don't call the driver's set_flags() if the flags did not change at all.

src/sync.c

@@ -351,7 +351,7 @@ sync_old( int tops, store_t *sctx, store_t *tctx, store_conf_t *tconf, FILE *jfp
 				aflags &= F_DELETED;
 				dflags = 0;
 			}
-			switch (tdriver->set_flags( tctx, tmsg, tuid, aflags, dflags )) {
+			switch ((aflags | dflags) ? tdriver->set_flags( tctx, tmsg, tuid, aflags, dflags ) : DRV_OK) {
 			case DRV_STORE_BAD: return pull ? SYNC_SLAVE_BAD : SYNC_MASTER_BAD;
 			case DRV_BOX_BAD: return SYNC_FAIL;
 			default: /* ok */ break;