XCM: Allow reclaim of assets dropped from holding (#3727)

* XCM: Introduce AssetTrap

* Revert reversions

* Remove attempts at weighing and add test

* Less storage use for asset trapping

* Add missing file

* Fixes

* Fixes

* Formatting

* Fixes

* Docs

* Filter types to allow runtimes to dictate which assets/origins should be trapped

* Formatting

* Tests

* Formatting

* Fixes

* Docs

polkadot/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()));

polkadot/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! {

polkadot/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! {

polkadot/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;
 }

polkadot/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

polkadot/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",

polkadot/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> {}
 
@@ -553,8 +569,47 @@ 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)
@@ -562,7 +617,6 @@ pub mod pallet {
 			false
 		}
 
-		/// Handler for receiving a `response` from `origin` relating to `query_id`.
 		fn on_response(
 			origin: &MultiLocation,
 			query_id: QueryId,

polkadot/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>;

polkadot/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
+			)))
+		);
+	});
+}

polkadot/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)

polkadot/xcm/src/v2/mod.rs

@@ -392,6 +392,8 @@ pub enum Instruction<Call> {
 	///   prioritized under standard asset ordering. Any others will remain in holding.
 	/// - `beneficiary`: The new owner for the assets.
 	///
+	/// Kind: *Instruction*
+	///
 	/// Errors:
 	DepositAsset { assets: MultiAssetFilter, max_assets: u32, beneficiary: MultiLocation },
 
@@ -411,6 +413,8 @@ pub enum Instruction<Call> {
 	/// - `xcm`: The orders that should follow the `ReserveAssetDeposited` instruction
 	///   which is sent onwards to `dest`.
 	///
+	/// Kind: *Instruction*
+	///
 	/// Errors:
 	DepositReserveAsset {
 		assets: MultiAssetFilter,
@@ -428,6 +432,8 @@ pub enum Instruction<Call> {
 	/// - `give`: The asset(s) to remove from holding.
 	/// - `receive`: The minimum amount of assets(s) which `give` should be exchanged for.
 	///
+	/// Kind: *Instruction*
+	///
 	/// Errors:
 	ExchangeAsset { give: MultiAssetFilter, receive: MultiAssets },
 
@@ -442,6 +448,8 @@ pub enum Instruction<Call> {
 	/// - `xcm`: The instructions to execute on the assets once withdrawn *on the reserve
 	///   location*.
 	///
+	/// Kind: *Instruction*
+	///
 	/// Errors:
 	InitiateReserveWithdraw { assets: MultiAssetFilter, reserve: MultiLocation, xcm: Xcm<()> },
 
@@ -456,6 +464,8 @@ pub enum Instruction<Call> {
 	/// NOTE: The `dest` location *MUST* respect this origin as a valid teleportation origin for all
 	/// `assets`. If it does not, then the assets may be lost.
 	///
+	/// Kind: *Instruction*
+	///
 	/// Errors:
 	InitiateTeleport { assets: MultiAssetFilter, dest: MultiLocation, xcm: Xcm<()> },
 
@@ -472,6 +482,8 @@ pub enum Instruction<Call> {
 	///   is sent as a reply may take to execute. NOTE: If this is unexpectedly large then the
 	///   response may not execute at all.
 	///
+	/// Kind: *Instruction*
+	///
 	/// Errors:
 	QueryHolding {
 		#[codec(compact)]
@@ -490,13 +502,20 @@ pub enum Instruction<Call> {
 	///   expected maximum weight of the total XCM to be executed for the
 	///   `AllowTopLevelPaidExecutionFrom` barrier to allow the XCM be executed.
 	///
+	/// Kind: *Instruction*
+	///
 	/// Errors:
 	BuyExecution { fees: MultiAsset, weight_limit: WeightLimit },
 
 	/// Refund any surplus weight previously bought with `BuyExecution`.
+	///
+	/// Kind: *Instruction*
+	///
+	/// Errors: None.
 	RefundSurplus,
 
