prepare indirect zfs transfer and multi-transfer

app.go

@@ -478,7 +478,7 @@ func (a *App) Transfer() error {
 	defer log.WithFields(log.Fields{"app": a.name}).Debugf("done")
 
 	for _, src := range a.sources {
-		backedUp := false
+		dests := make([]Addr, 0)
 		for _, dest := range a.destinations {
 			dest2 := dest.Append("/" + src.Box() + "/" + src.Path())
 			if dest2.Online() {
@@ -486,18 +486,17 @@ func (a *App) Transfer() error {
 					log.WithFields(log.Fields{"app": a.name, "call": "Mkdir", "attr": dest, "error": err}).Errorf("")
 					return err
 				}
-				if err := TransferZfs(src, dest2); err != nil {
-					log.WithFields(log.Fields{"app": a.name, "call": "TransferZfs", "src": src, "dest": dest, "error": err}).Errorf("")
-					return err
-				}
 				if err := dest2.SetManaged(true); err != nil {
 					log.WithFields(log.Fields{"app": a.name, "call": "SetManaged", "src": src, "dest": dest, "error": err}).Errorf("")
 					return err
 				}
-				backedUp = true
+				dests = append(dests, dest2)
 			}
 		}
-		if backedUp {
+		if n, err := TransferZfs(src, dests); err != nil {
+			log.WithFields(log.Fields{"app": a.name, "call": "TransferZfs", "src": src, "dests": dests, "error": err}).Errorf("")
+			return err
+		} else if n > 0 {
 			if err := src.SetBackedUp(true); err != nil {
 				log.WithFields(log.Fields{"app": a.name, "call": "SetBackedUp", "src": src, "error": err}).Errorf("")
 				return err