There is a race condition when two threads try to initialize a type.

Thread A is initializating the gtype, which may involve adding an interface to it. This will grab class_init_rec_mutex.

Thread B is inside a class initialization already for some other type, so it already holds class_init_rec_mutex. Then the class init needs the gtype, so it calls get_type() on it.

The race here is that A gets to start initialzating the gtype, but before its reached the add_interface call thread B starts the unrelated class init, so thread A is blocking on class_init_rec_mutex. But thread B then blocks in the get_type() waiting for thread B to finish initializating the gtype.

So, we deadlock

Here is a sample diagram:

Thread 1                                                          Thread 2
create GUnixSocketAddress
init GUnixSocketAddress type
init (parent) GSocketAddress type
g_once_init_enter (socket_address_id)
 Add socket_address_id on g_once_init_list
                                                                  Creating a GSocketClient
                                                                  Needs to initialize GSocketClient class
                                                                  g_type_class_ref (GSocketClient)
                                                                    g_rec_mutex_lock (&class_init_rec_mutex);
                                                                  Call actual GSocketClientClass init code
                                                                  Needs GSocketAddress gtype (for g_signal_new argument)
                                                                  g_socket_address_get_type ()
                                                                  g_once_init_enter (socket_address_id)
                                                                   socket_address_id is NULL, so we're not inited
                                                                   but the it is on g_once_init_list!
                                                                   BLOCK until g_once_init_leave for socked_address_id

Start initializing GSocketAddress
  g_type_add_interface_static on socket_address type
  g_rec_mutex_lock (&class_init_rec_mutex);
  BLOCKS, because thread 2 is creating a class