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/xcm/pallet-xcm/src/mock.rs

@@ -16,8 +16,8 @@
 
 use codec::Encode;
 use frame_support::{
-	construct_runtime, parameter_types,
-	traits::{ConstU32, Everything, Nothing},
+	construct_runtime, match_types, parameter_types,
+	traits::{ConstU32, Everything, EverythingBut, Nothing},
 	weights::Weight,
 };
 use frame_system::EnsureRoot;
@@ -25,14 +25,16 @@ use polkadot_parachain_primitives::primitives::Id as ParaId;
 use polkadot_runtime_parachains::origin;
 use sp_core::H256;
 use sp_runtime::{traits::IdentityLookup, AccountId32, BuildStorage};
-pub use sp_std::{cell::RefCell, fmt::Debug, marker::PhantomData};
+pub use sp_std::{
+	cell::RefCell, collections::btree_map::BTreeMap, fmt::Debug, marker::PhantomData,
+};
 use xcm::prelude::*;
 use xcm_builder::{
 	AccountId32Aliases, AllowKnownQueryResponses, AllowSubscriptionsFrom,
 	AllowTopLevelPaidExecutionFrom, Case, ChildParachainAsNative, ChildParachainConvertsVia,
 	ChildSystemParachainAsSuperuser, CurrencyAdapter as XcmCurrencyAdapter, FixedRateOfFungible,
 	FixedWeightBounds, IsConcrete, SignedAccountId32AsNative, SignedToAccountId32,
-	SovereignSignedViaLocation, TakeWeightCredit,
+	SovereignSignedViaLocation, TakeWeightCredit, XcmFeesToAccount,
 };
 use xcm_executor::XcmExecutor;
 
@@ -154,7 +156,7 @@ pub(crate) fn take_sent_xcm() -> Vec<(MultiLocation, Xcm<()>)> {
 		r
 	})
 }
-/// Sender that never returns error, always sends
+/// Sender that never returns error.
 pub struct TestSendXcm;
 impl SendXcm for TestSendXcm {
 	type Ticket = (MultiLocation, Xcm<()>);
@@ -193,6 +195,38 @@ impl SendXcm for TestSendXcmErrX8 {
 	}
 }
 
+parameter_types! {
+	pub Para3000: u32 = 3000;
+	pub Para3000Location: MultiLocation = Parachain(Para3000::get()).into();
+	pub Para3000PaymentAmount: u128 = 1;
+	pub Para3000PaymentMultiAssets: MultiAssets = MultiAssets::from(MultiAsset::from((Here, Para3000PaymentAmount::get())));
+}
+/// Sender only sends to `Parachain(3000)` destination requiring payment.
+pub struct TestPaidForPara3000SendXcm;
+impl SendXcm for TestPaidForPara3000SendXcm {
+	type Ticket = (MultiLocation, Xcm<()>);
+	fn validate(
+		dest: &mut Option<MultiLocation>,
+		msg: &mut Option<Xcm<()>>,
+	) -> SendResult<(MultiLocation, Xcm<()>)> {
+		if let Some(dest) = dest.as_ref() {
+			if !dest.eq(&Para3000Location::get()) {
+				return Err(SendError::NotApplicable)
+			}
+		} else {
+			return Err(SendError::NotApplicable)
+		}
+
+		let pair = (dest.take().unwrap(), msg.take().unwrap());
+		Ok((pair, Para3000PaymentMultiAssets::get()))
+	}
+	fn deliver(pair: (MultiLocation, Xcm<()>)) -> Result<XcmHash, SendError> {
+		let hash = fake_message_hash(&pair.1);
+		SENT_XCM.with(|q| q.borrow_mut().push(pair));
+		Ok(hash)
+	}
+}
+
 parameter_types! {
 	pub const BlockHashCount: u64 = 250;
 }
@@ -271,6 +305,14 @@ parameter_types! {
 	pub TrustedAssets: (MultiAssetFilter, MultiLocation) = (All.into(), Here.into());
 	pub const MaxInstructions: u32 = 100;
 	pub const MaxAssetsIntoHolding: u32 = 64;
+	pub XcmFeesTargetAccount: AccountId = AccountId::new([167u8; 32]);
+}
+
+pub const XCM_FEES_NOT_WAIVED_USER_ACCOUNT: [u8; 32] = [37u8; 32];
+match_types! {
+	pub type XcmFeesNotWaivedLocations: impl Contains<MultiLocation> = {
+		MultiLocation { parents: 0, interior: X1(Junction::AccountId32 {network: None, id: XCM_FEES_NOT_WAIVED_USER_ACCOUNT})}
+	};
 }
 
