Perform fee padding in a larger bit width of integers (#70)

* Modify fee calculation to use higher int size
* Add tests for various overflow cases
* Simplify behavior and add another overflow check

cmd/crates/soroban-rpc/src/txn.rs

@@ -313,15 +313,25 @@ pub fn assemble(
         }
     }
 
-    // update the fees of the actual transaction to meet the minimum resource fees.
-    let classic_transaction_fees = crate::DEFAULT_TRANSACTION_FEES;
-    // Pad the fees up by 15% for a bit of wiggle room.
-    tx.fee = (tx.fee.max(
-        classic_transaction_fees
-            + u32::try_from(simulation.min_resource_fee)
-                .map_err(|_| Error::LargeFee(simulation.min_resource_fee))?,
-    ) * 115)
-        / 100;
+    // Update transaction fees to meet the minimum resource fees.
+    let classic_tx_fee: u64 = crate::DEFAULT_TRANSACTION_FEES.into();
+
+    // Choose larger of existing fee or inclusion + resource fee.
+    let base_tx_fee: u64 = tx
+        .fee
+        .max(
+            u32::try_from(classic_tx_fee + simulation.min_resource_fee)
+                .map_err(|_| Error::LargeFee(simulation.min_resource_fee + classic_tx_fee))?,
+        )
+        .into(); // invariant: base_tx_fee <= u32::MAX
+
+    // Pad the total fee by up to 15% for a bit of wiggle room.
+    //
+    // We know for a fact because of the .min() call that we will not exceed
+    // U32_MAX, but we can't get clippy to shut the fuck up for this single
+    // line, so we have a redundant error check anyway.
+    tx.fee = u32::try_from((base_tx_fee * 115 / 100).min(u64::from(u32::MAX)))
+        .map_err(|_| Error::LargeFee(base_tx_fee))?;
 
     tx.operations = vec![op].try_into()?;
     tx.ext = TransactionExt::V1(transaction_data);
@@ -721,4 +731,66 @@ mod tests {
             r => panic!("expected UnexpectedSimulateTransactionResultSize error, got: {r:#?}"),
         }
     }
+
+    #[test]
+    fn test_assemble_transaction_overflow_behavior() {
+        //
+        // Test three separate cases:
+        //
+        //  1. Given a near-max (U32_MAX) resource fee, ensure the "wiggle room"
+        //     doesn't cause an overflow due to correct math order.
+        //     (Specifically, do U32_MAX - 15% - 100 - 1, so the final fee is
+        //     U32_MAX-1.)
+        //
+        //  2. Given a large resource fee that WILL exceed on its own with the
+        //     inclusion fee, ensure the overflow is caught with an error rather
+        //     than silently ignored.
+        //
+        //  3. Given a max resource fee that exceeds U32_MAX *only with* wiggle
+        //     room, ensure the overflow is ignored and we cap on the max.
+        //
+        let txn = single_contract_fn_transaction();
+        let mut response = simulation_response();
+
+        // sanity check so these can be adjusted if the above helper changes
+        assert_eq!(txn.fee, 100, "modified txn.fee: update the math below");
+
+        // 1: wiggle room math overflows but result fits
+        let mut resource_fee = (u64::from(u32::MAX) * 85 / 100) - 100 - 1;
+        assert_eq!(resource_fee, 3_650_722_099);
+        response.min_resource_fee = resource_fee;
+
+        match assemble(&txn, &response) {
+            Ok(asstxn) => {
+                let expected = (resource_fee + 100) * 115 / 100;
+                assert_eq!(asstxn.fee, u32::try_from(expected).unwrap());
+            }
+            r => panic!("expected success, got: {r:#?}"),
+        }
+
+        // 2: combo overflows, should throw
+        resource_fee = u64::from(u32::MAX);
+        assert_eq!(resource_fee, 4_294_967_295);
+        response.min_resource_fee = resource_fee;
+
+        match assemble(&txn, &response) {
+            Err(Error::LargeFee(fee)) => {
+                let expected = resource_fee + 100;
+                assert_eq!(expected, fee, "expected {expected} != {fee} actual");
+            }
+            r => panic!("expected LargeFee error, got: {r:#?}"),
+        }
+
+        // 2: combo works but wiggle room overflows, should cap
+        resource_fee = u64::from(u32::MAX) * 90 / 100;
+        assert_eq!(resource_fee, 3_865_470_565);
+        response.min_resource_fee = resource_fee;
+
+        match assemble(&txn, &response) {
+            Ok(asstxn) => {
+                assert_eq!(asstxn.fee, u32::MAX);
+            }
+            r => panic!("expected success, got: {r:#?}"),
+        }
+    }
 }