pallet-xcm: add new flexible transfer_assets() call/extrinsic (pari…

…tytech#2388)

# Motivation (+testing)

### Enable easy `ForeignAssets` transfers using `pallet-xcm` 

We had just previously added capabilities to teleport fees during
reserve-based transfers, but what about reserve-transferring fees when
needing to teleport some non-fee asset?

This PR aligns everything under either explicit reserve-transfer,
explicit teleport, or this new flexible `transfer_assets()` which can
mix and match as needed with fewer artificial constraints imposed to the
user.

This will enable, for example, a (non-system) parachain to teleport
their `ForeignAssets` assets to AssetHub while using DOT to pay fees.
(the assets are teleported - as foreign assets should from their owner
chain - while DOT used for fees can only be reserve-based transferred
between said parachain and AssetHub).

Added `xcm-emulator` tests for this scenario ^.

# Description

Reverts `(limited_)reserve_transfer_assets` to only allow reserve-based
transfers for all `assets` including fees.

Similarly `(limited_)teleport_assets` only allows teleports for all
`assets` including fees.
    
For complex combinations of asset transfers where assets and fees may
have different reserves or different reserve/teleport trust
configurations, users can use the newly added `transfer_assets()`
extrinsic which is more flexible in allowing more complex scenarios.

`assets` (excluding `fees`) must have same reserve location or otherwise
be teleportable to `dest`.
No limitations imposed on `fees`.

- for local reserve: transfer assets to sovereign account of destination
chain and forward a notification XCM to `dest` to mint and deposit
reserve-based assets to `beneficiary`.
- for destination reserve: burn local assets and forward a notification
to `dest` chain to withdraw the reserve assets from this chain's
sovereign account and deposit them to `beneficiary`.
- for remote reserve: burn local assets, forward XCM to reserve chain to
move reserves from this chain's SA to `dest` chain's SA, and forward
another XCM to `dest` to mint and deposit reserve-based assets to
`beneficiary`.
- for teleports: burn local assets and forward XCM to `dest` chain to
mint/teleport assets and deposit them to `beneficiary`.

## Review notes

Only around 500 lines are prod code (see `pallet_xcm/src/lib.rs`), the
rest of the PR is new tests and improving existing tests.

---------

Co-authored-by: command-bot <>

cumulus/parachains/integration-tests/emulated/chains/parachains/testing/penpal/src/lib.rs

@@ -15,6 +15,7 @@
 
 mod genesis;
 pub use genesis::{genesis, ED, PARA_ID_A, PARA_ID_B};
+pub use penpal_runtime::xcm_config::{LocalTeleportableToAssetHub, XcmConfig};
 
 // Substrate
 use frame_support::traits::OnInitialize;
@@ -67,6 +68,7 @@ decl_test_parachains! {
 
 // Penpal implementation
 impl_accounts_helpers_for_parachain!(PenpalA);
+impl_accounts_helpers_for_parachain!(PenpalB);
 impl_assets_helpers_for_parachain!(PenpalA, Rococo);
 impl_assets_helpers_for_parachain!(PenpalB, Westend);
 impl_assert_events_helpers_for_parachain!(PenpalA);

