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/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(())
 	}
 }