XCM: Allow reclaim of assets dropped from holding

runtime/common/src/claims.rs

@@ -1044,7 +1044,7 @@ mod tests {
 	}
 
 	#[test]
-	fn invalid_attest_transactions_are_recognised() {
+	fn invalid_attest_transactions_are_recognized() {
 		new_test_ext().execute_with(|| {
 			let p = PrevalidateAttests::<Test>::new();
 			let c = Call::Claims(ClaimsCall::attest(StatementKind::Regular.to_text().to_vec()));

runtime/kusama/src/lib.rs

@@ -1296,7 +1296,9 @@ impl xcm_executor::Config for XcmConfig {
 	type Weigher = FixedWeightBounds<BaseXcmWeight, Call, MaxInstructions>;
 	// The weight trader piggybacks on the existing transaction-fee conversion logic.
 	type Trader = UsingComponents<WeightToFee, KsmLocation, AccountId, Balances, ToAuthor<Runtime>>;
-	type ResponseHandler = ();
+	type ResponseHandler = XcmPallet;
+	type AssetTrap = XcmPallet;
+	type AssetClaims = XcmPallet;
 }
 
 parameter_types! {

runtime/rococo/src/lib.rs

@@ -669,7 +669,9 @@ impl xcm_executor::Config for XcmConfig {
 	type Barrier = Barrier;
 	type Weigher = FixedWeightBounds<BaseXcmWeight, Call, MaxInstructions>;
 	type Trader = UsingComponents<WeightToFee, RocLocation, AccountId, Balances, ToAuthor<Runtime>>;
-	type ResponseHandler = ();
+	type ResponseHandler = XcmPallet;
+	type AssetTrap = XcmPallet;
+	type AssetClaims = XcmPallet;
 }
 
 parameter_types! {

runtime/test-runtime/src/xcm_config.rs

@@ -86,4 +86,6 @@ impl xcm_executor::Config for XcmConfig {
 	type Weigher = FixedWeightBounds<super::BaseXcmWeight, super::Call, MaxInstructions>;
 	type Trader = DummyWeightTrader;
 	type ResponseHandler = super::Xcm;
+	type AssetTrap = super::Xcm;
+	type AssetClaims = super::Xcm;
 }

runtime/westend/src/lib.rs

@@ -938,7 +938,9 @@ impl xcm_executor::Config for XcmConfig {
 	type Barrier = Barrier;
 	type Weigher = FixedWeightBounds<BaseXcmWeight, Call, MaxInstructions>;
 	type Trader = UsingComponents<WeightToFee, WndLocation, AccountId, Balances, ToAuthor<Runtime>>;
-	type ResponseHandler = ();
+	type ResponseHandler = XcmPallet;
+	type AssetTrap = XcmPallet;
+	type AssetClaims = XcmPallet;
 }
 
 /// Type to convert an `Origin` type value into a `MultiLocation` value which represents an interior location

xcm/pallet-xcm/Cargo.toml

@@ -11,6 +11,7 @@ log = { version = "0.4.14", default-features = false }
 
 sp-std = { git = "https://github.com/paritytech/substrate", default-features = false, branch = "master" }
 sp-runtime = { git = "https://github.com/paritytech/substrate", default-features = false, branch = "master" }
+sp-core = { git = "https://github.com/paritytech/substrate", default-features = false, branch = "master" }
 frame-support = { git = "https://github.com/paritytech/substrate", default-features = false, branch = "master" }
 frame-system = { git = "https://github.com/paritytech/substrate", default-features = false, branch = "master" }
 
@@ -20,7 +21,6 @@ xcm-executor = { path = "../xcm-executor", default-features = false }
 [dev-dependencies]
 pallet-balances = { git = "https://github.com/paritytech/substrate", branch = "master" }
 polkadot-runtime-parachains = { path = "../../runtime/parachains" }
-sp-core = { git = "https://github.com/paritytech/substrate", branch = "master" }
 sp-io = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 xcm-builder = { path = "../xcm-builder" }
 polkadot-parachain = { path = "../../parachain" }
@@ -31,6 +31,7 @@ std = [
 	"codec/std",
 	"serde",
 	"sp-std/std",
+	"sp-core/std",
 	"sp-runtime/std",
 	"frame-support/std",
 	"frame-system/std",

xcm/pallet-xcm/src/lib.rs

@@ -50,8 +50,12 @@ pub mod pallet {
 		pallet_prelude::*,
 	};
 	use frame_system::{pallet_prelude::*, Config as SysConfig};
-	use sp_runtime::traits::{AccountIdConversion, BlockNumberProvider};
-	use xcm_executor::traits::{InvertLocation, OnResponse, WeightBounds};
+	use sp_core::H256;
+	use sp_runtime::traits::{AccountIdConversion, BlakeTwo256, BlockNumberProvider, Hash};
+	use xcm_executor::{
+		traits::{ClaimAssets, DropAssets, InvertLocation, OnResponse, WeightBounds},
+		Assets,
+	};
 
 	#[pallet::pallet]
 	#[pallet::generate_store(pub(super) trait Store)]
@@ -170,6 +174,10 @@ pub mod pallet {
 		///
 		/// \[ id \]
 		ResponseTaken(QueryId),
+		/// Some assets have been placed in an asset trap.
+		///
+		/// \[ hash, origin, assets \]
+		AssetsTrapped(H256, MultiLocation, VersionedMultiAssets),
 	}
 
 	#[pallet::origin]
@@ -236,6 +244,14 @@ pub mod pallet {
 	pub(super) type Queries<T: Config> =
 		StorageMap<_, Blake2_128Concat, QueryId, QueryStatus<T::BlockNumber>, OptionQuery>;
 
+	/// The existing asset traps.
+	///
+	/// Key is the blake2 256 hash of (origin, versioned `MultiAssets`) pair. Value is the number of
+	/// times this pair has been trapped (usually just 1 if it exists at all).
+	#[pallet::storage]
+	#[pallet::getter(fn asset_trap)]
+	pub(super) type AssetTraps<T: Config> = StorageMap<_, Identity, H256, u32, ValueQuery>;
+
 	#[pallet::hooks]
 	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}
 
@@ -553,16 +569,54 @@ pub mod pallet {
 		}
 	}
 
+	impl<T: Config> DropAssets for Pallet<T> {
+		fn drop_assets(origin: &MultiLocation, assets: Assets) -> Weight {
+			if assets.is_empty() {
+				return 0
+			}
+			let versioned = VersionedMultiAssets::from(MultiAssets::from(assets));
+			let hash = BlakeTwo256::hash_of(&(&origin, &versioned));
+			AssetTraps::<T>::mutate(hash, |n| *n += 1);
+			Self::deposit_event(Event::AssetsTrapped(hash, origin.clone(), versioned));
+			// TODO: Put the real weight in there.
+			0
+		}
+	}
+
+	impl<T: Config> ClaimAssets for Pallet<T> {
+		fn claim_assets(
+			origin: &MultiLocation,
+			ticket: &MultiLocation,
+			assets: &MultiAssets,
+		) -> bool {
+			let mut versioned = VersionedMultiAssets::from(assets.clone());
+			match (ticket.parents, &ticket.interior) {
+				(0, X1(GeneralIndex(i))) =>
+					versioned = match versioned.into_version(*i as u32) {
+						Ok(v) => v,
+						Err(()) => return false,
+					},
+				(0, Here) => (),
+				_ => return false,
+			};
+			let hash = BlakeTwo256::hash_of(&(origin, versioned));
+			match AssetTraps::<T>::get(hash) {
+				0 => return false,
+				1 => AssetTraps::<T>::remove(hash),
+				n => AssetTraps::<T>::insert(hash, n - 1),
+			}
+			return true
+		}
+	}
+
 	impl<T: Config> OnResponse for Pallet<T> {
-		/// Returns `true` if we are expecting a response from `origin` for query `query_id`.
 		fn expecting_response(origin: &MultiLocation, query_id: QueryId) -> bool {
 			if let Some(QueryStatus::Pending { responder, .. }) = Queries::<T>::get(query_id) {
 				return MultiLocation::try_from(responder).map_or(false, |r| origin == &r)
 			}
 			false
 		}
 
-		/// Handler for receiving a `response` from `origin` relating to `query_id`.
 		fn on_response(
 			origin: &MultiLocation,
 			query_id: QueryId,

xcm/pallet-xcm/src/mock.rs

@@ -251,6 +251,8 @@ impl xcm_executor::Config for XcmConfig {
 	type Weigher = FixedWeightBounds<BaseXcmWeight, Call, MaxInstructions>;
 	type Trader = FixedRateOfFungible<CurrencyPerSecond, ()>;
 	type ResponseHandler = XcmPallet;
+	type AssetTrap = XcmPallet;
+	type AssetClaims = XcmPallet;
 }
 
 pub type LocalOriginToLocation = SignedToAccountId32<Origin, AccountId, AnyNetwork>;

xcm/pallet-xcm/src/tests.rs

@@ -14,11 +14,12 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
-use crate::{mock::*, QueryStatus};
+use crate::{mock::*, AssetTraps, QueryStatus};
 use frame_support::{assert_noop, assert_ok, traits::Currency};
 use polkadot_parachain::primitives::{AccountIdConversion, Id as ParaId};
+use sp_runtime::traits::{BlakeTwo256, Hash};
 use std::convert::TryInto;
-use xcm::{latest::prelude::*, VersionedXcm};
+use xcm::{latest::prelude::*, VersionedMultiAssets, VersionedXcm};
 use xcm_executor::XcmExecutor;
 
 const ALICE: AccountId = AccountId::new([0u8; 32]);
@@ -295,3 +296,81 @@ fn execute_withdraw_to_deposit_works() {
 		);
 	});
 }
+
+/// Test drop/claim assets.
+#[test]
+fn trapped_assets_can_be_claimed() {
+	let balances = vec![(ALICE, INITIAL_BALANCE), (BOB, INITIAL_BALANCE)];
+	new_test_ext_with_balances(balances).execute_with(|| {
+		let weight = 6 * BaseXcmWeight::get();
+		let dest: MultiLocation =
+			Junction::AccountId32 { network: NetworkId::Any, id: BOB.into() }.into();
+
+		assert_ok!(XcmPallet::execute(
+			Origin::signed(ALICE),
+			Box::new(VersionedXcm::from(Xcm(vec![
+				WithdrawAsset((Here, SEND_AMOUNT).into()),
+				buy_execution((Here, SEND_AMOUNT)),
+				// Don't propagated the error into the result.
+				SetErrorHandler(Xcm(vec![ClearError])),
+				// This will make an error.
+				Trap(0),
+				// This would succeed, but we never get to it.
+				DepositAsset { assets: All.into(), max_assets: 1, beneficiary: dest.clone() },
+			]))),
+			weight
+		));
+		let source: MultiLocation =
+			Junction::AccountId32 { network: NetworkId::Any, id: ALICE.into() }.into();
+		let trapped = AssetTraps::<Test>::iter().collect::<Vec<_>>();
+		let vma = VersionedMultiAssets::from(MultiAssets::from((Here, SEND_AMOUNT)));
+		let hash = BlakeTwo256::hash_of(&(source.clone(), vma.clone()));
+		assert_eq!(
+			last_events(2),
+			vec![
+				Event::XcmPallet(crate::Event::AssetsTrapped(hash.clone(), source, vma)),
+				Event::XcmPallet(crate::Event::Attempted(Outcome::Complete(
+					5 * BaseXcmWeight::get()
+				)))
+			]
+		);
+		assert_eq!(Balances::total_balance(&ALICE), INITIAL_BALANCE - SEND_AMOUNT);
+		assert_eq!(Balances::total_balance(&BOB), INITIAL_BALANCE);
+
+		let expected = vec![(hash, 1u32)];
+		assert_eq!(trapped, expected);
+
+		let weight = 3 * BaseXcmWeight::get();
+		assert_ok!(XcmPallet::execute(
+			Origin::signed(ALICE),
+			Box::new(VersionedXcm::from(Xcm(vec![
+				ClaimAsset { assets: (Here, SEND_AMOUNT).into(), ticket: Here.into() },
+				buy_execution((Here, SEND_AMOUNT)),
+				DepositAsset { assets: All.into(), max_assets: 1, beneficiary: dest.clone() },
+			]))),
+			weight
+		));
+
+		assert_eq!(Balances::total_balance(&ALICE), INITIAL_BALANCE - SEND_AMOUNT);
+		assert_eq!(Balances::total_balance(&BOB), INITIAL_BALANCE + SEND_AMOUNT);
+		assert_eq!(AssetTraps::<Test>::iter().collect::<Vec<_>>(), vec![]);
+
+		let weight = 3 * BaseXcmWeight::get();
+		assert_ok!(XcmPallet::execute(
+			Origin::signed(ALICE),
+			Box::new(VersionedXcm::from(Xcm(vec![
+				ClaimAsset { assets: (Here, SEND_AMOUNT).into(), ticket: Here.into() },
+				buy_execution((Here, SEND_AMOUNT)),
+				DepositAsset { assets: All.into(), max_assets: 1, beneficiary: dest },
+			]))),
+			weight
+		));
+		assert_eq!(
+			last_event(),
+			Event::XcmPallet(crate::Event::Attempted(Outcome::Incomplete(
+				BaseXcmWeight::get(),
+				XcmError::UnknownClaim
+			)))
+		);
+	});
+}

xcm/src/lib.rs

@@ -217,6 +217,16 @@ pub enum VersionedMultiAssets {
 	V1(v1::MultiAssets),
 }
 
+impl VersionedMultiAssets {
+	pub fn into_version(self, n: u32) -> Result<Self, ()> {
+		Ok(match n {
+			0 => Self::V0(self.try_into()?),
+			1 | 2 => Self::V1(self.try_into()?),
+			_ => return Err(()),
+		})
+	}
+}
+
 impl From<Vec<v0::MultiAsset>> for VersionedMultiAssets {
 	fn from(x: Vec<v0::MultiAsset>) -> Self {
 		VersionedMultiAssets::V0(x)