Contracts: xcm host fn fixes (#3086)

## Xcm changes:
- Fix `pallet_xcm::execute`, move the logic into The `ExecuteController`
so it can be shared with anything that implement that trait.
- Make `ExecuteController::execute` retursn `DispatchErrorWithPostInfo`
instead of `DispatchError`, so that we don't charge the full
`max_weight` provided if the execution is incomplete (useful for
force_batch or contracts calls)
- Fix docstring for `pallet_xcm::execute`, to reflect the changes from
#2405
- Update the signature for `ExecuteController::execute`, we don't need
to return the `Outcome` anymore since we only care about
`Outcome::Complete`

## Contracts changes:

- Update host fn `xcm_exexute`, we don't need to write the `Outcome` to
the sandbox memory anymore. This was also not charged as well before so
it if fixes this too.
- One of the issue was that the dry_run of a contract that call
`xcm_execute` would exhaust the `gas_limit`.

This is because `XcmExecuteController::execute` takes a `max_weight`
argument, and since we don't want the user to specify it manually we
were passing everything left by pre-charghing
`ctx.ext.gas_meter().gas_left()`

- To fix it I added a `fn influence_lowest_limit` on the `Token` trait
and make it return false for `RuntimeCost::XcmExecute`.
- Got rid of the `RuntimeToken` indirection, we can just use
`RuntimeCost` directly.

---------

Co-authored-by: command-bot <>

substrate/frame/contracts/mock-network/src/lib.rs

@@ -26,7 +26,8 @@ use crate::primitives::{AccountId, UNITS};
 use sp_runtime::BuildStorage;
 use xcm::latest::prelude::*;
 use xcm_executor::traits::ConvertLocation;
-use xcm_simulator::{decl_test_network, decl_test_parachain, decl_test_relay_chain, TestExt};
+pub use xcm_simulator::TestExt;
+use xcm_simulator::{decl_test_network, decl_test_parachain, decl_test_relay_chain};
 
 // Accounts
 pub const ADMIN: sp_runtime::AccountId32 = sp_runtime::AccountId32::new([0u8; 32]);

substrate/frame/contracts/mock-network/src/tests.rs

@@ -21,7 +21,6 @@ use crate::{
 	primitives::{AccountId, CENTS},
 	relay_chain, MockNet, ParaA, ParachainBalances, Relay, ALICE, BOB, INITIAL_BALANCE,
 };
-use assert_matches::assert_matches;
 use codec::{Decode, Encode};
 use frame_support::{
 	assert_err,
@@ -31,11 +30,18 @@ use frame_support::{
 use pallet_balances::{BalanceLock, Reasons};
 use pallet_contracts::{Code, CollectEvents, DebugInfo, Determinism};
 use pallet_contracts_fixtures::compile_module;
+use pallet_contracts_uapi::ReturnErrorCode;
 use xcm::{v4::prelude::*, VersionedLocation, VersionedXcm};
 use xcm_simulator::TestExt;
 
 type ParachainContracts = pallet_contracts::Pallet<parachain::Runtime>;
 
+macro_rules! assert_return_code {
+	( $x:expr , $y:expr $(,)? ) => {{
+		assert_eq!(u32::from_le_bytes($x.data[..].try_into().unwrap()), $y as u32);
+	}};
+}
+
 /// Instantiate the tests contract, and fund it with some balance and assets.
 fn instantiate_test_contract(name: &str) -> AccountId {
 	let (wasm, _) = compile_module::<Runtime>(name).unwrap();
@@ -100,19 +106,57 @@ fn test_xcm_execute() {
 			DebugInfo::UnsafeDebug,
 			CollectEvents::UnsafeCollect,
 			Determinism::Enforced,
-		)
-		.result
-		.unwrap();
+		);
 
-		let mut data = &result.data[..];
-		let outcome = Outcome::decode(&mut data).expect("Failed to decode xcm_execute Outcome");
-		assert_matches!(outcome, Outcome::Complete { .. });
+		assert_eq!(result.gas_consumed, result.gas_required);
+		assert_return_code!(&result.result.unwrap(), ReturnErrorCode::Success);
 
 		// Check if the funds are subtracted from the account of Alice and added to the account of
 		// Bob.
 		let initial = INITIAL_BALANCE;
-		assert_eq!(parachain::Assets::balance(0, contract_addr), initial);
 		assert_eq!(ParachainBalances::free_balance(BOB), initial + amount);
+		assert_eq!(ParachainBalances::free_balance(&contract_addr), initial - amount);
+	});
+}
+
+#[test]
+fn test_xcm_execute_incomplete() {
+	MockNet::reset();
+
+	let contract_addr = instantiate_test_contract("xcm_execute");
+	let amount = 10 * CENTS;
+
+	// Execute XCM instructions through the contract.
+	ParaA::execute_with(|| {
+		// The XCM used to transfer funds to Bob.
+		let message: Xcm<()> = Xcm(vec![
+			WithdrawAsset(vec![(Here, amount).into()].into()),
+			// This will fail as the contract does not have enough balance to complete both
+			// withdrawals.
+			WithdrawAsset(vec![(Here, INITIAL_BALANCE).into()].into()),
+			DepositAsset {
+				assets: All.into(),
+				beneficiary: AccountId32 { network: None, id: BOB.clone().into() }.into(),
+			},
+		]);
+
+		let result = ParachainContracts::bare_call(
+			ALICE,
+			contract_addr.clone(),
+			0,
+			Weight::MAX,
+			None,
+			VersionedXcm::V4(message).encode(),
+			DebugInfo::UnsafeDebug,
+			CollectEvents::UnsafeCollect,
+			Determinism::Enforced,
+		);
+
+		assert_eq!(result.gas_consumed, result.gas_required);
+		assert_return_code!(&result.result.unwrap(), ReturnErrorCode::XcmExecutionFailed);
+
+		assert_eq!(ParachainBalances::free_balance(BOB), INITIAL_BALANCE);
+		assert_eq!(ParachainBalances::free_balance(&contract_addr), INITIAL_BALANCE - amount);
 	});
 }
 
@@ -182,17 +226,10 @@ fn test_xcm_execute_reentrant_call() {
 			DebugInfo::UnsafeDebug,
 			CollectEvents::UnsafeCollect,
 			Determinism::Enforced,
-		)
-		.result
-		.unwrap();
-
-		let mut data = &result.data[..];
-		let outcome = Outcome::decode(&mut data).expect("Failed to decode xcm_execute Outcome");
-		assert_matches!(
-			outcome,
-			Outcome::Incomplete { used: _, error: XcmError::ExpectationFalse }
 		);
 
+		assert_return_code!(&result.result.unwrap(), ReturnErrorCode::XcmExecutionFailed);
+
 		// Funds should not change hands as the XCM transact failed.
 		assert_eq!(ParachainBalances::free_balance(BOB), INITIAL_BALANCE);
 	});