XCM: Properly set the pricing for the DMP router (#6843)

* Properly set the pricing for the DMP router

* Publicize price types

* Use FixedU128 instead of Percent

* Add sp-arithmetic as a dependency for rococo runtime

* Add sp-arithmetic as a dependency to all runtimes

* Remove duplicate import

* Add missing import

* Fix tests

* Create an appropriate QueueDownwardMessageError variant

* Recalculate delivery fee factor based on past queue sizes

* Remove unused error variant

* Fixes

* Fixes

* Remove unused imports

* Rewrite fee factor update mechanism

* Remove unused imports

* Fixes

* Update runtime/parachains/src/dmp.rs

Co-authored-by: Squirrel <gilescope@gmail.com>

* Make DeliveryFeeFactor be a StorageMap keyed on ParaIds

* Fixes

* introduce limit for fee increase on dmp queue

* add message_size based fee factor to increment_fee_factor

* change message_size fee rate to correct value

* fix div by 0 error

* bind limit to variable

* fix message_size_factor and add DeliveryFeeFactor test

* add test for ExponentialPrice implementation

* make test formula based

* make delivery fee factor test formula based

* add max value test for DeliveryFeeFactor and move limit to config

* change threshold back to dynamic value and fix tests

* fmt

* suggested changes and fmt

* small stylistic change

* fmt

* change to tokenlocation

* small fixes

* fmt

* remove sp_arithmetic dependency

* Update runtime/parachains/src/dmp.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

---------

Co-authored-by: Squirrel <gilescope@gmail.com>
Co-authored-by: Just van Stam <just.van.stam@gmail.com>
Co-authored-by: Just van Stam <vstam1@users.noreply.github.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

polkadot/runtime/common/src/xcm_sender.rs

@@ -21,8 +21,9 @@ use parity_scale_codec::Encode;
 use primitives::Id as ParaId;
 use runtime_parachains::{
 	configuration::{self, HostConfiguration},
-	dmp,
+	dmp, FeeTracker,
 };
+use sp_runtime::FixedPointNumber;
 use sp_std::{marker::PhantomData, prelude::*};
 use xcm::prelude::*;
 use SendError::*;
@@ -47,6 +48,24 @@ impl<T: Get<MultiAssets>> PriceForParachainDelivery for ConstantPrice<T> {
 	}
 }
 
+/// Implementation of `PriceForParachainDelivery` which returns an exponentially increasing price.
+/// The `A` type parameter is used to denote the asset ID that will be used for paying the delivery
+/// fee.
+///
+/// The formula for the fee is based on the sum of a base fee plus a message length fee, multiplied
+/// by a specified factor. In mathematical form, it is `F * (B + encoded_msg_len * M)`.
+pub struct ExponentialPrice<A, B, M, F>(sp_std::marker::PhantomData<(A, B, M, F)>);
+impl<A: Get<AssetId>, B: Get<u128>, M: Get<u128>, F: FeeTracker> PriceForParachainDelivery
+	for ExponentialPrice<A, B, M, F>
+{
+	fn price_for_parachain_delivery(para: ParaId, msg: &Xcm<()>) -> MultiAssets {
+		let msg_fee = (msg.encoded_size() as u128).saturating_mul(M::get());
+		let fee_sum = B::get().saturating_add(msg_fee);
+		let amount = F::get_fee_factor(para).saturating_mul_int(fee_sum);
+		(A::get(), amount).into()
+	}
+}
+
 /// XCM sender for relay chain. It only sends downward message.
 pub struct ChildParachainRouter<T, W, P>(PhantomData<(T, W, P)>);
 
@@ -88,3 +107,61 @@ impl<T: configuration::Config + dmp::Config, W: xcm::WrapVersion, P: PriceForPar
 			.map_err(|_| SendError::Transport(&"Error placing into DMP queue"))
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use frame_support::parameter_types;
+	use runtime_parachains::FeeTracker;
+	use sp_runtime::FixedU128;
+
+	parameter_types! {
+		pub const BaseDeliveryFee: u128 = 300_000_000;
+		pub const TransactionByteFee: u128 = 1_000_000;
+		pub FeeAssetId: AssetId = Concrete(Here.into());
+	}
+
+	struct TestFeeTracker;
+	impl FeeTracker for TestFeeTracker {
+		fn get_fee_factor(_: ParaId) -> FixedU128 {
+			FixedU128::from_rational(101, 100)
+		}
+	}
+
+	type TestExponentialPrice =
+		ExponentialPrice<FeeAssetId, BaseDeliveryFee, TransactionByteFee, TestFeeTracker>;
+
+	#[test]
+	fn exponential_price_correct_price_calculation() {
+		let id: ParaId = 123.into();
+		let b: u128 = BaseDeliveryFee::get();
+		let m: u128 = TransactionByteFee::get();
+
+		// F * (B + msg_length * M)
+		// message_length = 1
+		let result: u128 = TestFeeTracker::get_fee_factor(id.clone()).saturating_mul_int(b + m);
+		assert_eq!(
+			TestExponentialPrice::price_for_parachain_delivery(id.clone(), &Xcm(vec![])),
+			(FeeAssetId::get(), result).into()
+		);
+
+		// message size = 2
+		let result: u128 =
+			TestFeeTracker::get_fee_factor(id.clone()).saturating_mul_int(b + (2 * m));
+		assert_eq!(
+			TestExponentialPrice::price_for_parachain_delivery(id.clone(), &Xcm(vec![ClearOrigin])),
+			(FeeAssetId::get(), result).into()
+		);
+
+		// message size = 4
+		let result: u128 =
+			TestFeeTracker::get_fee_factor(id.clone()).saturating_mul_int(b + (4 * m));
+		assert_eq!(
+			TestExponentialPrice::price_for_parachain_delivery(
+				id.clone(),
+				&Xcm(vec![SetAppendix(Xcm(vec![ClearOrigin]))])
+			),
+			(FeeAssetId::get(), result).into()
+		);
+	}
+}