cumulus/parachains/integration-tests/emulated/common/src/macros.rs

@@ -120,3 +120,102 @@ macro_rules! test_parachain_is_trusted_teleporter {
 		}
 	};
 }
+
+#[macro_export]
+macro_rules! include_penpal_create_foreign_asset_on_asset_hub {
+	( $penpal:ident, $asset_hub:ident, $relay_ed:expr, $weight_to_fee:expr) => {
+		$crate::impls::paste::paste! {
+			pub fn penpal_create_foreign_asset_on_asset_hub(
+				asset_id_on_penpal: u32,
+				foreign_asset_at_asset_hub: MultiLocation,
+				ah_as_seen_by_penpal: MultiLocation,
+				is_sufficient: bool,
+				asset_owner: AccountId,
+				prefund_amount: u128,
+			) {
+				use frame_support::weights::WeightToFee;
+				let ah_check_account = $asset_hub::execute_with(|| {
+					<$asset_hub as [<$asset_hub Pallet>]>::PolkadotXcm::check_account()
+				});
+				let penpal_check_account =
+					$penpal::execute_with(|| <$penpal as [<$penpal Pallet>]>::PolkadotXcm::check_account());
+				let penpal_as_seen_by_ah = $asset_hub::sibling_location_of($penpal::para_id());
+
+				// prefund SA of Penpal on AssetHub with enough native tokens to pay for creating
+				// new foreign asset, also prefund CheckingAccount with ED, because teleported asset
+				// itself might not be sufficient and CheckingAccount cannot be created otherwise
+				let sov_penpal_on_ah = $asset_hub::sovereign_account_id_of(penpal_as_seen_by_ah);
+				$asset_hub::fund_accounts(vec![
+					(sov_penpal_on_ah.clone().into(), $relay_ed * 100_000_000_000),
+					(ah_check_account.clone().into(), $relay_ed * 1000),
+				]);
+
+				// prefund SA of AssetHub on Penpal with native asset
+				let sov_ah_on_penpal = $penpal::sovereign_account_id_of(ah_as_seen_by_penpal);
+				$penpal::fund_accounts(vec![
+					(sov_ah_on_penpal.into(), $relay_ed * 1_000_000_000),
+					(penpal_check_account.clone().into(), $relay_ed * 1000),
+				]);
+
+				// Force create asset on $penpal and prefund [<$penpal Sender>]
+				$penpal::force_create_and_mint_asset(
+					asset_id_on_penpal,
+					ASSET_MIN_BALANCE,
+					is_sufficient,
+					asset_owner,
+					None,
+					prefund_amount,
+				);
+
+				let require_weight_at_most = Weight::from_parts(1_100_000_000_000, 30_000);
+				// `OriginKind::Xcm` required by ForeignCreators pallet-assets origin filter
+				let origin_kind = OriginKind::Xcm;
+				let call_create_foreign_assets =
+					<$asset_hub as Chain>::RuntimeCall::ForeignAssets(pallet_assets::Call::<
+						<$asset_hub as Chain>::Runtime,
+						pallet_assets::Instance2,
+					>::create {
+						id: foreign_asset_at_asset_hub,
+						min_balance: ASSET_MIN_BALANCE,
+						admin: sov_penpal_on_ah.into(),
+					})
+					.encode();
+				let buy_execution_fee_amount = $weight_to_fee::weight_to_fee(
+					&Weight::from_parts(10_100_000_000_000, 300_000),
+				);
+				let buy_execution_fee = MultiAsset {
+					id: Concrete(MultiLocation { parents: 1, interior: Here }),
+					fun: Fungible(buy_execution_fee_amount),
+				};
+				let xcm = VersionedXcm::from(Xcm(vec![
+					WithdrawAsset { 0: vec![buy_execution_fee.clone()].into() },
+					BuyExecution { fees: buy_execution_fee.clone(), weight_limit: Unlimited },
+					Transact { require_weight_at_most, origin_kind, call: call_create_foreign_assets.into() },
+					ExpectTransactStatus(MaybeErrorCode::Success),
+					RefundSurplus,
+					DepositAsset { assets: All.into(), beneficiary: penpal_as_seen_by_ah },
+				]));
+				// Send XCM message from penpal => asset_hub
+				let sudo_penpal_origin = <$penpal as Chain>::RuntimeOrigin::root();
+				$penpal::execute_with(|| {
+					assert_ok!(<$penpal as [<$penpal Pallet>]>::PolkadotXcm::send(
+						sudo_penpal_origin.clone(),
+						bx!(ah_as_seen_by_penpal.into()),
+						bx!(xcm),
+					));
+					type RuntimeEvent = <$penpal as Chain>::RuntimeEvent;
+					assert_expected_events!(
+						$penpal,
+						vec![
+							RuntimeEvent::PolkadotXcm(pallet_xcm::Event::Sent { .. }) => {},
+						]
+					);
+				});
+				$asset_hub::execute_with(|| {
+					type ForeignAssets = <$asset_hub as [<$asset_hub Pallet>]>::ForeignAssets;
+					assert!(ForeignAssets::asset_exists(foreign_asset_at_asset_hub));
+				});
+			}
+		}
+	};
+}