-	/// Set code that should be called in the case of an error happening.
+	/// Set the Error Handler Register. This is code that should be called in the case of an error
+	/// happening.
 	///
 	/// An error occurring within execution of this code will _NOT_ result in the error register
 	/// being set, nor will an error handler be called due to it. The error handler and appendix
@@ -505,10 +524,15 @@ pub enum Instruction<Call> {
 	/// The apparent weight of this instruction is inclusive of the inner `Xcm`; the executing
 	/// weight however includes only the difference between the previous handler and the new
 	/// handler, which can reasonably be negative, which would result in a surplus.
+	///
+	/// Kind: *Instruction*
+	///
+	/// Errors: None.
 	SetErrorHandler(Xcm<Call>),
 
-	/// Set code that should be called after code execution (including the error handler if any)
-	/// is finished. This will be called regardless of whether an error occurred.
+	/// Set the Appendix Register. This is code that should be called after code execution
+	/// (including the error handler if any) is finished. This will be called regardless of whether
+	/// an error occurred.
 	///
 	/// Any error occurring due to execution of this code will result in the error register being
 	/// set, and the error handler (if set) firing.
@@ -516,10 +540,38 @@ pub enum Instruction<Call> {
 	/// The apparent weight of this instruction is inclusive of the inner `Xcm`; the executing
 	/// weight however includes only the difference between the previous appendix and the new
 	/// appendix, which can reasonably be negative, which would result in a surplus.
+	///
+	/// Kind: *Instruction*
+	///
+	/// Errors: None.
 	SetAppendix(Xcm<Call>),
 
-	/// Clear the error register.
+	/// Clear the Error Register.
+	///
+	/// Kind: *Instruction*
+	///
+	/// Errors: None.
 	ClearError,
+
+	/// Create some assets which are being held on behalf of the origin.
+	///
+	/// - `assets`: The assets which are to be claimed. This must match exactly with the assets
+	///   claimable by the origin of the ticket.
+	/// - `ticket`: The ticket of the asset; this is an abstract identifier to help locate the
+	///   asset.
+	///
+	/// Kind: *Instruction*
+	///
+	/// Errors:
+	ClaimAsset { assets: MultiAssets, ticket: MultiLocation },
+
+	/// Always throws an error of type `Trap`.
+	///
+	/// Kind: *Instruction*
+	///
+	/// Errors:
+	/// - `Trap`: All circumstances, whose inner value is the same as this item's inner value.
+	Trap(u64),
 }
 
 impl<Call> Xcm<Call> {
@@ -572,6 +624,8 @@ impl<Call> Instruction<Call> {
 			SetErrorHandler(xcm) => SetErrorHandler(xcm.into()),
 			SetAppendix(xcm) => SetAppendix(xcm.into()),
 			ClearError => ClearError,
+			ClaimAsset { assets, ticket } => ClaimAsset { assets, ticket },
+			Trap(code) => Trap(code),
 		}
 	}
 }

polkadot/xcm/src/v2/traits.rs

@@ -94,6 +94,10 @@ pub enum Error {
 	Unroutable,
 	/// The weight required was not specified when it should have been.
 	UnknownWeightRequired,
+	/// An error was intentionally forced. A code is included.
+	Trap(u64),
+	/// The given claim could not be recognized/found.
+	UnknownClaim,
 }
 
