[Common] Fix show_file unhandled dbus error

If dbus org.freedesktop.FileManager1 service is missing then show_file
raised an unhandled exception. The service is not available on certain
desktop environments e.g. i3wm.

The solution is to fallback to xdg-open.

Fixes: #3272

deluge/common.py

@@ -81,6 +81,9 @@ TORRENT_STATE = [
 # The output formatting for json.dump
 JSON_FORMAT = {'indent': 4, 'sort_keys': True, 'ensure_ascii': False}
 
+DBUS_FM_ID = 'org.freedesktop.FileManager1'
+DBUS_FM_PATH = '/org/freedesktop/FileManager1'
+
 PY2 = sys.version_info.major == 2
 
 
@@ -355,16 +358,19 @@ def show_file(path, timestamp=None):
             timestamp,
             timestamp,
         )
+
         if dbus:
             bus = dbus.SessionBus()
-            filemanager1 = bus.get_object(
-                'org.freedesktop.FileManager1', '/org/freedesktop/FileManager1'
-            )
-            paths = [urljoin('file:', pathname2url(path))]
-            filemanager1.ShowItems(
-                paths, startup_id, dbus_interface='org.freedesktop.FileManager1'
-            )
+            try:
+                filemanager1 = bus.get_object(DBUS_FM_ID, DBUS_FM_PATH)
+            except dbus.exceptions.DBusException as ex:
+                log.debug('Unable to get dbus file manager: %s', ex)
+                # Fallback to xdg-open
             else:
+                paths = [urljoin('file:', pathname2url(path))]
+                filemanager1.ShowItems(paths, startup_id, dbus_interface=DBUS_FM_ID)
+                return
+
         env = os.environ.copy()
         env['DESKTOP_STARTUP_ID'] = startup_id.replace('dbus', 'xdg-open')
         # No option in xdg to highlight a file so just open parent folder.