pallet-xcm: enhance reserve_transfer_assets to support remote reserves (#1672)

## Motivation

`pallet-xcm` is the main user-facing interface for XCM functionality,
including assets manipulation functions like `teleportAssets()` and
`reserve_transfer_assets()` calls.

While `teleportAsset()` works both ways, `reserve_transfer_assets()`
works only for sending reserve-based assets to a remote destination and
beneficiary when the reserve is the _local chain_.

## Solution

This PR enhances `pallet_xcm::(limited_)reserve_withdraw_assets` to
support transfers when reserves are other chains.
This will allow complete, **bi-directional** reserve-based asset
transfers user stories using `pallet-xcm`.

Enables following scenarios:
- transferring assets with local reserve (was previously supported iff
asset used as fee also had local reserve - now it works in all cases),
- transferring assets with reserve on destination,
- transferring assets with reserve on remote/third-party chain (iff
assets and fees have same remote reserve),
- transferring assets with reserve different than the reserve of the
asset to be used as fees - meaning can be used to transfer random asset
with local/dest reserve while using DOT for fees on all involved chains,
even if DOT local/dest reserve doesn't match asset reserve,
- transferring assets with any type of local/dest reserve while using
fees which can be teleported between involved chains.

All of the above is done by pallet inner logic without the user having
to specify which scenario/reserves/teleports/etc. The correct scenario
and corresponding XCM programs are identified, and respectively, built
automatically based on runtime configuration of trusted teleporters and
trusted reserves.

#### Current limitations:
- while `fees` and "non-fee" `assets` CAN have different reserves (or
fees CAN be teleported), the remaining "non-fee" `assets` CANNOT, among
themselves, have different reserve locations (this is also implicitly
enforced by `MAX_ASSETS_FOR_TRANSFER=2`, but this can be safely
increased in the future).
- `fees` and "non-fee" `assets` CANNOT have **different remote**
reserves (this could also be supported in the future, but adds even more
complexity while possibly not being worth it - we'll see what the future
holds).

Fixes https://github.com/paritytech/polkadot-sdk/issues/1584
Fixes https://github.com/paritytech/polkadot-sdk/issues/2055

---------

Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Branislav Kontur <bkontur@gmail.com>

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

@@ -434,7 +434,7 @@ parameter_types! {
 // pub type AssetsForceOrigin =
 // 	EnsureOneOf<EnsureRoot<AccountId>, EnsureXcm<IsMajorityOfBody<KsmLocation, ExecutiveBody>>>;
 
-impl pallet_assets::Config for Runtime {
+impl pallet_assets::Config<pallet_assets::Instance1> for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type Balance = Balance;
 	type AssetId = AssetId;
@@ -577,7 +577,12 @@ impl pallet_asset_tx_payment::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type Fungibles = Assets;
 	type OnChargeAssetTransaction = pallet_asset_tx_payment::FungiblesAdapter<
-		pallet_assets::BalanceToAssetBalance<Balances, Runtime, ConvertInto>,
+		pallet_assets::BalanceToAssetBalance<
+			Balances,
+			Runtime,
+			ConvertInto,
+			pallet_assets::Instance1,
+		>,
 		AssetsToBlockAuthor<Runtime>,
 	>;
 }
@@ -619,7 +624,7 @@ construct_runtime!(
 		MessageQueue: pallet_message_queue::{Pallet, Call, Storage, Event<T>} = 34,
 
 		// The main stage.
-		Assets: pallet_assets::{Pallet, Call, Storage, Event<T>} = 50,
+		Assets: pallet_assets::<Instance1>::{Pallet, Call, Storage, Event<T>} = 50,
 
