logging PickForWrite error

https://github.com/seaweedfs/seaweedfs/issues/3886

weed/server/master_grpc_server_assign.go

@@ -90,7 +90,12 @@ func (ms *MasterServer) Assign(ctx context.Context, req *master_pb.AssignRequest
 
 	for time.Now().Sub(startTime) < maxTimeout {
 		fid, count, dnList, err := ms.Topo.PickForWrite(req.Count, option)
-		if err == nil {
+		if err != nil {
+			glog.Warningf("PickForWrite error: %+v", err)
+			lastErr = err
+			time.Sleep(200 * time.Millisecond)
+			continue
+		}
 		dn := dnList.Head()
 		var replicas []*master_pb.Location
 		for _, r := range dnList.Rest() {
@@ -114,9 +119,5 @@ func (ms *MasterServer) Assign(ctx context.Context, req *master_pb.AssignRequest
 			Replicas: replicas,
 		}, nil
 	}
-		//glog.V(4).Infoln("waiting for volume growing...")
-		lastErr = err
-		time.Sleep(200 * time.Millisecond)
-	}
 	return nil, lastErr
 }