 impl From<()> for Error {

polkadot/xcm/xcm-builder/src/barriers.rs

@@ -30,7 +30,7 @@ use xcm_executor::traits::{OnResponse, ShouldExecute};
 pub struct TakeWeightCredit;
 impl ShouldExecute for TakeWeightCredit {
 	fn should_execute<Call>(
-		_origin: &Option<MultiLocation>,
+		_origin: &MultiLocation,
 		_top_level: bool,
 		_message: &mut Xcm<Call>,
 		max_weight: Weight,
@@ -49,19 +49,21 @@ impl ShouldExecute for TakeWeightCredit {
 pub struct AllowTopLevelPaidExecutionFrom<T>(PhantomData<T>);
 impl<T: Contains<MultiLocation>> ShouldExecute for AllowTopLevelPaidExecutionFrom<T> {
 	fn should_execute<Call>(
-		origin: &Option<MultiLocation>,
+		origin: &MultiLocation,
 		top_level: bool,
 		message: &mut Xcm<Call>,
 		max_weight: Weight,
 		_weight_credit: &mut Weight,
 	) -> Result<(), ()> {
-		let origin = origin.as_ref().ok_or(())?;
 		ensure!(T::contains(origin), ());
 		ensure!(top_level, ());
 		let mut iter = message.0.iter_mut();
 		let i = iter.next().ok_or(())?;
 		match i {
-			ReceiveTeleportedAsset(..) | WithdrawAsset(..) | ReserveAssetDeposited(..) => (),
+			ReceiveTeleportedAsset(..) |
+			WithdrawAsset(..) |
+			ReserveAssetDeposited(..) |
+			ClaimAsset { .. } => (),
 			_ => return Err(()),
 		}
 		let mut i = iter.next().ok_or(())?;
@@ -87,13 +89,12 @@ impl<T: Contains<MultiLocation>> ShouldExecute for AllowTopLevelPaidExecutionFro
 pub struct AllowUnpaidExecutionFrom<T>(PhantomData<T>);
 impl<T: Contains<MultiLocation>> ShouldExecute for AllowUnpaidExecutionFrom<T> {
 	fn should_execute<Call>(
-		origin: &Option<MultiLocation>,
+		origin: &MultiLocation,
 		_top_level: bool,
 		_message: &mut Xcm<Call>,
 		_max_weight: Weight,
 		_weight_credit: &mut Weight,
 	) -> Result<(), ()> {
-		let origin = origin.as_ref().ok_or(())?;
 		ensure!(T::contains(origin), ());
 		Ok(())
 	}
@@ -115,13 +116,12 @@ impl<ParaId: IsSystem + From<u32>> Contains<MultiLocation> for IsChildSystemPara
 pub struct AllowKnownQueryResponses<ResponseHandler>(PhantomData<ResponseHandler>);
 impl<ResponseHandler: OnResponse> ShouldExecute for AllowKnownQueryResponses<ResponseHandler> {
 	fn should_execute<Call>(
-		origin: &Option<MultiLocation>,
+		origin: &MultiLocation,
 		_top_level: bool,
 		message: &mut Xcm<Call>,
 		_max_weight: Weight,
 		_weight_credit: &mut Weight,
 	) -> Result<(), ()> {
-		let origin = origin.as_ref().ok_or(())?;
 		match message.0.first() {
 			Some(QueryResponse { query_id, .. })
 				if ResponseHandler::expecting_response(origin, *query_id) =>

polkadot/xcm/xcm-builder/src/mock.rs

@@ -35,6 +35,7 @@ pub use sp_std::{
 	marker::PhantomData,
 };
 pub use xcm::latest::prelude::*;
+use xcm_executor::traits::{ClaimAssets, DropAssets};
 pub use xcm_executor::{
 	traits::{ConvertOrigin, FilterAssetLocation, InvertLocation, OnResponse, TransactAsset},
 	Assets, Config,
@@ -269,6 +270,37 @@ pub type TestBarrier = (
 	AllowUnpaidExecutionFrom<IsInVec<AllowUnpaidFrom>>,
 );
 
+parameter_types! {
+	pub static TrappedAssets: Vec<(MultiLocation, MultiAssets)> = vec![];
+}
+
+pub struct TestAssetTrap;
+
+impl DropAssets for TestAssetTrap {
+	fn drop_assets(origin: &MultiLocation, assets: Assets) -> Weight {
+		let mut t: Vec<(MultiLocation, MultiAssets)> = TrappedAssets::get();
+		t.push((origin.clone(), assets.into()));
+		TrappedAssets::set(t);
+		5
+	}
+}
+
+impl ClaimAssets for TestAssetTrap {
+	fn claim_assets(origin: &MultiLocation, ticket: &MultiLocation, what: &MultiAssets) -> bool {
+		let mut t: Vec<(MultiLocation, MultiAssets)> = TrappedAssets::get();
+		if let (0, X1(GeneralIndex(i))) = (ticket.parents, &ticket.interior) {
+			if let Some((l, a)) = t.get(*i as usize) {
+				if l == origin && a == what {
+					t.swap_remove(*i as usize);
+					TrappedAssets::set(t);
+					return true
+				}
+			}
+		}
+		false
+	}
+}
+
 pub struct TestConfig;
 impl Config for TestConfig {
 	type Call = TestCall;
@@ -282,4 +314,6 @@ impl Config for TestConfig {
 	type Weigher = FixedWeightBounds<UnitWeightCost, TestCall, MaxInstructions>;
 	type Trader = FixedRateOfFungible<WeightPrice, ()>;
 	type ResponseHandler = TestResponseHandler;
+	type AssetTrap = TestAssetTrap;
+	type AssetClaims = TestAssetTrap;
 }

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

@@ -58,7 +58,7 @@ fn take_weight_credit_barrier_should_work() {
 		Xcm::<()>(vec![TransferAsset { assets: (Parent, 100).into(), beneficiary: Here.into() }]);
 	let mut weight_credit = 10;
 	let r = TakeWeightCredit::should_execute(
-		&Some(Parent.into()),
+		&Parent.into(),
 		true,
 		&mut message,
 		10,
@@ -68,7 +68,7 @@ fn take_weight_credit_barrier_should_work() {
 	assert_eq!(weight_credit, 0);
 
 	let r = TakeWeightCredit::should_execute(
-		&Some(Parent.into()),
+		&Parent.into(),
 		true,
 		&mut message,
 		10,
@@ -86,7 +86,7 @@ fn allow_unpaid_should_work() {
 	AllowUnpaidFrom::set(vec![Parent.into()]);
 
 	let r = AllowUnpaidExecutionFrom::<IsInVec<AllowUnpaidFrom>>::should_execute(
-		&Some(Parachain(1).into()),
+		&Parachain(1).into(),
 		true,
 		&mut message,
 		10,
@@ -95,7 +95,7 @@ fn allow_unpaid_should_work() {
 	assert_eq!(r, Err(()));
 
 	let r = AllowUnpaidExecutionFrom::<IsInVec<AllowUnpaidFrom>>::should_execute(
-		&Some(Parent.into()),
+		&Parent.into(),
 		true,
 		&mut message,
 		10,
@@ -112,7 +112,7 @@ fn allow_paid_should_work() {
 		Xcm::<()>(vec![TransferAsset { assets: (Parent, 100).into(), beneficiary: Here.into() }]);
 
 	let r = AllowTopLevelPaidExecutionFrom::<IsInVec<AllowPaidFrom>>::should_execute(
-		&Some(Parachain(1).into()),
+		&Parachain(1).into(),
 		true,
 		&mut message,
 		10,
@@ -128,7 +128,7 @@ fn allow_paid_should_work() {
 	]);
 
 	let r = AllowTopLevelPaidExecutionFrom::<IsInVec<AllowPaidFrom>>::should_execute(
-		&Some(Parent.into()),
+		&Parent.into(),
 		true,
 		&mut underpaying_message,
 		30,
@@ -144,7 +144,7 @@ fn allow_paid_should_work() {
 	]);
 
 	let r = AllowTopLevelPaidExecutionFrom::<IsInVec<AllowPaidFrom>>::should_execute(
-		&Some(Parachain(1).into()),
+		&Parachain(1).into(),
 		true,
 		&mut paying_message,
 		30,
@@ -153,7 +153,7 @@ fn allow_paid_should_work() {
 	assert_eq!(r, Err(()));
 
 	let r = AllowTopLevelPaidExecutionFrom::<IsInVec<AllowPaidFrom>>::should_execute(
-		&Some(Parent.into()),
+		&Parent.into(),
 		true,
 		&mut paying_message,
 		30,
@@ -202,6 +202,119 @@ fn transfer_should_work() {
 	assert_eq!(sent_xcm(), vec![]);
 }
 
+#[test]
+fn basic_asset_trap_should_work() {
+	// we'll let them have message execution for free.
+	AllowUnpaidFrom::set(vec![X1(Parachain(1)).into(), X1(Parachain(2)).into()]);
+
+	// Child parachain #1 owns 1000 tokens held by us in reserve.
+	add_asset(1001, (Here, 1000));
+	// They want to transfer 100 of them to their sibling parachain #2 but have a problem
+	let r = XcmExecutor::<TestConfig>::execute_xcm(
+		Parachain(1).into(),
+		Xcm(vec![
+			WithdrawAsset((Here, 100).into()),
+			DepositAsset {
+				assets: Wild(All),
+				max_assets: 0, //< Whoops!
+				beneficiary: AccountIndex64 { index: 3, network: Any }.into(),
+			},
+		]),
+		20,
+	);
+	assert_eq!(r, Outcome::Complete(25));
+	assert_eq!(assets(1001), vec![(Here, 900).into()]);
+	assert_eq!(assets(3), vec![]);
+
+	// Incorrect ticket doesn't work.
+	let old_trapped_assets = TrappedAssets::get();
+	let r = XcmExecutor::<TestConfig>::execute_xcm(
+		Parachain(1).into(),
+		Xcm(vec![
+			ClaimAsset { assets: (Here, 100).into(), ticket: GeneralIndex(1).into() },
+			DepositAsset {
+				assets: Wild(All),
+				max_assets: 1,
+				beneficiary: AccountIndex64 { index: 3, network: Any }.into(),
+			},
+		]),
+		20,
+	);
+	assert_eq!(r, Outcome::Incomplete(10, XcmError::UnknownClaim));
+	assert_eq!(assets(1001), vec![(Here, 900).into()]);
+	assert_eq!(assets(3), vec![]);
+	assert_eq!(old_trapped_assets, TrappedAssets::get());
+
+	// Incorrect origin doesn't work.
+	let old_trapped_assets = TrappedAssets::get();
+	let r = XcmExecutor::<TestConfig>::execute_xcm(
+		Parachain(2).into(),
+		Xcm(vec![
+			ClaimAsset { assets: (Here, 100).into(), ticket: GeneralIndex(0).into() },
+			DepositAsset {
+				assets: Wild(All),
+				max_assets: 1,
+				beneficiary: AccountIndex64 { index: 3, network: Any }.into(),
+			},
+		]),
+		20,
+	);
+	assert_eq!(r, Outcome::Incomplete(10, XcmError::UnknownClaim));
+	assert_eq!(assets(1001), vec![(Here, 900).into()]);
+	assert_eq!(assets(3), vec![]);
+	assert_eq!(old_trapped_assets, TrappedAssets::get());
+
+	// Incorrect assets doesn't work.
+	let old_trapped_assets = TrappedAssets::get();
+	let r = XcmExecutor::<TestConfig>::execute_xcm(
+		Parachain(1).into(),
+		Xcm(vec![
+			ClaimAsset { assets: (Here, 101).into(), ticket: GeneralIndex(0).into() },
+			DepositAsset {
+				assets: Wild(All),
+				max_assets: 1,
+				beneficiary: AccountIndex64 { index: 3, network: Any }.into(),
+			},
+		]),
+		20,
+	);
+	assert_eq!(r, Outcome::Incomplete(10, XcmError::UnknownClaim));
+	assert_eq!(assets(1001), vec![(Here, 900).into()]);
+	assert_eq!(assets(3), vec![]);
+	assert_eq!(old_trapped_assets, TrappedAssets::get());
+
+	let r = XcmExecutor::<TestConfig>::execute_xcm(
+		Parachain(1).into(),
+		Xcm(vec![
+			ClaimAsset { assets: (Here, 100).into(), ticket: GeneralIndex(0).into() },
+			DepositAsset {
+				assets: Wild(All),
+				max_assets: 1,
+				beneficiary: AccountIndex64 { index: 3, network: Any }.into(),
+			},
+		]),
+		20,
+	);
+	assert_eq!(r, Outcome::Complete(20));
+	assert_eq!(assets(1001), vec![(Here, 900).into()]);
+	assert_eq!(assets(3), vec![(Here, 100).into()]);
+
+	// Same again doesn't work :-)
+	let r = XcmExecutor::<TestConfig>::execute_xcm(
+		Parachain(1).into(),
+		Xcm(vec![
+			ClaimAsset { assets: (Here, 100).into(), ticket: GeneralIndex(0).into() },
+			DepositAsset {
+				assets: Wild(All),
+				max_assets: 1,
+				beneficiary: AccountIndex64 { index: 3, network: Any }.into(),
+			},
+		]),
+		20,
+	);
+	assert_eq!(r, Outcome::Incomplete(10, XcmError::UnknownClaim));
+}
+
 #[test]
 fn errors_should_return_unused_weight() {
 	// we'll let them have message execution for free.

