GtkPrintJob API wart
gtk_print_job_set_source_fd
doesn't take ownership of the passed file descriptor, so one has to manually track the ownership from outside. Every other way that priv->spool_io
is being set creates it with the close_on_unref
flag set.
Since fixing that would be an API break, I guess this can't be fixed before gtk5.
gtk-3-24 (also present on master).