cumulus/parachains/integration-tests/emulated/common/src/xcm_helpers.rs

@@ -59,3 +59,17 @@ pub fn xcm_transact_unpaid_execution(
 		Transact { require_weight_at_most, origin_kind, call },
 	]))
 }
+
+/// Helper method to get the non-fee asset used in multiple assets transfer
+pub fn non_fee_asset(assets: &MultiAssets, fee_idx: usize) -> Option<(MultiLocation, u128)> {
+	let asset = assets.inner().into_iter().enumerate().find(|a| a.0 != fee_idx)?.1.clone();
+	let asset_id = match asset.id {
+		Concrete(id) => id,
+		_ => return None,
+	};
+	let asset_amount = match asset.fun {
+		Fungible(amount) => amount,
+		_ => return None,
+	};
+	Some((asset_id, asset_amount))
+}

cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-rococo/Cargo.toml

@@ -30,5 +30,4 @@ asset-test-utils = { path = "../../../../../runtimes/assets/test-utils" }
 parachains-common = { path = "../../../../../../parachains/common" }
 asset-hub-rococo-runtime = { path = "../../../../../runtimes/assets/asset-hub-rococo" }
 emulated-integration-tests-common = { path = "../../../common", default-features = false }
-penpal-runtime = { path = "../../../../../runtimes/testing/penpal" }
 rococo-system-emulated-network = { path = "../../../networks/rococo-system" }

cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-rococo/src/tests/mod.rs

@@ -18,3 +18,11 @@ mod send;
 mod set_xcm_versions;
 mod swap;
 mod teleport;
+
+use crate::*;
+emulated_integration_tests_common::include_penpal_create_foreign_asset_on_asset_hub!(
+	PenpalA,
+	AssetHubRococo,
+	ROCOCO_ED,
+	parachains_common::rococo::fee::WeightToFee
+);

cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-rococo/src/tests/reserve_transfer.rs

@@ -15,14 +15,12 @@
 
 use crate::*;
 use asset_hub_rococo_runtime::xcm_config::XcmConfig as AssetHubRococoXcmConfig;
-use penpal_runtime::xcm_config::XcmConfig as PenpalRococoXcmConfig;
 use rococo_runtime::xcm_config::XcmConfig as RococoXcmConfig;
+use rococo_system_emulated_network::penpal_emulated_chain::XcmConfig as PenpalRococoXcmConfig;
 
 fn relay_to_para_sender_assertions(t: RelayToParaTest) {
 	type RuntimeEvent = <Rococo as Chain>::RuntimeEvent;
-
 	Rococo::assert_xcm_pallet_attempted_complete(Some(Weight::from_parts(864_610_000, 8_799)));
-
 	assert_expected_events!(
 		Rococo,
 		vec![
@@ -42,12 +40,10 @@ fn relay_to_para_sender_assertions(t: RelayToParaTest) {
 
 fn system_para_to_para_sender_assertions(t: SystemParaToParaTest) {
 	type RuntimeEvent = <AssetHubRococo as Chain>::RuntimeEvent;
-
 	AssetHubRococo::assert_xcm_pallet_attempted_complete(Some(Weight::from_parts(
 		864_610_000,
 		8_799,
 	)));
-
 	assert_expected_events!(
 		AssetHubRococo,
 		vec![
@@ -80,9 +76,7 @@ fn para_receiver_assertions<Test>(_: Test) {
 
 fn para_to_system_para_sender_assertions(t: ParaToSystemParaTest) {
 	type RuntimeEvent = <PenpalA as Chain>::RuntimeEvent;
-
 	PenpalA::assert_xcm_pallet_attempted_complete(Some(Weight::from_parts(864_610_000, 8_799)));
-
 	assert_expected_events!(
 		PenpalA,
 		vec![
@@ -99,15 +93,13 @@ fn para_to_system_para_sender_assertions(t: ParaToSystemParaTest) {
 
 fn para_to_system_para_receiver_assertions(t: ParaToSystemParaTest) {
 	type RuntimeEvent = <AssetHubRococo as Chain>::RuntimeEvent;
-
 	let sov_penpal_on_ahr = AssetHubRococo::sovereign_account_id_of(
 		AssetHubRococo::sibling_location_of(PenpalA::para_id()),
 	);
-
 	assert_expected_events!(
 		AssetHubRococo,
 		vec![
-			// Amount to reserve transfer is transferred to Parachain's Sovereign account
+			// Amount to reserve transfer is withdrawn from Parachain's Sovereign account
 			RuntimeEvent::Balances(
 				pallet_balances::Event::Withdraw { who, amount }
 			) => {
@@ -124,12 +116,10 @@ fn para_to_system_para_receiver_assertions(t: ParaToSystemParaTest) {
 
 fn system_para_to_para_assets_sender_assertions(t: SystemParaToParaTest) {
 	type RuntimeEvent = <AssetHubRococo as Chain>::RuntimeEvent;
-
 	AssetHubRococo::assert_xcm_pallet_attempted_complete(Some(Weight::from_parts(
 		864_610_000,
 		8799,
 	)));
-
 	assert_expected_events!(
 		AssetHubRococo,
 		vec![
@@ -162,7 +152,7 @@ fn system_para_to_para_assets_receiver_assertions<Test>(_: Test) {
 	);
 }
 
