xcm: SovereignPaidRemoteExporter: remove unused RefundSurplus instruction (#2312)

Refunding surplus happens anyway on xcm_executor::post_process(),
automatically refunding surplus to original_origin at the end of
execution. Since SovereignPaidRemoteExporter doesn't ClearOrigin, it can
simply rely on the automatic mechanism.

Furthermore, RefundSurplus instruction refunds _surplus_. Surplus exists
only as a result of Transact, SetErrorHandler or SetAppendix
instructions, none of which being part of the
SovereignPaidRemoteExporter XCM program. So surplus is always zero here
anyway.

cumulus/parachains/runtimes/bridge-hubs/test-utils/src/test_cases.rs

@@ -909,7 +909,6 @@ where
 				]),
 			]),
 		},
-		RefundSurplus,
 		DepositAsset {
 			assets: Wild(All),
 			beneficiary: MultiLocation { parents: 1, interior: X1(Parachain(1000)) },

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

@@ -44,7 +44,7 @@ fn sending_to_bridged_chain_works() {
 	maybe_with_topic(|| {
 		let msg = Xcm(vec![Trap(1)]);
 		let dest = (Parent, Parent, Remote::get(), Parachain(1)).into();
-		assert_eq!(send_xcm::<Router>(dest, msg).unwrap().1, (Here, 100).into());
+		assert_eq!(send_xcm::<Router>(dest, msg).unwrap().1, Price::get());
 		assert_eq!(TheBridge::service(), 1);
 		assert_eq!(
 			take_received_remote_messages(),
@@ -78,7 +78,7 @@ fn sending_to_parachain_of_bridged_chain_works() {
 	maybe_with_topic(|| {
 		let msg = Xcm(vec![Trap(1)]);
 		let dest = (Parent, Parent, Remote::get(), Parachain(1000)).into();
-		assert_eq!(send_xcm::<Router>(dest, msg).unwrap().1, (Here, 100).into());
+		assert_eq!(send_xcm::<Router>(dest, msg).unwrap().1, Price::get());
 		assert_eq!(TheBridge::service(), 1);
 		let expected = vec![(
 			(Parent, Parachain(1000)).into(),
@@ -110,7 +110,7 @@ fn sending_to_relay_chain_of_bridged_chain_works() {
 	maybe_with_topic(|| {
 		let msg = Xcm(vec![Trap(1)]);
 		let dest = (Parent, Parent, Remote::get()).into();
-		assert_eq!(send_xcm::<Router>(dest, msg).unwrap().1, (Here, 100).into());
+		assert_eq!(send_xcm::<Router>(dest, msg).unwrap().1, Price::get());
 		assert_eq!(TheBridge::service(), 1);
 		let expected = vec![(
 			Parent.into(),

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

@@ -41,7 +41,7 @@ fn sending_to_bridged_chain_works() {
 		let msg = Xcm(vec![Trap(1)]);
 		assert_eq!(
 			send_xcm::<Router>((Parent, Remote::get()).into(), msg).unwrap().1,
-			(Here, 100).into()
+			Price::get()
 		);
 		assert_eq!(TheBridge::service(), 1);
 		let expected = vec![(
@@ -68,7 +68,7 @@ fn sending_to_parachain_of_bridged_chain_works() {
 	maybe_with_topic(|| {
 		let msg = Xcm(vec![Trap(1)]);
 		let dest = (Parent, Remote::get(), Parachain(1000)).into();
-		assert_eq!(send_xcm::<Router>(dest, msg).unwrap().1, (Here, 100).into());
+		assert_eq!(send_xcm::<Router>(dest, msg).unwrap().1, Price::get());
 		assert_eq!(TheBridge::service(), 1);
 		let expected = vec![(
 			Parachain(1000).into(),

