xcm-executor: DepositReserveAsset charges delivery fees from inner as…

…sets

This fix aims to solve an issue in Kusama that resulted in failed
reserve asset transfers.

During multi-hop XCMs, like reserve asset transfers where the reserve
is not the sender nor the destination, but a third remote chain, the
origin is not available to pay for delivery fees out of their account
directly, so delivery fees should be paid out of transferred assets.

This commit also adds an xcm-emulator regression test that validates
this scenario is now working.

Signed-off-by: Adrian Catangiu <[email protected]>

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

@@ -822,6 +822,20 @@ impl<Config: config::Config> XcmExecutor<Config> {
 			DepositReserveAsset { assets, dest, xcm } => {
 				let old_holding = self.holding.clone();
 				let result = Config::TransactionalProcessor::process(|| {
+					// we need to do this take/put cycle to solve wildcards and get exact assets to
+					// be weighed
+					let to_weigh = self.holding.saturating_take(assets.clone());
+					self.holding.subsume_assets(to_weigh.clone());
+
+					let mut message_to_weigh =
+						vec![ReserveAssetDeposited(to_weigh.into()), ClearOrigin];
+					message_to_weigh.extend(xcm.0.clone().into_iter());
+					let (_, fee) =
+						validate_send::<Config::XcmSender>(dest.clone(), Xcm(message_to_weigh))?;
+					// set aside fee to be charged by XcmSender
+					let transport_fee = self.holding.saturating_take(fee.into());
+
+					// now take assets to deposit (excluding transport_fee)
 					let deposited = self.holding.saturating_take(assets);
 					for asset in deposited.assets_iter() {
 						Config::AssetTransactor::deposit_asset(&asset, &dest, Some(&self.context))?;
@@ -832,6 +846,8 @@ impl<Config: config::Config> XcmExecutor<Config> {
 					let assets = Self::reanchored(deposited, &dest, None);
 					let mut message = vec![ReserveAssetDeposited(assets), ClearOrigin];
 					message.extend(xcm.0.into_iter());
+					// put back transport_fee in holding register to be charged by XcmSender
+					self.holding.subsume_assets(transport_fee);
 					self.send(dest, Xcm(message), FeeReason::DepositReserveAsset)?;
 					Ok(())
 				});