Introduce XcmFeesToAccount fee manager (#1234)

Combination of paritytech/polkadot#7005, its addon PR
paritytech/polkadot#7585 and its companion paritytech/cumulus#2433.

This PR introduces a new XcmFeesToAccount struct which implements the
`FeeManager` trait, and assigns this struct as the `FeeManager` in the
XCM config for all runtimes.

The struct simply deposits all fees handled by the XCM executor to a
specified account. In all runtimes, the specified account is configured
as the treasury account.

XCM __delivery__ fees are now being introduced (unless the root origin
is sending a message to a system parachain on behalf of the originating
chain).

# Note for reviewers

Most file changes are tests that had to be modified to account for the
new fees.
Main changes are in:
- cumulus/pallets/xcmp-queue/src/lib.rs <- To make it track the delivery
fees exponential factor
- polkadot/xcm/xcm-builder/src/fee_handling.rs <- Added. Has the
FeeManager implementation
- All runtime xcm_config files <- To add the FeeManager to the XCM
configuration

# Important note

After this change, instructions that create and send a new XCM (Query*,
Report*, ExportMessage, InitiateReserveWithdraw, InitiateTeleport,
DepositReserveAsset, TransferReserveAsset, LockAsset and RequestUnlock)
will require the corresponding origin account in the origin register to
pay for transport delivery fees, and the onward message will fail to be
sent if the origin account does not have the required amount. This
delivery fee is on top of what we already collect as tx fees in
pallet-xcm and XCM BuyExecution fees!

Wallet UIs that want to expose the new delivery fee can do so using the
formula:

```
delivery_fee_factor * (base_fee + encoded_msg_len * per_byte_fee)
```

where the delivery fee factor can be obtained from the corresponding
pallet based on which transport you are using (UMP, HRMP or bridges),
the base fee is a constant, the encoded message length from the message
itself and the per byte fee is the same as the configured per byte fee
for txs (i.e. `TransactionByteFee`).

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Giles Cope <gilescope@gmail.com>
Co-authored-by: command-bot <>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

polkadot/runtime/parachains/src/dmp.rs

@@ -17,12 +17,12 @@
 //! To prevent Out of Memory errors on the `DownwardMessageQueue`, an
 //! exponential fee factor (`DeliveryFeeFactor`) is set. The fee factor
 //! increments exponentially after the number of messages in the
-//! `DownwardMessageQueue` pass a threshold. This threshold is set as:
+//! `DownwardMessageQueue` passes a threshold. This threshold is set as:
 //!
 //! ```ignore
 //! // Maximum max sized messages that can be send to
 //! // the DownwardMessageQueue before it runs out of memory
-//! max_messsages = MAX_POSSIBLE_ALLOCATION / max_downward_message_size
+//! max_messages = MAX_POSSIBLE_ALLOCATION / max_downward_message_size
 //! threshold = max_messages / THRESHOLD_FACTOR
 //! ```
 //! Based on the THRESHOLD_FACTOR, the threshold is set as a fraction of the
@@ -144,7 +144,7 @@ pub mod pallet {
 		FixedU128::from_u32(1)
 	}
 
-	/// The number to multiply the base delivery fee by.
+	/// The factor to multiply the base delivery fee by.
 	#[pallet::storage]
 	pub(crate) type DeliveryFeeFactor<T: Config> =
 		StorageMap<_, Twox64Concat, ParaId, FixedU128, ValueQuery, InitialFactor>;
@@ -243,10 +243,9 @@ impl<T: Config> Pallet<T> {
 		let threshold =
 			Self::dmq_max_length(config.max_downward_message_size).saturating_div(THRESHOLD_FACTOR);
 		if q_len > (threshold as usize) {
-			let message_size_factor =
-				FixedU128::from_u32(serialized_len.saturating_div(1024) as u32)
-					.saturating_mul(MESSAGE_SIZE_FEE_BASE);
-			Self::increment_fee_factor(para, message_size_factor);
+			let message_size_factor = FixedU128::from((serialized_len / 1024) as u128)
+				.saturating_mul(MESSAGE_SIZE_FEE_BASE);
+			Self::increase_fee_factor(para, message_size_factor);
 		}
 
