datetime: Re-implment backup/restore for timezone

Closes: #2326.

Earlier /etc/timezone was used to store timezone. Now, we use /etc/localtime
symlink instead. Since the change, backup/restore for timezone has not been
working. Implement this by backing up and restoring the new symlink. Borg
understands symlinks so it properly backs them up and restore them.

When the symlink is restored, timedatectl does not immediately show the new
timezone. This is because a DBus activated daemon 'systemd-timedated' which
supplies the information for timedatectl needs to be reloaded. Add this service
to list of services that backup framework needs to restart.

Tests:

- Set a new timezone. Backup datetime app. Set another timezone. Restore the
datetime app. Visiting the datetime app shows the restored timezone as expected
and timedatectl on command line also immediately shows the expected timezone.

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

plinth/modules/datetime/__init__.py

@@ -9,7 +9,7 @@ from django.utils.translation import gettext_lazy as _
 
 from plinth import app as app_module
 from plinth import menu
-from plinth.daemon import Daemon
+from plinth.daemon import Daemon, RelatedDaemon
 from plinth.modules.backups.components import BackupRestore
 from plinth.package import Packages
 
@@ -75,6 +75,9 @@ class DateTimeApp(app_module.App):
         packages = Packages('packages-datetime', ['systemd-timesyncd'])
         self.add(packages)
 
+        daemon = RelatedDaemon('daemon-datetime', 'systemd-timedated')
+        self.add(daemon)
+
         if self._is_time_managed():
             daemon = Daemon('daemon-datetime', 'systemd-timesyncd')
             self.add(daemon)

plinth/modules/datetime/manifest.py

@@ -3,4 +3,9 @@
 Application manifest for datetime.
 """
 
-backup = {'data': {'files': ['/etc/timezone']}}
+backup = {
+    'data': {
+        'files': ['/etc/localtime']
+    },
+    'services': ['systemd-timedated'],
+}