define stub tasks and implement setup_file_sftp_userdir task

- create new module osusers.tasks
- define helper functions _build_sftp_directory_name and
  _build_mail_directory_name
- implement task setup_file_sftp_userdir
- define stubs for delete_file_sftp_userdir, setup_file_mail_userdir and
  delete_file_mail_userdir

gvafile/osusers/tasks.py

@@ -0,0 +1,82 @@
+"""
+This module defines `Celery`_ tasks to manage file system entities.
+
+.. _Celery: http://www.celeryproject.org/
+
+"""
+from __future__ import absolute_import, unicode_literals
+
+import os
+import subprocess
+
+from django.conf import settings
+
+from celery import shared_task
+from celery.utils.log import get_task_logger
+from celery.exceptions import Reject
+
+
+_logger = get_task_logger(__name__)
+
+
+def _build_sftp_directory_name(username):
+    """
+    Constructs the SFTP directory name for a given username.
+
+    """
+    return os.path.join(settings.GVAFILE_SFTP_DIRECTORY, username)
+
+
+def _build_mail_directory_name(username):
+    """
+    Constructs the mailbox directory name for a given username.
+
+    """
+    return os.path.join(settings.GVAFILE_MAIL_DIRECTORY, username)
+
+
+@shared_task
+def setup_file_sftp_userdir(username):
+    """
+    This task creates the home directory for an SFTP user if it does not exist
+    yet.
+
+    The task is rejected if the directory creation fails.
+
+    :param str username: the user name
+    :raises celery.exceptions.Reject: if the SFTP directory of the user cannot
+        be created
+    :return: the created directory name
+    :rtype: str
+
+    """
+    sftp_directory = _build_sftp_directory_name(username)
+    try:
+        subprocess.check_output([
+            'sudo', 'install', '-o', username, '-g', username, '-m', '0750',
+            '-d', sftp_directory], stderr=subprocess.STDOUT)
+        subprocess.check_output([
+            'sudo' 'setfacl', '-r', '-m', 'www-data:--x', sftp_directory],
+            stderr=subprocess.STDOUT)
+    except subprocess.CalledProcessError as exc:
+        _logger.exception(
+            'could not create SFTP directory for user {0}: {1}',
+            username, exc.output
+        )
+        raise Reject(exc, requeue=False)
+    return sftp_directory
+
+
+@shared_task
+def delete_file_sftp_userdir(username):
+    pass
+
+
+@shared_task
+def setup_file_mail_userdir(username):
+    pass
+
+
+@shared_task
+def delete_file_mail_userdir(username):
+    pass