swap: Don't defer mutex unlock in a loop.

Using defer to unlock the mutex means that the mutex can be locked
repeatedly during the loop and only unlocked when the function returns,
rather than being unlocked at the end of the current loop iteration.

This can lead to the mutex being held for longer than required.

Author: jholdstock

server/swap/swap.go

@@ -1103,11 +1103,11 @@ func (s *Swapper) processBlock(ctx context.Context, block *blockNotification) {
 		// Lock the matchTracker so the following checks and updates are atomic
 		// with respect to Status.
 		match.mtx.RLock()
-		defer match.mtx.RUnlock()
 
 		switch match.Status {
 		case order.MakerSwapCast:
 			if match.makerStatus.swapAsset != block.assetID {
+				match.mtx.RUnlock()
 				break
 			}
 			// If the maker has broadcast their transaction, the taker's
@@ -1118,6 +1118,7 @@ func (s *Swapper) processBlock(ctx context.Context, block *blockNotification) {
 			}
 		case order.TakerSwapCast:
 			if match.takerStatus.swapAsset != block.assetID {
+				match.mtx.RUnlock()
 				break
 			}
 			// If the taker has broadcast their transaction, the maker's
@@ -1126,6 +1127,8 @@ func (s *Swapper) processBlock(ctx context.Context, block *blockNotification) {
 			if s.tryConfirmSwap(ctx, match.takerStatus, block.time) {
 				s.unlockOrderCoins(match.Taker)
 			}
+
+			match.mtx.RUnlock()
 		}
 	}
 }