updated snapshot name extraction

zfs-snapshot-backup.py

@@ -68,10 +68,19 @@ def gzip_filename(current, last=None):
 
 
 def extract_snapshot_name(filename):
-    parts = filename.split(".")
-    if len(parts) in (2, 4):
-        # snapshot1.gz or snapshot1.to.snapshot2.gz
-        return parts[-2]
+    # elabfs-LukasMetzler@auto-20190807.0200-1w.gz
+    if not filename.endswith(".gz"):
+        raise ValueError(f"Not a gzip file: {filename}")
+    if not filename.startswith(ZFS_ELAB_PREFIX):
+        raise ValueError(f"Not an elabfs snapshot: {filename}")
+    snapshot = filename[:-3]
+    if snapshot.count(".") == 1 and snapshot.count("@") == 1:
+        # elabfs-...@auto-20190807.0200-1w
+        return snapshot
+    elif snapshot.count(".to.") == 1 and snapshot.count("@") == 2:
+        # elabfs-...@auto-20190806.0200-1w.to.elabfs-...@auto-20190807.0200-1w
+        old_snapshot, new_snapshot = snapshot.split(".to.")
+        return new_snapshot
     else:
         raise ValueError(f"Unknown Filename Format: {filename}")