storagenode/{orders,piecestore}: Always unlock unsent orders file, even with an empty order.

When we call ordersStore.BeginEnqueue, the unsent orders file for that
satellite and hour is prevented from being sent. It is freed when the
commit callback returned by BeginEnqueue is used. This change ensures
that we always call the commit callback, even when we have an empty
order or an order with Amount <= 0.

Change-Id: Ic4678f7eaa1e6957dd77d4bb5a23bb35d25b1e93

storagenode/orders/store.go

@@ -102,6 +102,11 @@ func (store *FileStore) BeginEnqueue(satelliteID storj.NodeID, createdAt time.Ti
 		// always remove the in flight operation
 		defer store.enqueueFinishedLocked(satelliteID, createdAt)
 
+		// caller wants to abort; free file for sending and return with no error
+		if info == nil {
+			return nil
+		}
+
 		// check that the info matches what the enqueue was begun with
 		if info.Limit.SatelliteId != satelliteID || !info.Limit.OrderCreation.Equal(createdAt) {
 			return OrderError.New("invalid info passed in to enqueue commit")

storagenode/piecestore/endpoint.go

@@ -660,12 +660,20 @@ func (endpoint *Endpoint) beginSaveOrder(limit *pb.OrderLimit) (_commit func(ctx
 
 	done := false
 	return func(ctx context.Context, order *pb.Order) {
-		// TODO: do this in a goroutine
-		if order == nil || order.Amount <= 0 || done {
+		if done {
 			return
 		}
 		done = true
 
+		if order == nil || order.Amount <= 0 {
+			// free unsent orders file for sending without writing anything
+			err = commit(nil)
+			if err != nil {
+				endpoint.log.Error("failed to unlock orders file", zap.Error(err))
+			}
+			return
+		}
+
 		err = commit(&orders.Info{Limit: limit, Order: order})
 		if err != nil {
 			endpoint.log.Error("failed to add order", zap.Error(err))