improvement(exchange-rate): make the scale conversion after the exchange rate is applied

Author: gakonst

crates/interledger-service-util/src/exchange_rates_service.rs

@@ -90,26 +90,25 @@ where
                 .build()));
             };
 
-            // Note that this is not an overflow safe operation. Given realistic
-            // assumptions, the asset scale will be <=18 and >=1.
-            // It is doubtful that the exchange rate between two assets,
-            // multiplied by 18 would exceed std::f64::MAX
-            let scaled_rate = rate.normalize_scale(ConvertDetails {
+            // Can we overflow here?
+            let outgoing_amount = (request.prepare.amount() as f64) * rate;
+            let outgoing_amount = outgoing_amount.normalize_scale(ConvertDetails {
                 from: request.from.asset_scale(),
                 to: request.to.asset_scale(),
             });
 
-            match scaled_rate {
-                Ok(scaled_rate) => {
-                    let outgoing_amount = (request.prepare.amount() as f64) * scaled_rate;
+            match outgoing_amount {
+                Ok(outgoing_amount) => {
+                    // NOTE: If the f64 amount could not fit in a u64, then the
+                    // typecast makes the outgoing amount 0.
                     request.prepare.set_amount(outgoing_amount as u64);
                     trace!("Converted incoming amount of: {} {} (scale {}) from account {} to outgoing amount of: {} {} (scale {}) for account {}",
                         request.original_amount, request.from.asset_code(), request.from.asset_scale(), request.from.id(),
                         outgoing_amount, request.to.asset_code(), request.to.asset_scale(), request.to.id());
                 }
                 Err(_) => {
                     return Box::new(err(RejectBuilder {
-                        code: ErrorCode::F02_UNREACHABLE,
+                        code: ErrorCode::F08_AMOUNT_TOO_LARGE,
                         message: format!(
                             "Could not convert exchange rate from {}:{} to: {}:{}",
                             request.from.asset_code(),