solana: add comment about additional grace period

Author: a5-pickle

solana/programs/matching-engine/src/processor/auction/execute_fast_order/mod.rs

@@ -54,21 +54,27 @@ fn prepare_order_execution<'info>(
 
     let (user_amount, new_status, beneficiary) = {
         let auction_info = auction.info.as_ref().unwrap();
-
         let current_slot = Clock::get().unwrap().slot;
+
+        // We extend the grace period for locally executed orders. Reserving a sequence number for
+        // the fast fill will most likely require an additional transaction, so this buffer allows
+        // the best offer participant to perform his duty without the risk of getting slashed by
+        // another executor.
+        let additional_grace_period = match auction.target_protocol {
+            MessageProtocol::Local { .. } => {
+                crate::EXECUTE_FAST_ORDER_LOCAL_ADDITIONAL_GRACE_PERIOD.into()
+            }
+            _ => None,
+        };
+
         let DepositPenalty {
             penalty,
             user_reward,
         } = utils::auction::compute_deposit_penalty(
             config,
             auction_info,
             current_slot,
-            match auction.target_protocol {
-                MessageProtocol::Local { .. } => {
-                    crate::EXECUTE_FAST_ORDER_LOCAL_ADDITIONAL_GRACE_PERIOD.into()
-                }
-                _ => None,
-            },
+            additional_grace_period,
         );
 
         let init_auction_fee = order.init_auction_fee();