 		Sudo: pallet_sudo::{Pallet, Call, Storage, Event<T>, Config<T>} = 255,
 	}

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

@@ -38,6 +38,7 @@ use frame_support::{
 };
 use frame_system::EnsureRoot;
 use pallet_asset_tx_payment::HandleCredit;
+use pallet_assets::Instance1;
 use pallet_xcm::XcmPassthrough;
 use polkadot_parachain_primitives::primitives::Sibling;
 use polkadot_runtime_common::impls::ToAuthor;
@@ -48,9 +49,10 @@ use xcm_builder::{
 	AllowSubscriptionsFrom, AllowTopLevelPaidExecutionFrom, AsPrefixedGeneralIndex,
 	ConvertedConcreteId, CurrencyAdapter, DenyReserveTransferToRelayChain, DenyThenTry,
 	EnsureXcmOrigin, FixedWeightBounds, FungiblesAdapter, IsConcrete, LocalMint, NativeAsset,
-	ParentIsPreset, RelayChainAsNative, SiblingParachainAsNative, SiblingParachainConvertsVia,
-	SignedAccountId32AsNative, SignedToAccountId32, SovereignSignedViaLocation, TakeWeightCredit,
-	TrailingSetTopicAsId, UsingComponents, WithComputedOrigin, WithUniqueTopic,
+	ParentAsSuperuser, ParentIsPreset, RelayChainAsNative, SiblingParachainAsNative,
+	SiblingParachainConvertsVia, SignedAccountId32AsNative, SignedToAccountId32,
+	SovereignSignedViaLocation, TakeWeightCredit, TrailingSetTopicAsId, UsingComponents,
+	WithComputedOrigin, WithUniqueTopic,
 };
 use xcm_executor::{traits::JustTry, XcmExecutor};
 
@@ -126,6 +128,9 @@ pub type XcmOriginToTransactDispatchOrigin = (
 	// Native converter for sibling Parachains; will convert to a `SiblingPara` origin when
 	// recognized.
 	SiblingParachainAsNative<cumulus_pallet_xcm::Origin, RuntimeOrigin>,
+	// Superuser converter for the Relay-chain (Parent) location. This will allow it to issue a
+	// transaction from the Root origin.
+	ParentAsSuperuser<RuntimeOrigin>,
 	// Native signed account converter; this just converts an `AccountId32` origin into a normal
 	// `RuntimeOrigin::Signed` origin of the same 32-byte value.
 	SignedAccountId32AsNative<RelayNetwork, RuntimeOrigin>,
@@ -182,14 +187,25 @@ pub type Barrier = TrailingSetTopicAsId<
 /// Type alias to conveniently refer to `frame_system`'s `Config::AccountId`.
 pub type AccountIdOf<R> = <R as frame_system::Config>::AccountId;
 
-/// Asset filter that allows all assets from a certain location.
+/// Asset filter that allows all assets from a certain location matching asset id.
 pub struct AssetsFrom<T>(PhantomData<T>);
 impl<T: Get<MultiLocation>> ContainsPair<MultiAsset, MultiLocation> for AssetsFrom<T> {
 	fn contains(asset: &MultiAsset, origin: &MultiLocation) -> bool {
 		let loc = T::get();
 		&loc == origin &&
 			matches!(asset, MultiAsset { id: AssetId::Concrete(asset_loc), fun: Fungible(_a) }
-			if asset_loc.match_and_split(&loc).is_some())
+			if asset_loc.starts_with(&loc))
+	}
+}
+
+/// Asset filter that allows native/relay asset if coming from a certain location.
+pub struct NativeAssetFrom<T>(PhantomData<T>);
+impl<T: Get<MultiLocation>> ContainsPair<MultiAsset, MultiLocation> for NativeAssetFrom<T> {
+	fn contains(asset: &MultiAsset, origin: &MultiLocation) -> bool {
+		let loc = T::get();
+		&loc == origin &&
+			matches!(asset, MultiAsset { id: AssetId::Concrete(asset_loc), fun: Fungible(_a) }
+			if *asset_loc == MultiLocation::from(Parent))
 	}
 }
 
@@ -208,56 +224,19 @@ where
 /// A `HandleCredit` implementation that naively transfers the fees to the block author.
 /// Will drop and burn the assets in case the transfer fails.
 pub struct AssetsToBlockAuthor<R>(PhantomData<R>);
-impl<R> HandleCredit<AccountIdOf<R>, pallet_assets::Pallet<R>> for AssetsToBlockAuthor<R>
+impl<R> HandleCredit<AccountIdOf<R>, pallet_assets::Pallet<R, Instance1>> for AssetsToBlockAuthor<R>
 where
-	R: pallet_authorship::Config + pallet_assets::Config,
+	R: pallet_authorship::Config + pallet_assets::Config<Instance1>,
 	AccountIdOf<R>: From<polkadot_primitives::AccountId> + Into<polkadot_primitives::AccountId>,
 {
-	fn handle_credit(credit: Credit<AccountIdOf<R>, pallet_assets::Pallet<R>>) {
+	fn handle_credit(credit: Credit<AccountIdOf<R>, pallet_assets::Pallet<R, Instance1>>) {
 		if let Some(author) = pallet_authorship::Pallet::<R>::author() {
 			// In case of error: Will drop the result triggering the `OnDrop` of the imbalance.
-			let _ = pallet_assets::Pallet::<R>::resolve(&author, credit);
+			let _ = pallet_assets::Pallet::<R, Instance1>::resolve(&author, credit);
 		}
 	}
 }
 
