[pallet-xcm] fix transport fees for remote reserve transfers (#3792)

Currently `transfer_assets` from pallet-xcm covers 4 main different
transfer types:
- `localReserve`
- `DestinationReserve`
- `Teleport`
- `RemoteReserve`

For the first three, the local execution and the remote message sending
are separated, and fees are deducted in pallet-xcm itself:
https://github.com/paritytech/polkadot-sdk/blob/3410dfb3929462da88be2da813f121d8b1cf46b3/polkadot/xcm/pallet-xcm/src/lib.rs#L1758.

For the 4th case `RemoteReserve`, pallet-xcm is still relying on the
xcm-executor itself to send the message (through the
`initiateReserveWithdraw` instruction). In this case, if delivery fees
need to be charged, it is not possible to do so because the
`jit_withdraw` mode has not being set.

This PR proposes to still use the `initiateReserveWithdraw` but
prepending a `setFeesMode { jit_withdraw: true }` to make sure delivery
fees can be paid.

A test-case is also added to present the aforementioned case

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>

cumulus/parachains/runtimes/assets/test-utils/src/test_cases.rs

@@ -186,7 +186,7 @@ pub fn teleports_for_native_asset_works<
 
 				// Mint funds into account to ensure it has enough balance to pay delivery fees
 				let delivery_fees =
-					xcm_helpers::transfer_assets_delivery_fees::<XcmConfig::XcmSender>(
+					xcm_helpers::teleport_assets_delivery_fees::<XcmConfig::XcmSender>(
 						(native_asset_id.clone(), native_asset_to_teleport_away.into()).into(),
 						0,
 						Unlimited,
@@ -579,7 +579,7 @@ pub fn teleports_for_foreign_assets_works<
 
 				// Make sure the target account has enough native asset to pay for delivery fees
 				let delivery_fees =
-					xcm_helpers::transfer_assets_delivery_fees::<XcmConfig::XcmSender>(
+					xcm_helpers::teleport_assets_delivery_fees::<XcmConfig::XcmSender>(
 						(foreign_asset_id_location_latest.clone(), asset_to_teleport_away).into(),
 						0,
 						Unlimited,

cumulus/parachains/runtimes/assets/test-utils/src/xcm_helpers.rs

@@ -18,11 +18,10 @@
 
 use xcm::latest::prelude::*;
 
-/// Returns the delivery fees amount for pallet xcm's `teleport_assets` and
-/// `reserve_transfer_assets` extrinsics.
+/// Returns the delivery fees amount for pallet xcm's `teleport_assets` extrinsics.
 /// Because it returns only a `u128`, it assumes delivery fees are only paid
 /// in one asset and that asset is known.
-pub fn transfer_assets_delivery_fees<S: SendXcm>(
+pub fn teleport_assets_delivery_fees<S: SendXcm>(
 	assets: Assets,
 	fee_asset_item: u32,
 	weight_limit: WeightLimit,

cumulus/parachains/runtimes/testing/penpal/src/lib.rs

@@ -57,7 +57,6 @@ use parachains_common::{
 	impls::{AssetsToBlockAuthor, NonZeroIssuance},
 	message_queue::{NarrowOriginToSibling, ParaIdToSibling},
 };
-use polkadot_runtime_common::xcm_sender::NoPriceForMessageDelivery;
 use smallvec::smallvec;
 use sp_api::impl_runtime_apis;
 pub use sp_consensus_aura::sr25519::AuthorityId as AuraId;
@@ -85,7 +84,7 @@ use weights::{BlockExecutionWeight, ExtrinsicBaseWeight, RocksDbWeight};
 
 // XCM Imports
 use parachains_common::{AccountId, Signature};
-use xcm::latest::prelude::BodyId;
+use xcm::latest::prelude::{AssetId as AssetLocationId, BodyId};
 
 /// Balance of an account.
 pub type Balance = u128;
@@ -545,6 +544,20 @@ impl pallet_message_queue::Config for Runtime {
 
 impl cumulus_pallet_aura_ext::Config for Runtime {}
 
+parameter_types! {
+	/// The asset ID for the asset that we use to pay for message delivery fees.
+	pub FeeAssetId: AssetLocationId = AssetLocationId(xcm_config::RelayLocation::get());
+	/// The base fee for the message delivery fees (3 CENTS).
+	pub const BaseDeliveryFee: u128 = (1_000_000_000_000u128 / 100).saturating_mul(3);
+}
+
+pub type PriceForSiblingParachainDelivery = polkadot_runtime_common::xcm_sender::ExponentialPrice<
+	FeeAssetId,
+	BaseDeliveryFee,
+	TransactionByteFee,
+	XcmpQueue,
+>;
+
 impl cumulus_pallet_xcmp_queue::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type ChannelInfo = ParachainSystem;
@@ -555,7 +568,7 @@ impl cumulus_pallet_xcmp_queue::Config for Runtime {
 	type ControllerOrigin = EnsureRoot<AccountId>;
 	type ControllerOriginConverter = XcmOriginToTransactDispatchOrigin;
 	type WeightInfo = ();
-	type PriceForSiblingDelivery = NoPriceForMessageDelivery<ParaId>;
+	type PriceForSiblingDelivery = PriceForSiblingParachainDelivery;
 }
 
