backups: repository: Introduce a prepare method

Useful for abstracting the mount() operation that is not available on non-remote
repositories.

- On non-remote repositories do nothing.

- On remote repositories, mount the location in preparation for operations on
the repository.

Tests performed:

- When an SSH repository is unmounted before a schedule backup, it is
automatically mounted for listing, backup and cleanup at scheduled time.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/backups/repository.py

@@ -126,6 +126,10 @@ class BaseBorgRepository(abc.ABC):
         """Return the repository that the backups action script should use."""
         return self._path
 
+    @staticmethod
+    def prepare():
+        """Prepare the repository for operations."""
+
     def get_info(self):
         """Return Borg information about a repository."""
         output = self.run(['info', '--path', self.borg_path])
@@ -373,6 +377,13 @@ class SshBorgRepository(BaseBorgRepository):
         """
         return self._mountpoint
 
+    def prepare(self):
+        """Prepare the repository for operations by mounting."""
+        if not self.is_usable():
+            raise errors.SshfsError('Remote host not verified')
+
+        self.mount()
+
     @property
     def hostname(self):
         """Return hostname from the remote path."""