polkadot/xcm/xcm-builder/tests/mock/mod.rs

@@ -166,7 +166,9 @@ impl xcm_executor::Config for XcmConfig {
 	type Barrier = Barrier;
 	type Weigher = FixedWeightBounds<BaseXcmWeight, Call, MaxInstructions>;
 	type Trader = FixedRateOfFungible<KsmPerSecond, ()>;
-	type ResponseHandler = ();
+	type ResponseHandler = XcmPallet;
+	type AssetTrap = XcmPallet;
+	type AssetClaims = XcmPallet;
 }
 
 pub type LocalOriginToLocation = SignedToAccountId32<Origin, AccountId, KusamaNetwork>;

polkadot/xcm/xcm-executor/src/assets.rs

@@ -94,6 +94,11 @@ impl Assets {
 		self.fungible.len() + self.non_fungible.len()
 	}
 
+	/// Returns `true` if `self` contains no assets.
+	pub fn is_empty(&self) -> bool {
+		self.fungible.is_empty() && self.non_fungible.is_empty()
+	}
+
 	/// A borrowing iterator over the fungible assets.
 	pub fn fungible_assets_iter<'a>(&'a self) -> impl Iterator<Item = MultiAsset> + 'a {
 		self.fungible

polkadot/xcm/xcm-executor/src/config.rs

@@ -15,8 +15,8 @@
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
 use crate::traits::{
-	ConvertOrigin, FilterAssetLocation, InvertLocation, OnResponse, ShouldExecute, TransactAsset,
-	WeightBounds, WeightTrader,
+	ClaimAssets, ConvertOrigin, DropAssets, FilterAssetLocation, InvertLocation, OnResponse,
+	ShouldExecute, TransactAsset, WeightBounds, WeightTrader,
 };
 use frame_support::{
 	dispatch::{Dispatchable, Parameter},
@@ -58,4 +58,11 @@ pub trait Config {
 
 	/// What to do when a response of a query is found.
 	type ResponseHandler: OnResponse;
+
+	/// The general asset trap - handler for when assets are left in the Holding Register at the
+	/// end of execution.
+	type AssetTrap: DropAssets;
+
+	/// The handler for when there is an instruction to claim assets.
+	type AssetClaims: ClaimAssets;
 }

polkadot/xcm/xcm-executor/src/lib.rs

@@ -31,8 +31,8 @@ use xcm::latest::{
 
 pub mod traits;
 use traits::{
-	ConvertOrigin, FilterAssetLocation, InvertLocation, OnResponse, ShouldExecute, TransactAsset,
-	WeightBounds, WeightTrader,
+	ClaimAssets, ConvertOrigin, DropAssets, FilterAssetLocation, InvertLocation, OnResponse,
+	ShouldExecute, TransactAsset, WeightBounds, WeightTrader,
 };
 
 mod assets;
@@ -86,7 +86,6 @@ impl<Config: config::Config> ExecuteXcm<Config::Call> for XcmExecutor<Config> {
 		if xcm_weight > weight_limit {
 			return Outcome::Error(XcmError::WeightLimitReached(xcm_weight))
 		}
-		let origin = Some(origin);
 
 		if let Err(_) = Config::Barrier::should_execute(
 			&origin,
@@ -98,7 +97,7 @@ impl<Config: config::Config> ExecuteXcm<Config::Call> for XcmExecutor<Config> {
 			return Outcome::Error(XcmError::Barrier)
 		}
 
-		let mut vm = Self::new(origin);
+		let mut vm = Self::new(origin.clone());
 
 		while !message.0.is_empty() {
 			let result = vm.execute(message);
@@ -116,9 +115,12 @@ impl<Config: config::Config> ExecuteXcm<Config::Call> for XcmExecutor<Config> {
 		vm.refund_surplus();
 		drop(vm.trader);
 
-		// TODO #2841: Do something with holding? (Fail-safe AssetTrap?)
+		let mut weight_used = xcm_weight.saturating_sub(vm.total_surplus);
+
+		if !vm.holding.is_empty() {
+			weight_used.saturating_accrue(Config::AssetTrap::drop_assets(&origin, vm.holding));
+		};
 
