email: Code quality fixes

- flake8: Delete unused imports
- lock.Mutex: Set file mode (rw-rw----) and ownership (plinth:plinth)

plinth/modules/email_server/__init__.py

@@ -14,7 +14,7 @@ from plinth.modules.firewall.components import Firewall
 from . import audit
 from . import manifest
 
-version = 31
+version = 1
 managed_packages = ['postfix', 'dovecot-pop3d', 'dovecot-imapd',
                     'dovecot-lmtpd', 'dovecot-ldap', 'dovecot-managesieved']
 managed_services = ['postfix', 'dovecot']

plinth/modules/email_server/lock.py

@@ -1,8 +1,11 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 import contextlib
+import errno
 import fcntl
 import os
+import pwd
 import threading
+import time
 
 
 class Mutex:
@@ -25,15 +28,50 @@ class Mutex:
     def lock_all(self):
         """Acquire both the thread lock and the file lock"""
         with self.lock_threads_only():
-            fd = open(self.lock_path, 'wb')
-            # FIXME: Who can lock?
-            try:
-                os.fchmod(fd.fileno(), 0o666)  # rw-rw-rw-
-            except OSError:
-                pass
+            # Set up
+            fd = self._open_lock_file()
             fcntl.lockf(fd, fcntl.LOCK_EX)
+            self._chmod_and_chown(fd)
+            # Enter context
             try:
                 yield
             finally:
+                # Clean up
                 fcntl.lockf(fd, fcntl.LOCK_UN)
                 fd.close()
+
+    def _open_lock_file(self):
+        """Attempt to open lock file for R&W. Raises OSError on failure"""
+        attempts = 10
+        errno = -1
+        fd = None
+        # Simulate a spin lock
+        while attempts > 0:
+            errno, fd = self._try(lambda: open(self.lock_path, 'wb'))
+            if errno == 0:
+                return fd
+            else:
+                attempts -= 1
+                time.sleep(0.25)
+        raise OSError(errno, os.strerror(errno))
+
+    def _chmod_and_chown(self, fd):
+        """If the process UID is root, set fd's mode and ownership to
+        appropriate values. If we are not root, only set the mode"""
+        if os.getuid() == 0:
+            user_info = pwd.getpwnam('plinth')
+            os.fchown(fd.fileno(), 0, 0)
+            os.fchmod(fd.fileno(), 0o660)  # rw-rw----
+            fd.truncate(0)
+            os.fchown(fd.fileno(), user_info.pw_uid, user_info.pw_gid)
+        else:
+            self._try(lambda: os.fchmod(fd.fileno(), 0o660))  # rw-rw----
+
+    def _try(self, function):
+        try:
+            return 0, function()
+        except OSError as error:
+            if error.errno in (errno.EACCES, errno.EPERM):
+                return error.errno, None
+            else:
+                raise

plinth/modules/email_server/postconf.py

@@ -4,7 +4,6 @@
 import dataclasses
 import re
 import subprocess
-import plinth.actions
 from .lock import Mutex
 
 postconf_mutex = Mutex('plinth-email-postconf.lock')