Distinguish when MinXcmFee is not registered and FeeNotEnough

traits/src/get_by_key.rs

@@ -1,3 +1,5 @@
+use xcm::v1::MultiLocation;
+
 /// A trait for querying a value by a key.
 pub trait GetByKey<Key, Value> {
 	/// Return the value.
@@ -55,3 +57,11 @@ mod tests {
 		assert_eq!(Test::get(&3), 2);
 	}
 }
+
+// Default implementation for xTokens::MinXcmFee
+pub struct DisabledParachainFee;
+impl GetByKey<MultiLocation, Option<u128>> for DisabledParachainFee {
+    fn get(_key: &MultiLocation) -> Option<u128> {
+        None
+    }
+}

xtokens/README.md

@@ -51,22 +51,22 @@ Parachains should implements config `MinXcmFee` in `xtokens` module config:
 
 ```rust
 parameter_type_with_key! {
-	pub ParachainMinFee: |location: MultiLocation| -> u128 {
+	pub ParachainMinFee: |location: MultiLocation| -> Option<u128> {
 		#[allow(clippy::match_ref_pats)] // false positive
 		match (location.parents, location.first_interior()) {
-			(1, Some(Parachain(parachains::statemine::ID))) => 4_000_000_000,
-			_ => u128::MAX,
+			(1, Some(Parachain(parachains::statemine::ID))) => Some(4_000_000_000),
+			_ => None,
 		}
 	};
 }
 ```
 
-If Parachain don't want have this case, can simply return Max value:
+If Parachain don't want have this case, can simply return None:
 
 ```rust
 parameter_type_with_key! {
-	pub ParachainMinFee: |_location: MultiLocation| -> u128 {
-		u128::MAX
+	pub ParachainMinFee: |_location: MultiLocation| -> Option<u128> {
+		None
 	};
 }
 ```

xtokens/src/lib.rs

@@ -89,7 +89,7 @@ pub mod module {
 		type SelfLocation: Get<MultiLocation>;
 
 		/// Minimum xcm execution fee paid on destination chain.
-		type MinXcmFee: GetByKey<MultiLocation, u128>;
+		type MinXcmFee: GetByKey<MultiLocation, Option<u128>>;
 
 		/// XCM executor.
 		type XcmExecutor: ExecuteXcm<Self::Call>;
@@ -173,6 +173,8 @@ pub mod module {
 		FeeNotEnough,
 		/// Not supported MultiLocation
 		NotSupportedMultiLocation,
+		/// MinXcmFee not registered for certain reserve location
+		MinXcmFeeNotDefined,
 	}
 
 	#[pallet::hooks]
@@ -538,7 +540,7 @@ pub mod module {
 				ensure!(non_fee_reserve == dest.chain_part(), Error::<T>::InvalidAsset);
 
 				let reserve_location = non_fee_reserve.clone().ok_or(Error::<T>::AssetHasNoReserve)?;
-				let min_xcm_fee = T::MinXcmFee::get(&reserve_location);
+				let min_xcm_fee = T::MinXcmFee::get(&reserve_location).ok_or(Error::<T>::MinXcmFeeNotDefined)?;
 
 				// min xcm fee should less than user fee
 				let fee_to_dest: MultiAsset = (fee.id.clone(), min_xcm_fee).into();

xtokens/src/mock/para.rs

@@ -272,11 +272,11 @@ match_types! {
 }
 
 parameter_type_with_key! {
-	pub ParachainMinFee: |location: MultiLocation| -> u128 {
+	pub ParachainMinFee: |location: MultiLocation| -> Option<u128> {
 		#[allow(clippy::match_ref_pats)] // false positive
 		match (location.parents, location.first_interior()) {
-			(1, Some(Parachain(2))) => 40,
-			_ => u128::MAX,
+			(1, Some(Parachain(2))) => Some(40),
+			_ => None,
 		}
 	};
 }

xtokens/src/mock/para_relative_view.rs

@@ -339,11 +339,11 @@ match_types! {
 }
 
 parameter_type_with_key! {
-	pub ParachainMinFee: |location: MultiLocation| -> u128 {
+	pub ParachainMinFee: |location: MultiLocation| -> Option<u128> {
 		#[allow(clippy::match_ref_pats)] // false positive
 		match (location.parents, location.first_interior()) {
-			(1, Some(Parachain(2))) => 40,
-			_ => u128::MAX,
+			(1, Some(Parachain(2))) => Some(40),
+			_ => None,
 		}
 	};
 }

xtokens/src/tests.rs

@@ -873,7 +873,7 @@ fn transfer_asset_with_relay_fee_failed() {
 				),
 				40,
 			),
-			Error::<para::Runtime>::FeeNotEnough
+			Error::<para::Runtime>::MinXcmFeeNotDefined
 		);
 	});
 }