-		let weight_used = xcm_weight.saturating_sub(vm.total_surplus);
 		match vm.error {
 			None => Outcome::Complete(weight_used),
 			// TODO: #2841 #REALWEIGHT We should deduct the cost of any instructions following
@@ -129,10 +131,10 @@ impl<Config: config::Config> ExecuteXcm<Config::Call> for XcmExecutor<Config> {
 }
 
 impl<Config: config::Config> XcmExecutor<Config> {
-	fn new(origin: Option<MultiLocation>) -> Self {
+	fn new(origin: MultiLocation) -> Self {
 		Self {
 			holding: Assets::new(),
-			origin,
+			origin: Some(origin),
 			trader: Config::Trader::new(),
 			error: None,
 			total_surplus: 0,
@@ -183,7 +185,7 @@ impl<Config: config::Config> XcmExecutor<Config> {
 	/// Drop the registered error handler and refund its weight.
 	fn drop_error_handler(&mut self) {
 		self.error_handler = Xcm::<Config::Call>(vec![]);
-		self.total_surplus = self.total_surplus.saturating_add(self.error_handler_weight);
+		self.total_surplus.saturating_accrue(self.error_handler_weight);
 		self.error_handler_weight = 0;
 	}
 
@@ -199,7 +201,7 @@ impl<Config: config::Config> XcmExecutor<Config> {
 	fn refund_surplus(&mut self) {
 		let current_surplus = self.total_surplus.saturating_sub(self.total_refunded);
 		if current_surplus > 0 {
-			self.total_refunded = self.total_refunded.saturating_add(current_surplus);
+			self.total_refunded.saturating_accrue(current_surplus);
 			if let Some(w) = self.trader.refund_weight(current_surplus) {
 				self.holding.subsume(w);
 			}
@@ -300,7 +302,7 @@ impl<Config: config::Config> XcmExecutor<Config> {
 				// reported back to the caller and this ensures that they account for the total
 				// weight consumed correctly (potentially allowing them to do more operations in a
 				// block than they otherwise would).
-				self.total_surplus = self.total_surplus.saturating_add(surplus);
+				self.total_surplus.saturating_accrue(surplus);
 				Ok(())
 			},
 			QueryResponse { query_id, response, max_weight } => {
@@ -390,14 +392,14 @@ impl<Config: config::Config> XcmExecutor<Config> {
 			},
 			SetErrorHandler(mut handler) => {
 				let handler_weight = Config::Weigher::weight(&mut handler)?;
-				self.total_surplus = self.total_surplus.saturating_add(self.error_handler_weight);
+				self.total_surplus.saturating_accrue(self.error_handler_weight);
 				self.error_handler = handler;
 				self.error_handler_weight = handler_weight;
 				Ok(())
 			},
 			SetAppendix(mut appendix) => {
 				let appendix_weight = Config::Weigher::weight(&mut appendix)?;
-				self.total_surplus = self.total_surplus.saturating_add(self.appendix_weight);
+				self.total_surplus.saturating_accrue(self.appendix_weight);
 				self.appendix = appendix;
 				self.appendix_weight = appendix_weight;
 				Ok(())
@@ -406,6 +408,16 @@ impl<Config: config::Config> XcmExecutor<Config> {
 				self.error = None;
 				Ok(())
 			},
+			ClaimAsset { assets, ticket } => {
+				let origin = self.origin.as_ref().ok_or(XcmError::BadOrigin)?;
+				let ok = Config::AssetClaims::claim_assets(origin, &ticket, &assets);
+				ensure!(ok, XcmError::UnknownClaim);
+				for asset in assets.drain().into_iter() {
+					self.holding.subsume(asset);
+				}
+				Ok(())
+			},
+			Trap(code) => Err(XcmError::Trap(code)),
 			ExchangeAsset { .. } => Err(XcmError::Unimplemented),
 			HrmpNewChannelOpenRequest { .. } => Err(XcmError::Unimplemented),
 			HrmpChannelAccepted { .. } => Err(XcmError::Unimplemented),

