backup: only allow finished backups as base snapshot

If the datastore holds broken backups for some reason, do not attempt to
base following snapshots on those. This would lead to an error on
/previous, leaving the client no choice but to upload all chunks, even
though there might be potential for incremental savings.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/backup/backup_info.rs

@@ -313,9 +313,13 @@ impl BackupInfo {
     }
 
     /// Finds the latest backup inside a backup group
-    pub fn last_backup(base_path: &Path, group: &BackupGroup) -> Result<Option<BackupInfo>, Error> {
+    pub fn last_backup(base_path: &Path, group: &BackupGroup, only_finished: bool)
+        -> Result<Option<BackupInfo>, Error>
+    {
         let backups = group.list_backups(base_path)?;
-        Ok(backups.into_iter().max_by_key(|item| item.backup_dir.backup_time()))
+        Ok(backups.into_iter()
+            .filter(|item| !only_finished || item.is_finished())
+            .max_by_key(|item| item.backup_dir.backup_time()))
     }
 
     pub fn sort_list(list: &mut Vec<BackupInfo>, ascendending: bool) {