Don't print tracebacks when desktop notifications fail

Without dbus-python, or without a session bus (e.g. when running
from a timer), each notification printed a traceback. Skip
notifications if dbus is not installed, and log a single warning
if sending one fails. Remove the now unneeded NameError handling
around _notify calls.

simple_backup/simple_backup.py

@@ -15,7 +15,6 @@ Classes:
 from __future__ import annotations
 
 # Import libraries
-import traceback
 import sys
 import os
 import pwd
@@ -51,7 +50,7 @@ except ImportError:
 try:
     import dbus
 except ImportError:
-    pass
+    dbus = None
 
 
 load_dotenv()
@@ -316,10 +315,7 @@ class Backup:
                 logger.critical(
                     'Cannot access the backup directory. Permission denied')
 
-                try:
                 _notify('Backup failed (check log for details)')
-                except NameError:
-                    pass
 
                 sys.exit(3)
 
@@ -500,10 +496,7 @@ class Backup:
 
         logger.info('Starting backup...')
 
-        try:
         _notify('Starting backup...')
-        except NameError:
-            pass
 
         self.define_backup_dir()
         self.find_last_backup()
@@ -525,10 +518,7 @@ class Backup:
             logger.info(
                 'No existing files or directories specified for backup. Nothing to do')
 
-            try:
             _notify('Backup finished. No files copied')
-            except NameError:
-                pass
 
             return 1
 
@@ -615,17 +605,11 @@ class Backup:
             if output == 'ok':
                 logger.info('Backup completed')
 
-                try:
                 _notify('Backup completed')
-                except NameError:
-                    pass
             else:
                 logger.error('Backup failed')
 
-                try:
                 _notify('Backup failed (check log for details)')
-                except NameError:
-                    pass
 
             if self._ssh:
                 self._ssh.close()
@@ -634,20 +618,14 @@ class Backup:
                 logger.error(
                     'Some errors occurred while performing the backup')
 
-                try:
                 _notify(
                     'Some errors occurred while performing the backup. Check log for details')
-                except NameError:
-                    pass
 
                 return 4
 
             logger.info('Backup completed')
 
-            try:
             _notify('Backup completed')
-            except NameError:
-                pass
 
             return 0
 
@@ -844,12 +822,20 @@ def _target_uid() -> int | None:
     return None
 
 
+_notify_failed = False
+
+
 def _notify(text: str) -> None:
+    global _notify_failed
+
+    if dbus is None or _notify_failed:
+        return
+
     euid = os.geteuid()
     uid = _target_uid()
 
     if uid is None:
-        print('notify: no target uid (neither SUDO_UID nor DOAS_USER set)', file=sys.stderr)
+        logger.debug('Cannot send desktop notification: user not found')
 
         return
 
@@ -862,8 +848,10 @@ def _notify(text: str) -> None:
                                            '/org/freedesktop/Notifications')
         obj = dbus.Interface(obj, 'org.freedesktop.Notifications')
         obj.Notify('simple_backup', 0, '', 'simple_backup', text, [], {'urgency': dbus.Byte(1)}, 10000)
-    except Exception:
-        traceback.print_exc()
+    except Exception as e:
+        # Report the failure only once (e.g. no session bus available), then stop trying
+        _notify_failed = True
+        logger.warning('Cannot send desktop notification: %s', e)
     finally:
         os.seteuid(euid)