 pub type Barrier = (
@@ -283,7 +325,7 @@ pub type Barrier = (
 pub struct XcmConfig;
 impl xcm_executor::Config for XcmConfig {
 	type RuntimeCall = RuntimeCall;
-	type XcmSender = TestSendXcm;
+	type XcmSender = (TestPaidForPara3000SendXcm, TestSendXcm);
 	type AssetTransactor = LocalAssetTransactor;
 	type OriginConverter = LocalOriginConverter;
 	type IsReserve = ();
@@ -300,7 +342,12 @@ impl xcm_executor::Config for XcmConfig {
 	type SubscriptionService = XcmPallet;
 	type PalletInstancesInfo = AllPalletsWithSystem;
 	type MaxAssetsIntoHolding = MaxAssetsIntoHolding;
-	type FeeManager = ();
+	type FeeManager = XcmFeesToAccount<
+		Self,
+		EverythingBut<XcmFeesNotWaivedLocations>,
+		AccountId,
+		XcmFeesTargetAccount,
+	>;
 	type MessageExporter = ();
 	type UniversalAliases = Nothing;
 	type CallDispatcher = RuntimeCall;
@@ -322,7 +369,7 @@ parameter_types! {
 impl pallet_xcm::Config for Test {
 	type RuntimeEvent = RuntimeEvent;
 	type SendXcmOrigin = xcm_builder::EnsureXcmOrigin<RuntimeOrigin, LocalOriginToLocation>;
-	type XcmRouter = (TestSendXcmErrX8, TestSendXcm);
+	type XcmRouter = (TestSendXcmErrX8, TestPaidForPara3000SendXcm, TestSendXcm);
 	type ExecuteXcmOrigin = xcm_builder::EnsureXcmOrigin<RuntimeOrigin, LocalOriginToLocation>;
 	type XcmExecuteFilter = Everything;
 	type XcmExecutor = XcmExecutor<XcmConfig>;

polkadot/xcm/pallet-xcm/src/tests.rs

@@ -522,6 +522,74 @@ fn reserve_transfer_assets_works() {
 	});
 }
 
+/// Test `reserve_transfer_assets_with_paid_router_works`
+///
+/// Asserts that the sender's balance is decreased and the beneficiary's balance
+/// is increased. Verifies the correct message is sent and event is emitted.
+/// Verifies that XCM router fees (`SendXcm::validate` -> `MultiAssets`) are withdrawn from correct
+/// user account and deposited to a correct target account (`XcmFeesTargetAccount`).
+#[test]
+fn reserve_transfer_assets_with_paid_router_works() {
+	let user_account = AccountId::from(XCM_FEES_NOT_WAIVED_USER_ACCOUNT);
+	let paid_para_id = Para3000::get();
+	let balances = vec![
+		(user_account.clone(), INITIAL_BALANCE),
+		(ParaId::from(paid_para_id).into_account_truncating(), INITIAL_BALANCE),
+		(XcmFeesTargetAccount::get(), INITIAL_BALANCE),
+	];
+	new_test_ext_with_balances(balances).execute_with(|| {
+		let xcm_router_fee_amount = Para3000PaymentAmount::get();
+		let weight = BaseXcmWeight::get() * 2;
+		let dest: MultiLocation =
+			Junction::AccountId32 { network: None, id: user_account.clone().into() }.into();
+		assert_eq!(Balances::total_balance(&user_account), INITIAL_BALANCE);
+		assert_ok!(XcmPallet::reserve_transfer_assets(
+			RuntimeOrigin::signed(user_account.clone()),
+			Box::new(Parachain(paid_para_id).into()),
+			Box::new(dest.into()),
+			Box::new((Here, SEND_AMOUNT).into()),
+			0,
+		));
+		// check event
+		assert_eq!(
+			last_event(),
+			RuntimeEvent::XcmPallet(crate::Event::Attempted { outcome: Outcome::Complete(weight) })
+		);
+
+		// XCM_FEES_NOT_WAIVED_USER_ACCOUNT spent amount
+		assert_eq!(
+			Balances::free_balance(user_account),
+			INITIAL_BALANCE - SEND_AMOUNT - xcm_router_fee_amount
+		);
+		// Destination account (parachain account) has amount
+		let para_acc: AccountId = ParaId::from(paid_para_id).into_account_truncating();
+		assert_eq!(Balances::free_balance(para_acc), INITIAL_BALANCE + SEND_AMOUNT);
+		// XcmFeesTargetAccount where should lend xcm_router_fee_amount
+		assert_eq!(
+			Balances::free_balance(XcmFeesTargetAccount::get()),
+			INITIAL_BALANCE + xcm_router_fee_amount
+		);
+		assert_eq!(
+			sent_xcm(),
+			vec![(
+				Parachain(paid_para_id).into(),
+				Xcm(vec![
+					ReserveAssetDeposited((Parent, SEND_AMOUNT).into()),
+					ClearOrigin,
+					buy_execution((Parent, SEND_AMOUNT)),
+					DepositAsset { assets: AllCounted(1).into(), beneficiary: dest },
+				]),
+			)]
+		);
+		let versioned_sent = VersionedXcm::from(sent_xcm().into_iter().next().unwrap().1);
+		let _check_v2_ok: xcm::v2::Xcm<()> = versioned_sent.try_into().unwrap();
+		assert_eq!(
+			last_event(),
+			RuntimeEvent::XcmPallet(crate::Event::Attempted { outcome: Outcome::Complete(weight) })
+		);
+	});
+}
+
 /// Test `limited_reserve_transfer_assets`
 ///
 /// Asserts that the sender's balance is decreased and the beneficiary's balance