-pub trait Reserve {
-	/// Returns assets reserve location.
-	fn reserve(&self) -> Option<MultiLocation>;
-}
-
-// Takes the chain part of a MultiAsset
-impl Reserve for MultiAsset {
-	fn reserve(&self) -> Option<MultiLocation> {
-		if let AssetId::Concrete(location) = self.id {
-			let first_interior = location.first_interior();
-			let parents = location.parent_count();
-			match (parents, first_interior) {
-				(0, Some(Parachain(id))) => Some(MultiLocation::new(0, X1(Parachain(*id)))),
-				(1, Some(Parachain(id))) => Some(MultiLocation::new(1, X1(Parachain(*id)))),
-				(1, _) => Some(MultiLocation::parent()),
-				_ => None,
-			}
-		} else {
-			None
-		}
-	}
-}
-
-/// A `FilterAssetLocation` implementation. Filters multi native assets whose
-/// reserve is same with `origin`.
-pub struct MultiNativeAsset;
-impl ContainsPair<MultiAsset, MultiLocation> for MultiNativeAsset {
-	fn contains(asset: &MultiAsset, origin: &MultiLocation) -> bool {
-		if let Some(ref reserve) = asset.reserve() {
-			if reserve == origin {
-				return true
-			}
-		}
-		false
-	}
-}
-
 parameter_types! {
 	/// The location that this chain recognizes as the Relay network's Asset Hub.
 	pub SystemAssetHubLocation: MultiLocation = MultiLocation::new(1, X1(Parachain(1000)));
@@ -268,7 +247,8 @@ parameter_types! {
 	pub CheckingAccount: AccountId = PolkadotXcm::check_account();
 }
 
-pub type Reserves = (NativeAsset, AssetsFrom<SystemAssetHubLocation>);
+pub type Reserves =
+	(NativeAsset, AssetsFrom<SystemAssetHubLocation>, NativeAssetFrom<SystemAssetHubLocation>);
 
 pub struct XcmConfig;
 impl xcm_executor::Config for XcmConfig {
@@ -277,7 +257,8 @@ impl xcm_executor::Config for XcmConfig {
 	// How to withdraw and deposit an asset.
 	type AssetTransactor = AssetTransactors;
 	type OriginConverter = XcmOriginToTransactDispatchOrigin;
-	type IsReserve = MultiNativeAsset; // TODO: maybe needed to be replaced by Reserves
+	type IsReserve = Reserves;
+	// no teleport trust established with other chains
 	type IsTeleporter = NativeAsset;
 	type UniversalLocation = UniversalLocation;
 	type Barrier = Barrier;
@@ -312,11 +293,6 @@ pub type XcmRouter = WithUniqueTopic<(
 	XcmpQueue,
 )>;
 
-#[cfg(feature = "runtime-benchmarks")]
-parameter_types! {
-	pub ReachableDest: Option<MultiLocation> = Some(Parent.into());
-}
-
 impl pallet_xcm::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type SendXcmOrigin = EnsureXcmOrigin<RuntimeOrigin, LocalOriginToLocation>;
@@ -342,8 +318,6 @@ impl pallet_xcm::Config for Runtime {
 	type SovereignAccountOf = LocationToAccountId;
 	type MaxLockers = ConstU32<8>;
 	type WeightInfo = pallet_xcm::TestWeightInfo;
-	#[cfg(feature = "runtime-benchmarks")]
-	type ReachableDest = ReachableDest;
 	type AdminOrigin = EnsureRoot<AccountId>;
 	type MaxRemoteLockConsumers = ConstU32<0>;
 	type RemoteLockConsumerIdentifier = ();