In g_io_channel_get_type():

    our_type = g_boxed_type_register_static ("GIOChannel",
                                             (GBoxedCopyFunc) g_io_channel_ref,
                                             (GBoxedFreeFunc)
g_io_channel_unref);

The GBoxed code relies on the GBoxedCopyFunc associated with a type to
return a gpointer. However, g_io_channel_ref returns void, so the value is
undefined.

Possible solutions are:

1) API change, make g_io_channel_ref return its GIOChannel * instead of void.
2) Implement a private copy function in gsourceclosure.c to wrap
g_io_channel_ref and return the GIOChannel * itself.

I prefer solution #1, but since it's an API change I'm filing it here for
others to comment instead of just committing a fix.