Always close the SSH connection

The connection was only closed at the end of a backup that ran to
completion, not on early returns (e.g. missing output folder or no
inputs) or exceptions, and a client that failed to log in was left
open. Close it in all cases, and remove the unused _password field.

simple_backup/simple_backup.py

@@ -155,7 +155,6 @@ class Backup:
         self._remote = False
         self._ssh = None
         self._password_auth = False
-        self._password = None
         self._removed_count = 0
 
     def check_params(self, homedir: str = '') -> int:
@@ -379,6 +378,18 @@ class Backup:
 
             return None
 
+        connected = None
+
+        try:
+            connected = self._ssh_login(ssh, homedir)
+
+            return connected
+        finally:
+            # Don't leave a half-open connection (e.g. after failed authentication)
+            if connected is None:
+                ssh.close()
+
+    def _ssh_login(self, ssh: paramiko.client.SSHClient, homedir: str) -> Optional[paramiko.client.SSHClient]:
         try:
             ssh.load_host_keys(filename=f'{homedir}/.ssh/known_hosts')
         except FileNotFoundError:
@@ -545,6 +556,13 @@ class Backup:
         finally:
             self._remove_temp_files()
 
+    def close(self) -> None:
+        """Close the SSH connection, if open"""
+
+        if self._ssh is not None:
+            self._ssh.close()
+            self._ssh = None
+
     def _remove_temp_files(self) -> None:
         for path in [self._inputs_path, self._exclude_path]:
             if path != '':
@@ -665,11 +683,6 @@ class Backup:
             else:
                 logger.warning('Backup not completed successfully. Old backups will not be removed')
 
-        if self._remote:
-            assert self._ssh is not None
-
-            self._ssh.close()
-
         # Files vanishing during the transfer is normal for a live system (return code 24)
         if returncode not in [0, 24]:
             logger.error(
@@ -1012,12 +1025,15 @@ def simple_backup() -> int:
     backup = Backup(inputs, output, exclude, keep, rsync_options, ssh_host, ssh_user, ssh_keyfile,
                     remote_sudo, remove_before=args.remove_before_backup, verbose=args.verbose)
 
+    try:
         return_code = backup.check_params(homedir)
 
         if return_code == 0:
             return backup.run()
 
         return return_code
+    finally:
+        backup.close()
 
 
 if __name__ == '__main__':