add: steps towards sending first snapshot

zfs-backup.py

@@ -75,12 +75,24 @@ class Manager:
             remote_dataset = self._search_matching_dataset_in_remote_pool(
                 local_dataset, self._external_pool
             )
+            if remote_dataset:
+                stream = self._send_incremental(local_dataset, remote_dataset)
+            else:
+                # TODO ask for confirmation before sending
+                stream = self._send_absolute(local_dataset)
+            ZFS.receive(stream, self._external_pool)
+
+    def _send_incremental(self, local_dataset: Dataset, remote_dataset: Dataset) -> Popen:
         start_snapshot = self._get_last_common_snapshot(
             local_dataset, remote_dataset
         )
         end_snapshot = self._get_last_snapshot_with_backup_tag(local_dataset)
         stream = ZFS.send_incremental(start_snapshot, end_snapshot)
-            ZFS.receive(stream, self._external_pool)
+        return stream
+
+    def _send_absolute(self, local_dataset: Dataset) -> Popen:
+        snapshot = self._get_last_snapshot_with_backup_tag(local_dataset)
+        return ZFS.send_absolute(snapshot)
 
     def _get_last_snapshot_with_backup_tag(self, local_dataset):
         snapshots = self._find_snapshots_with_backup_tag(local_dataset)
@@ -106,7 +118,7 @@ class Manager:
         ]
 
     @staticmethod
-    def _search_matching_dataset_in_remote_pool(dataset: Dataset, pool: Pool):
+    def _search_matching_dataset_in_remote_pool(dataset: Dataset, pool: Pool) -> Dataset | None:
         dataset_to_search = dataset.replace_pool(pool.name)
         for dataset in pool.datasets:
             if dataset == dataset_to_search: