_outbound_queue_merkle_tree::MerkleProof;
 use sp_core::{H256, U256};
 use sp_runtime::{
 	traits::{CheckedDiv, Hash},
-	DigestItem,
+	DigestItem, Saturating,
 };
 use sp_std::prelude::*;
 pub use types::{CommittedMessage, ProcessMessageOriginOf};
@@ -366,8 +379,9 @@ pub mod pallet {
 			// downcast to u128
 			let fee: u128 = fee.try_into().defensive_unwrap_or(u128::MAX);
 
-			// convert to local currency
+			// multiply by multiplier and convert to local currency
 			let fee = FixedU128::from_inner(fee)
+				.saturating_mul(params.multiplier)
 				.checked_div(&params.exchange_rate)
 				.expect("exchange rate is not zero; qed")
 				.into_inner();

bridges/snowbridge/pallets/outbound-queue/src/mock.rs

@@ -77,7 +77,8 @@ parameter_types! {
 	pub Parameters: PricingParameters<u128> = PricingParameters {
 		exchange_rate: FixedU128::from_rational(1, 400),
 		fee_per_gas: gwei(20),
-		rewards: Rewards { local: DOT, remote: meth(1) }
+		rewards: Rewards { local: DOT, remote: meth(1) },
+		multiplier: FixedU128::from_rational(4, 3),
 	};
 }
 

bridges/snowbridge/pallets/outbound-queue/src/test.rs

@@ -268,28 +268,34 @@ fn encode_digest_item() {
 }
 
 #[test]
-fn validate_messages_with_fees() {
+fn test_calculate_fees_with_unit_multiplier() {
 	new_tester().execute_with(|| {
-		let message = mock_message(1000);
-		let (_, fee) = OutboundQueue::validate(&message).unwrap();
+		let gas_used: u64 = 250000;
+		let price_params: PricingParameters<<Test as Config>::Balance> = PricingParameters {
+			exchange_rate: FixedU128::from_rational(1, 400),
+			fee_per_gas: 10000_u32.into(),
+			rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() },
+			multiplier: FixedU128::from_rational(1, 1),
+		};
+		let fee = OutboundQueue::calculate_fee(gas_used, price_params);
 		assert_eq!(fee.local, 698000000);
-		assert_eq!(fee.remote, 2680000000000);
+		assert_eq!(fee.remote, 1000000);
 	});
 }
 
 #[test]
-fn test_calculate_fees() {
+fn test_calculate_fees_with_multiplier() {
 	new_tester().execute_with(|| {
 		let gas_used: u64 = 250000;
-		let illegal_price_params: PricingParameters<<Test as Config>::Balance> =
-			PricingParameters {
-				exchange_rate: FixedU128::from_rational(1, 400),
-				fee_per_gas: 10000_u32.into(),
-				rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() },
-			};
-		let fee = OutboundQueue::calculate_fee(gas_used, illegal_price_params);
+		let price_params: PricingParameters<<Test as Config>::Balance> = PricingParameters {
+			exchange_rate: FixedU128::from_rational(1, 400),
+			fee_per_gas: 10000_u32.into(),
+			rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() },
+			multiplier: FixedU128::from_rational(4, 3),
+		};
+		let fee = OutboundQueue::calculate_fee(gas_used, price_params);
 		assert_eq!(fee.local, 698000000);
-		assert_eq!(fee.remote, 1000000);
+		assert_eq!(fee.remote, 1333333);
 	});
 }
 
@@ -297,13 +303,13 @@ fn test_calculate_fees() {
 fn test_calculate_fees_with_valid_exchange_rate_but_remote_fee_calculated_as_zero() {
 	new_tester().execute_with(|| {
 		let gas_used: u64 = 250000;
-		let illegal_price_params: PricingParameters<<Test as Config>::Balance> =
-			PricingParameters {
-				exchange_rate: FixedU128::from_rational(1, 1),
-				fee_per_gas: 1_u32.into(),
-				rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() },
-			};
-		let fee = OutboundQueue::calculate_fee(gas_used, illegal_price_params.clone());
+		let price_params: PricingParameters<<Test as Config>::Balance> = PricingParameters {
+			exchange_rate: FixedU128::from_rational(1, 1),
+			fee_per_gas: 1_u32.into(),
+			rewards: Rewards { local: 1_u32.into(), remote: 1_u32.into() },
+			multiplier: FixedU128::from_rational(1, 1),
+		};
+		let fee = OutboundQueue::calculate_fee(gas_used, price_params.clone());
 		assert_eq!(fee.local, 698000000);
 		// Though none zero pricing params the remote fee calculated here is invalid
 		// which should be avoided

bridges/snowbridge/pallets/system/src/lib.rs

@@ -159,6 +159,7 @@ pub mod pallet {
 		type DefaultPricingParameters: Get<PricingParametersOf<Self>>;
 
