Fix remote removal of old backups deleting the wrong folders

Old backups on the server were listed with ls, which also lists files
and prints nothing on errors. A stray file made one backup too many
get removed, and an empty or unreadable backup folder was counted as
one backup with an empty name, which made rm -r delete the whole
simple_backup folder. List only directories with find, and don't
remove anything if listing fails.

simple_backup/simple_backup.py

@@ -248,10 +248,23 @@ class Backup:
         if self._remote:
             assert self._ssh is not None
 
-            _, stdout, _ = self._ssh.exec_command(
+            backup_dir = shlex.quote(self.output + '/simple_backup/')
-                f'ls {shlex.quote(self.output + "/simple_backup")}')
 
-            dirs = stdout.read().decode('utf-8').strip().split('\n')
+            # List only directories, and nothing (without errors) if no backup was made yet
+            _, stdout, stderr = self._ssh.exec_command(
+                f'if [ -d {backup_dir} ]; then find {backup_dir} -mindepth 1 -maxdepth 1 -type d; fi')
+
+            output = stdout.read().decode('utf-8')
+            err = stderr.read().decode('utf-8').strip()
+
+            if err != '':
+                # Don't risk removing the wrong directories if the list may be incomplete
+                logger.error('Cannot list old backups on the server. Old backups will not be removed')
+                logger.error(err)
+
+                return
+
+            dirs = [os.path.basename(d) for d in output.split('\n') if d != '']
 
             n_backup = len(dirs)
 
@@ -276,7 +289,7 @@ class Backup:
 
                     if err != '':
                         logger.error(
-                            'Error while removing backup %s.', {dirs[i]})
+                            'Error while removing backup %s.', dirs[i])
                         logger.error(err)
                     else:
                         count += 1