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/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>;