 		Ok(())
@@ -304,7 +303,7 @@ impl<T: Config> Pallet<T> {
 		let threshold =
 			Self::dmq_max_length(config.max_downward_message_size).saturating_div(THRESHOLD_FACTOR);
 		if q_len <= (threshold as usize) {
-			Self::decrement_fee_factor(para);
+			Self::decrease_fee_factor(para);
 		}
 		T::DbWeight::get().reads_writes(1, 1)
 	}
@@ -337,32 +336,26 @@ impl<T: Config> Pallet<T> {
 	) -> Vec<InboundDownwardMessage<BlockNumberFor<T>>> {
 		DownwardMessageQueues::<T>::get(&recipient)
 	}
+}
 
-	/// Raise the delivery fee factor by a multiplicative factor and stores the resulting value.
-	///
-	/// Returns the new delivery fee factor after the increment.
-	pub(crate) fn increment_fee_factor(para: ParaId, message_size_factor: FixedU128) -> FixedU128 {
-		<DeliveryFeeFactor<T>>::mutate(para, |f| {
-			*f = f.saturating_mul(EXPONENTIAL_FEE_BASE + message_size_factor);
+impl<T: Config> FeeTracker for Pallet<T> {
+	type Id = ParaId;
+
+	fn get_fee_factor(id: Self::Id) -> FixedU128 {
+		DeliveryFeeFactor::<T>::get(id)
+	}
+
+	fn increase_fee_factor(id: Self::Id, message_size_factor: FixedU128) -> FixedU128 {
+		<DeliveryFeeFactor<T>>::mutate(id, |f| {
+			*f = f.saturating_mul(EXPONENTIAL_FEE_BASE.saturating_add(message_size_factor));
 			*f
 		})
 	}
 
-	/// Reduce the delivery fee factor by a multiplicative factor and stores the resulting value.
-	///
-	/// Does not reduce the fee factor below the initial value, which is currently set as 1.
-	///
-	/// Returns the new delivery fee factor after the decrement.
-	pub(crate) fn decrement_fee_factor(para: ParaId) -> FixedU128 {
-		<DeliveryFeeFactor<T>>::mutate(para, |f| {
+	fn decrease_fee_factor(id: Self::Id) -> FixedU128 {
+		<DeliveryFeeFactor<T>>::mutate(id, |f| {
 			*f = InitialFactor::get().max(*f / EXPONENTIAL_FEE_BASE);
 			*f
 		})
 	}
 }
-
-impl<T: Config> FeeTracker for Pallet<T> {
-	fn get_fee_factor(para: ParaId) -> FixedU128 {
-		DeliveryFeeFactor::<T>::get(para)
-	}
-}

polkadot/runtime/parachains/src/dmp/tests.rs

@@ -233,7 +233,7 @@ fn verify_dmq_mqc_head_is_externally_accessible() {
 }
 
 #[test]
-fn verify_fee_increment_and_decrement() {
+fn verify_fee_increase_and_decrease() {
 	let a = ParaId::from(123);
 	let mut genesis = default_genesis_config();
 	genesis.configuration.config.max_downward_message_size = 16777216;

polkadot/runtime/parachains/src/lib.rs

@@ -59,7 +59,21 @@ use sp_runtime::{DispatchResult, FixedU128};
 
 /// Trait for tracking message delivery fees on a transport protocol.
 pub trait FeeTracker {
-	fn get_fee_factor(para: ParaId) -> FixedU128;
+	/// Type used for assigning different fee factors to different destinations
+	type Id;
+	/// Returns the evolving exponential fee factor which will be used to calculate the delivery
+	/// fees.
+	fn get_fee_factor(id: Self::Id) -> FixedU128;
+	/// Increases the delivery fee factor by a factor based on message size and records the result.
+	///
+	/// Returns the new delivery fee factor after the increase.
+	fn increase_fee_factor(id: Self::Id, message_size_factor: FixedU128) -> FixedU128;
+	/// Decreases the delivery fee factor by a constant factor and records the result.
+	///
+	/// Does not reduce the fee factor below the initial value, which is currently set as 1.
+	///
+	/// Returns the new delivery fee factor after the decrease.
+	fn decrease_fee_factor(id: Self::Id) -> FixedU128;
 }
 
 /// Schedule a para to be initialized at the start of the next session with the given genesis data.