dbus how to reconnect to a new session bus daemon

Question

As far as I understand it, a dbus user connects to the session bus daemon if the environment defines two variables like:

DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-1cAnv4I,guid=3a453c40b5034f
DBUS_SESSION_BUS_PID=102

If the daemon stops and we start a new daemon by dbus-launch, we will get new values for the environment. In this case how the new values can be propagated into a running user process without restart?

Answer 1

The logic to reconnect to a new dbus daemon should involve a few components and events:

• A bus-monitor which tells the application whether the bus is still up, or down, or restarted.

  • When the bus is restarted, the new bus address will be sent to the application.

• The application which responses to the down and restart events:

  • If the bus is down or restarted, close everything and go back to the beginning of the bus cycle before the call to dbus_bus_get().
  • If the bus is down, wait till bus is restarted.
  • If the bus is restarted, modify the process environment variable DBUS_SESSION_BUS_ADDRESS to the new dbus address, then proceed the new bus cycle by calling dbus_bus_get().

If you read the code in dbus/dbus_bus.c and follow the functions dbus_bus_get() -> internal_bus_get() -> init_connections_unlocked(), you may see the above logic is what you can do. Or you may see other approaches of doing the same thing. Please leave a comment if you know other approaches or have other thinking.

Edit: Verified the approach with more details:

• After dbus_bus_get(), call dbus_connection_set_exit_on_disconnect(conn,FALSE) so that the application will keep running after the dbus daemon disconnects.
• Add a dumb signal receiver that calls dbus_connection_read_write(conn,0). If the dbus connection is lost it returns a FALSE. That is used to decide when to close the dbus cycle. Drain the message queue with dbus_connection_pop_message(conn) despite the connection is closed.
• Close dbus cycle by dbus_connection_unref(conn) then dbus_shutdown(). If it is not shutdown the call to dbus_bus_get() will not make use of the new environment variable.
• Use setenv(char * name, char * value) to set session bus address.

Answer 2

One way to get address after restart is to query corresponding X11 property:

dbus-daemon sets _DBUS_SESSION_BUS_ADDRESS property of _DBUS_SESSION_BUS_SELECTION_[hostname]_[uuid] selection owner (uuid is content of /var/lib/dbus/machine-id ). This is the code I use to read the this value: https://github.com/sidorares/node-dbus/blob/master/lib/address-x11.js