[Gnucash-changes] Eliminate a double free of memory.

[David Hampton]

On Thu, 2005-06-02 at 17:33 -0400, Derek Atkins wrote:
> Quoting David Hampton <hampton-gnucash at rainbolthampton.net>:
> 
> > 
> > I think this will work because gtk_widget_destroy correctly handles
> > recursive invocation.  If the widget is already in the process of being
> > destroyed then the call to gtk_widget_destroy from
> > gnc_customer_window_close_handler is a noop, otherwise it will actually
> > destroy the dialog.
> 
> What if the gtk_widget_destroy() finishes and returns before the CM callback
> happens?  For example, if the CM events are suspended, the CM system will cache
> the event and then return, causing gtk_widget_destroy() to finish.  Then later
> when the events are resumed it will call the CM callback which will call
> gtk_widget_destroy() on the already-freed widget.  Wouldn't that be a problem?

It might be possible to add a call to g_signal_connect_after(cw->dialog,
"destroy", gtk_widget_destroyed, &cw->dialog) when the dialog is created
to resolve this case.   If the window destruction completes before the
CM close function is called, then this new callback will have zeroed the
cw->dialog pointer.  

Of course, since the gnc_customer_window_close_handler function frees
cw, it would have to remove this callback so the original invocation
order we were discussing won't write to freed memory.  I don't know if
this will work though as I don't know if glib handles deleting callback
functions from within a callback.  It appears that glib builds an
ordered list of all handlers before calling the first handler, so the
above solution will not work for all cases.

I suppose this could be flipped and the gnc_customer_window_destroy_cb
callback could be connected "after" the gtk_widget_destroyed handler.
In this case if the dialog has been closed via the window manager close
button cw->dialog will always be NULL regardless of whether the CM
callback runs immediately or at some later time.  If the dialog has been
closed some other way, then cw->dialog will be non-NULL and this
function can call gtk_widget_destroy.  I think this works. 

David