Use rounded up computation when calculating fee (#1511)

* Use perbill floating point mult when calculating weight

* Cosmetic changes

* Rename test

* Always round up fractional computation

* Comments

* fmt

* comments

Author: girazoki

pallets/xcm-transactor/src/lib.rs

@@ -742,10 +742,17 @@ pub mod pallet {
 		}
 
 		/// Returns the fee for a given set of parameters
+		/// We always round up in case of fractional division
 		pub fn calculate_fee_per_second(weight: Weight, fee_per_second: u128) -> u128 {
-			let weight_fee =
-				fee_per_second.saturating_mul(weight as u128) / (WEIGHT_PER_SECOND as u128);
-			return weight_fee;
+			// grab WEIGHT_PER_SECOND as u128
+			let weight_per_second_u128 = WEIGHT_PER_SECOND as u128;
+
+			// we add WEIGHT_PER_SECOND -1 after multiplication to make sure that
+			// if there is a fractional part we round up the result
+			let fee_mul_rounded_up = (fee_per_second.saturating_mul(weight as u128))
+				.saturating_add(weight_per_second_u128 - 1);
+
+			fee_mul_rounded_up / weight_per_second_u128
 		}
 	}
 }

pallets/xcm-transactor/src/tests.rs

@@ -315,6 +315,23 @@ fn test_fee_calculation_works() {
 		})
 }
 
+// Kusama case
+#[test]
+fn test_fee_calculation_works_kusama_0_9_20_case() {
+	ExtBuilder::default()
+		.with_balances(vec![])
+		.build()
+		.execute_with(|| {
+			// 38620923000 * 319324000/1e12 = 12332587.6161
+			// integer arithmetic would round this to 12332587
+			// we test here that it rounds up to 12332588 instead
+			assert_eq!(
+				XcmTransactor::calculate_fee_per_second(319324000, 38620923000),
+				12332588
+			);
+		})
+}
+
 #[test]
 fn test_max_transact_weight_migration_works() {
 	ExtBuilder::default()