polkadot/xcm/xcm-executor/src/traits/drop_assets.rs

@@ -0,0 +1,79 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// 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::Assets;
+use core::marker::PhantomData;
+use frame_support::{traits::Contains, weights::Weight};
+use xcm::latest::{MultiAssets, MultiLocation};
+
+/// Define a handler for when some non-empty `Assets` value should be dropped.
+pub trait DropAssets {
+	/// Handler for receiving dropped assets. Returns the weight consumed by this operation.
+	fn drop_assets(origin: &MultiLocation, assets: Assets) -> Weight;
+}
+impl DropAssets for () {
+	fn drop_assets(_origin: &MultiLocation, _assets: Assets) -> Weight {
+		0
+	}
+}
+
+/// Morph a given `DropAssets` implementation into one which can filter based on assets. This can
+/// be used to ensure that `Assets` values which hold no value are ignored.
+pub struct FilterAssets<D, A>(PhantomData<(D, A)>);
+
+impl<D: DropAssets, A: Contains<Assets>> DropAssets for FilterAssets<D, A> {
+	fn drop_assets(origin: &MultiLocation, assets: Assets) -> Weight {
+		if A::contains(&assets) {
+			D::drop_assets(origin, assets)
+		} else {
+			0
+		}
+	}
+}
+
+/// Morph a given `DropAssets` implementation into one which can filter based on origin. This can
+/// be used to ban origins which don't have proper protections/policies against misuse of the
+/// asset trap facility don't get to use it.
+pub struct FilterOrigin<D, O>(PhantomData<(D, O)>);
+
+impl<D: DropAssets, O: Contains<MultiLocation>> DropAssets for FilterOrigin<D, O> {
+	fn drop_assets(origin: &MultiLocation, assets: Assets) -> Weight {
+		if O::contains(origin) {
+			D::drop_assets(origin, assets)
+		} else {
+			0
+		}
+	}
+}
+
+/// Define any handlers for the `AssetClaim` instruction.
+pub trait ClaimAssets {
+	/// Claim any assets available to `origin` and return them in a single `Assets` value, together
+	/// with the weight used by this operation.
+	fn claim_assets(origin: &MultiLocation, ticket: &MultiLocation, what: &MultiAssets) -> bool;
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl ClaimAssets for Tuple {
+	fn claim_assets(origin: &MultiLocation, ticket: &MultiLocation, what: &MultiAssets) -> bool {
+		for_tuples!( #(
+			if Tuple::claim_assets(origin, ticket, what) {
+				return true;
+			}
+		)* );
+		false
+	}
+}

polkadot/xcm/xcm-executor/src/traits/mod.rs

@@ -18,6 +18,8 @@
 
 mod conversion;
 pub use conversion::{Convert, ConvertOrigin, Decoded, Encoded, Identity, InvertLocation, JustTry};
+mod drop_assets;
+pub use drop_assets::{ClaimAssets, DropAssets};
 mod filter_asset_location;
 pub use filter_asset_location::FilterAssetLocation;
 mod matches_fungible;

polkadot/xcm/xcm-executor/src/traits/should_execute.rs

@@ -34,7 +34,7 @@ pub trait ShouldExecute {
 	///   message may utilize in its execution. Typically non-zero only because of prior fee
 	///   payment, but could in principle be due to other factors.
 	fn should_execute<Call>(
-		origin: &Option<MultiLocation>,
+		origin: &MultiLocation,
 		top_level: bool,
 		message: &mut Xcm<Call>,
 		max_weight: Weight,
@@ -45,7 +45,7 @@ pub trait ShouldExecute {
 #[impl_trait_for_tuples::impl_for_tuples(30)]
 impl ShouldExecute for Tuple {
 	fn should_execute<Call>(
-		origin: &Option<MultiLocation>,
+		origin: &MultiLocation,
 		top_level: bool,
 		message: &mut Xcm<Call>,
 		max_weight: Weight,

polkadot/xcm/xcm-simulator/example/src/parachain.rs

@@ -143,6 +143,8 @@ impl Config for XcmConfig {
 	type Weigher = FixedWeightBounds<UnitWeightCost, Call, MaxInstructions>;
 	type Trader = FixedRateOfFungible<KsmPerSecond, ()>;
 	type ResponseHandler = ();
+	type AssetTrap = ();
+	type AssetClaims = ();
 }
 
 #[frame_support::pallet]

polkadot/xcm/xcm-simulator/example/src/relay_chain.rs

@@ -133,6 +133,8 @@ impl Config for XcmConfig {
 	type Weigher = FixedWeightBounds<BaseXcmWeight, Call, MaxInstructions>;
 	type Trader = FixedRateOfFungible<KsmPerSecond, ()>;
 	type ResponseHandler = ();
+	type AssetTrap = ();
+	type AssetClaims = ();
 }
 
 pub type LocalOriginToLocation = SignedToAccountId32<Origin, AccountId, KusamaNetwork>;