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

@@ -24,6 +24,9 @@
 use super::*;
 
 parameter_types! {
+	// 100 to use the bridge (export) and 80 for the remote execution weight (4 instructions x (10 +
+	// 10) weight each).
+	pub SendOverBridgePrice: u128 = 180u128 + if UsingTopic::get() { 20 } else { 0 };
 	pub UniversalLocation: Junctions = X2(GlobalConsensus(Local::get()), Parachain(100));
 	pub RelayUniversalLocation: Junctions = X1(GlobalConsensus(Local::get()));
 	pub RemoteUniversalLocation: Junctions = X1(GlobalConsensus(Remote::get()));
@@ -32,11 +35,9 @@ parameter_types! {
 			Remote::get(),
 			None,
 			MultiLocation::parent(),
-			Some((Parent, 200u128 + if UsingTopic::get() { 20 } else { 0 }).into())
+			Some((Parent, SendOverBridgePrice::get()).into())
 		)
 	];
-	// ^^^ 100 to use the bridge (export) and 100 for the remote execution weight (5 instructions
-	//     x (10 + 10) weight each).
 }
 type TheBridge =
 	TestBridge<BridgeBlobDispatcher<TestRemoteIncomingRouter, RemoteUniversalLocation, ()>>;
@@ -68,7 +69,7 @@ fn sending_to_bridged_chain_works() {
 		clear_assets(Parachain(100));
 		add_asset(Parachain(100), (Here, 1000u128));
 
-		let price = 200u128 + if UsingTopic::get() { 20 } else { 0 };
+		let price = SendOverBridgePrice::get();
 
 		let msg = Xcm(vec![Trap(1)]);
 		assert_eq!(send_xcm::<LocalRouter>(dest, msg).unwrap().1, (Parent, price).into());
@@ -86,7 +87,7 @@ fn sending_to_bridged_chain_works() {
 		)];
 		assert_eq!(take_received_remote_messages(), expected);
 
-		// The export cost 50 ref time and 50 proof size weight units (and thus 100 units of
+		// The export cost 40 ref time and 40 proof size weight units (and thus 80 units of
 		// balance).
 		assert_eq!(asset_list(Parachain(100)), vec![(Here, 1000u128 - price).into()]);
 
@@ -104,11 +105,10 @@ fn sending_to_bridged_chain_works() {
 						destination: Here,
 						xcm: xcm_with_topic([0; 32], vec![Trap(1)]),
 					},
-					RefundSurplus,
 					DepositAsset { assets: Wild(All), beneficiary: Parachain(100).into() },
 				],
 			),
-			outcome: Outcome::Complete(test_weight(5)),
+			outcome: Outcome::Complete(test_weight(4)),
 			paid: true,
 		};
 		assert_eq!(RoutingLog::take(), vec![entry]);
@@ -143,7 +143,7 @@ fn sending_to_parachain_of_bridged_chain_works() {
 		clear_assets(Parachain(100));
 		add_asset(Parachain(100), (Here, 1000u128));
 
-		let price = 200u128 + if UsingTopic::get() { 20 } else { 0 };
+		let price = SendOverBridgePrice::get();
 
 		let msg = Xcm(vec![Trap(1)]);
 		assert_eq!(send_xcm::<LocalRouter>(dest, msg).unwrap().1, (Parent, price).into());
@@ -161,7 +161,7 @@ fn sending_to_parachain_of_bridged_chain_works() {
 		)];
 		assert_eq!(take_received_remote_messages(), expected);
 
-		// The export cost 50 ref time and 50 proof size weight units (and thus 100 units of
+		// The export cost 40 ref time and 40 proof size weight units (and thus 80 units of
 		// balance).
 		assert_eq!(asset_list(Parachain(100)), vec![(Here, 1000u128 - price).into()]);
 
@@ -179,11 +179,10 @@ fn sending_to_parachain_of_bridged_chain_works() {
 						destination: Parachain(100).into(),
 						xcm: xcm_with_topic([0; 32], vec![Trap(1)]),
 					},
-					RefundSurplus,
 					DepositAsset { assets: Wild(All), beneficiary: Parachain(100).into() },
 				],
 			),
-			outcome: Outcome::Complete(test_weight(5)),
+			outcome: Outcome::Complete(test_weight(4)),
 			paid: true,
 		};
 		assert_eq!(RoutingLog::take(), vec![entry]);

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

@@ -305,7 +305,6 @@ impl<Bridges: ExporterFor, Router: SendXcm, UniversalLocation: Get<InteriorMulti
 				WithdrawAsset(fees.clone().into()),
 				BuyExecution { fees, weight_limit: Unlimited },
 				export_instruction,
-				RefundSurplus,
 				DepositAsset { assets: All.into(), beneficiary: local_from_bridge },
 			]
 		} else {