 		/// Cost of delivering a message from Ethereum
+		#[pallet::constant]
 		type InboundDeliveryCost: Get<BalanceOf<Self>>;
 
 		type WeightInfo: WeightInfo;
@@ -334,6 +335,7 @@ pub mod pallet {
 			let command = Command::SetPricingParameters {
 				exchange_rate: params.exchange_rate.into(),
 				delivery_cost: T::InboundDeliveryCost::get().saturated_into::<u128>(),
+				multiplier: params.multiplier.into(),
 			};
 			Self::send(PRIMARY_GOVERNANCE_CHANNEL, command, PaysFee::<T>::No)?;
 

bridges/snowbridge/pallets/system/src/mock.rs

@@ -193,7 +193,8 @@ parameter_types! {
 	pub Parameters: PricingParameters<u128> = PricingParameters {
 		exchange_rate: FixedU128::from_rational(1, 400),
 		fee_per_gas: gwei(20),
-		rewards: Rewards { local: DOT, remote: meth(1) }
+		rewards: Rewards { local: DOT, remote: meth(1) },
+		multiplier: FixedU128::from_rational(4, 3)
 	};
 	pub const InboundDeliveryCost: u128 = 1_000_000_000;
 

bridges/snowbridge/primitives/core/src/outbound.rs

@@ -136,6 +136,8 @@ mod v1 {
 			exchange_rate: UD60x18,
 			// Cost of delivering a message from Ethereum to BridgeHub, in ROC/KSM/DOT
 			delivery_cost: u128,
+			// Fee multiplier
+			multiplier: UD60x18,
 		},
 	}
 
@@ -203,10 +205,11 @@ mod v1 {
 					Token::Uint(U256::from(*transfer_asset_xcm)),
 					Token::Uint(*register_token),
 				])]),
-				Command::SetPricingParameters { exchange_rate, delivery_cost } =>
+				Command::SetPricingParameters { exchange_rate, delivery_cost, multiplier } =>
 					ethabi::encode(&[Token::Tuple(vec![
 						Token::Uint(exchange_rate.clone().into_inner()),
 						Token::Uint(U256::from(*delivery_cost)),
+						Token::Uint(multiplier.clone().into_inner()),
 					])]),
 			}
 		}
@@ -273,7 +276,8 @@ mod v1 {
 	}
 }
 
-#[cfg_attr(feature = "std", derive(PartialEq, Debug))]
+#[derive(Clone, Encode, Decode, RuntimeDebug, TypeInfo)]
+#[cfg_attr(feature = "std", derive(PartialEq))]
 /// Fee for delivering message
 pub struct Fee<Balance>
 where
@@ -346,12 +350,13 @@ pub trait GasMeter {
 	/// the command within the message
 	const MAXIMUM_BASE_GAS: u64;
 
+	/// Total gas consumed at most, including verification & dispatch
 	fn maximum_gas_used_at_most(command: &Command) -> u64 {
 		Self::MAXIMUM_BASE_GAS + Self::maximum_dispatch_gas_used_at_most(command)
 	}
 
-	/// Measures the maximum amount of gas a command payload will require to dispatch, AFTER
-	/// validation & verification.
+	/// Measures the maximum amount of gas a command payload will require to *dispatch*, NOT
+	/// including validation & verification.
 	fn maximum_dispatch_gas_used_at_most(command: &Command) -> u64;
 }
 

bridges/snowbridge/primitives/core/src/pricing.rs

@@ -13,6 +13,8 @@ pub struct PricingParameters<Balance> {
 	pub rewards: Rewards<Balance>,
 	/// Ether (wei) fee per gas unit
 	pub fee_per_gas: U256,
+	/// Fee multiplier
+	pub multiplier: FixedU128,
 }
 
 #[derive(Clone, Encode, Decode, PartialEq, RuntimeDebug, MaxEncodedLen, TypeInfo)]
@@ -43,6 +45,9 @@ where
 		if self.rewards.remote.is_zero() {
 			return Err(InvalidPricingParameters)
 		}
+		if self.multiplier == FixedU128::zero() {
+			return Err(InvalidPricingParameters)
+		}
 		Ok(())
 	}
 }

cumulus/parachains/runtimes/bridge-hubs/bridge-hub-rococo/src/lib.rs

@@ -38,7 +38,9 @@ pub mod xcm_config;
 use cumulus_pallet_parachain_system::RelayNumberMonotonicallyIncreases;
 use snowbridge_beacon_primitives::{Fork, ForkVersions};
 use snowbridge_core::{
-	gwei, meth, outbound::Message, AgentId, AllowSiblingsOnly, PricingParameters, Rewards,
+	gwei, meth,
+	outbound::{Command, Fee},
+	AgentId, AllowSiblingsOnly, PricingParameters, Rewards,
 };
 use snowbridge_router_primitives::inbound::MessageToXcm;
 use sp_api::impl_runtime_apis;
@@ -503,7 +505,8 @@ parameter_types! {
 	pub Parameters: PricingParameters<u128> = PricingParameters {
 		exchange_rate: FixedU128::from_rational(1, 400),
 		fee_per_gas: gwei(20),
-		rewards: Rewards { local: 1 * UNITS, remote: meth(1) }
+		rewards: Rewards { local: 1 * UNITS, remote: meth(1) },
+		multiplier: FixedU128::from_rational(1, 1),
 	};
 }
 
@@ -1022,8 +1025,8 @@ impl_runtime_apis! {
 			snowbridge_pallet_outbound_queue::api::prove_message::<Runtime>(leaf_index)
 		}
 
-		fn calculate_fee(message: Message) -> Option<Balance> {
-			snowbridge_pallet_outbound_queue::api::calculate_fee::<Runtime>(message)
+		fn calculate_fee(command: Command, parameters: Option<PricingParameters<Balance>>) -> Fee<Balance> {
+			snowbridge_pallet_outbound_queue::api::calculate_fee::<Runtime>(command, parameters)
 		}
 	}