 parameter_types! {

cumulus/parachains/runtimes/testing/penpal/src/xcm_config.rs

@@ -28,6 +28,7 @@ use super::{
 	ParachainSystem, PolkadotXcm, Runtime, RuntimeCall, RuntimeEvent, RuntimeOrigin, WeightToFee,
 	XcmpQueue,
 };
+use crate::{BaseDeliveryFee, FeeAssetId, TransactionByteFee};
 use core::marker::PhantomData;
 use frame_support::{
 	parameter_types,
@@ -36,10 +37,10 @@ use frame_support::{
 };
 use frame_system::EnsureRoot;
 use pallet_xcm::XcmPassthrough;
-use parachains_common::xcm_config::AssetFeeAsExistentialDepositMultiplier;
+use parachains_common::{xcm_config::AssetFeeAsExistentialDepositMultiplier, TREASURY_PALLET_ID};
 use polkadot_parachain_primitives::primitives::Sibling;
-use polkadot_runtime_common::impls::ToAuthor;
-use sp_runtime::traits::ConvertInto;
+use polkadot_runtime_common::{impls::ToAuthor, xcm_sender::ExponentialPrice};
+use sp_runtime::traits::{AccountIdConversion, ConvertInto};
 use xcm::latest::prelude::*;
 use xcm_builder::{
 	AccountId32Aliases, AllowKnownQueryResponses, AllowSubscriptionsFrom,
@@ -49,6 +50,7 @@ use xcm_builder::{
 	SiblingParachainAsNative, SiblingParachainConvertsVia, SignedAccountId32AsNative,
 	SignedToAccountId32, SovereignSignedViaLocation, StartsWith, TakeWeightCredit,
 	TrailingSetTopicAsId, UsingComponents, WithComputedOrigin, WithUniqueTopic,
+	XcmFeeManagerFromComponents, XcmFeeToAccount,
 };
 use xcm_executor::{traits::JustTry, XcmExecutor};
 
@@ -59,6 +61,7 @@ parameter_types! {
 	pub const RelayNetwork: Option<NetworkId> = None;
 	pub RelayChainOrigin: RuntimeOrigin = cumulus_pallet_xcm::Origin::Relay.into();
 	pub UniversalLocation: InteriorLocation = [Parachain(ParachainInfo::parachain_id().into())].into();
+	pub TreasuryAccount: AccountId = TREASURY_PALLET_ID.into_account_truncating();
 }
 
 /// Type for specifying how a `Location` can be converted into an `AccountId`. This is used
@@ -331,7 +334,10 @@ impl xcm_executor::Config for XcmConfig {
 	type MaxAssetsIntoHolding = MaxAssetsIntoHolding;
 	type AssetLocker = ();
 	type AssetExchanger = ();
-	type FeeManager = ();
+	type FeeManager = XcmFeeManagerFromComponents<
+		(),
+		XcmFeeToAccount<Self::AssetTransactor, AccountId, TreasuryAccount>,
+	>;
 	type MessageExporter = ();
 	type UniversalAliases = Nothing;
 	type CallDispatcher = RuntimeCall;
@@ -355,11 +361,14 @@ pub type ForeignAssetFeeAsExistentialDepositMultiplierFeeCharger =
 /// No local origins on this chain are allowed to dispatch XCM sends/executions.
 pub type LocalOriginToLocation = SignedToAccountId32<RuntimeOrigin, AccountId, RelayNetwork>;
 
+pub type PriceForParentDelivery =
+	ExponentialPrice<FeeAssetId, BaseDeliveryFee, TransactionByteFee, ParachainSystem>;
+
 /// The means for routing XCM messages which are not for local execution into the right message
 /// queues.
 pub type XcmRouter = WithUniqueTopic<(
 	// Two routers - use UMP to communicate with the relay chain:
-	cumulus_primitives_utility::ParentAsUmp<ParachainSystem, PolkadotXcm, ()>,
+	cumulus_primitives_utility::ParentAsUmp<ParachainSystem, PolkadotXcm, PriceForParentDelivery>,
 	// ..and XCMP to communicate with the sibling chains.
 	XcmpQueue,
 )>;