-fn relay_to_para_limited_reserve_transfer_assets(t: RelayToParaTest) -> DispatchResult {
+fn relay_to_para_reserve_transfer_assets(t: RelayToParaTest) -> DispatchResult {
 	<Rococo as RococoPallet>::XcmPallet::limited_reserve_transfer_assets(
 		t.signed_origin,
 		bx!(t.args.dest.into()),
@@ -173,7 +163,7 @@ fn relay_to_para_limited_reserve_transfer_assets(t: RelayToParaTest) -> Dispatch
 	)
 }
 
-fn system_para_to_para_limited_reserve_transfer_assets(t: SystemParaToParaTest) -> DispatchResult {
+fn system_para_to_para_reserve_transfer_assets(t: SystemParaToParaTest) -> DispatchResult {
 	<AssetHubRococo as AssetHubRococoPallet>::PolkadotXcm::limited_reserve_transfer_assets(
 		t.signed_origin,
 		bx!(t.args.dest.into()),
@@ -184,7 +174,7 @@ fn system_para_to_para_limited_reserve_transfer_assets(t: SystemParaToParaTest)
 	)
 }
 
-fn para_to_system_para_limited_reserve_transfer_assets(t: ParaToSystemParaTest) -> DispatchResult {
+fn para_to_system_para_reserve_transfer_assets(t: ParaToSystemParaTest) -> DispatchResult {
 	<PenpalA as PenpalAPallet>::PolkadotXcm::limited_reserve_transfer_assets(
 		t.signed_origin,
 		bx!(t.args.dest.into()),
@@ -285,7 +275,7 @@ fn reserve_transfer_native_asset_from_relay_to_para() {
 
 	test.set_assertion::<Rococo>(relay_to_para_sender_assertions);
 	test.set_assertion::<PenpalA>(para_receiver_assertions);
-	test.set_dispatchable::<Rococo>(relay_to_para_limited_reserve_transfer_assets);
+	test.set_dispatchable::<Rococo>(relay_to_para_reserve_transfer_assets);
 	test.assert();
 
 	let delivery_fees = Rococo::execute_with(|| {
@@ -329,7 +319,7 @@ fn reserve_transfer_native_asset_from_system_para_to_para() {
 
 	test.set_assertion::<AssetHubRococo>(system_para_to_para_sender_assertions);
 	test.set_assertion::<PenpalA>(para_receiver_assertions);
-	test.set_dispatchable::<AssetHubRococo>(system_para_to_para_limited_reserve_transfer_assets);
+	test.set_dispatchable::<AssetHubRococo>(system_para_to_para_reserve_transfer_assets);
 	test.assert();
 
 	let sender_balance_after = test.sender.balance;
@@ -379,7 +369,7 @@ fn reserve_transfer_native_asset_from_para_to_system_para() {
 
 	test.set_assertion::<PenpalA>(para_to_system_para_sender_assertions);
 	test.set_assertion::<AssetHubRococo>(para_to_system_para_receiver_assertions);
-	test.set_dispatchable::<PenpalA>(para_to_system_para_limited_reserve_transfer_assets);
+	test.set_dispatchable::<PenpalA>(para_to_system_para_reserve_transfer_assets);
 	test.assert();
 
 	let sender_balance_after = test.sender.balance;
@@ -474,7 +464,7 @@ fn reserve_transfer_assets_from_system_para_to_para() {
 
 	test.set_assertion::<AssetHubRococo>(system_para_to_para_assets_sender_assertions);
 	test.set_assertion::<PenpalA>(system_para_to_para_assets_receiver_assertions);
-	test.set_dispatchable::<AssetHubRococo>(system_para_to_para_limited_reserve_transfer_assets);
+	test.set_dispatchable::<AssetHubRococo>(system_para_to_para_reserve_transfer_assets);
 	test.assert();
 
 	let sender_balance_after = test.sender.balance;