Threaded animation with cairo_gl backend over a GtkWidget

Hi!
I've made a modified version [1] of the example of threaded animation using
Cairo [2].

First I've used the method to create a cairo_gl_surface from the
test/gl_source_surface.c file adapted to create the surface for a given
widget instead for a defined size.

Also, for simplicity, I removed all the handled signals except the destroy
one.

To handle the drawing for the window widget using the cairo_gl backend I
use the create_gl_surface_for_window function passing it the proper widget
information.

So far, for a single drawing execution it works as expected:
1) In main() create the window_surface
2) The do_draw function creates a context based on window_surface
3) Once done the drawings with Cairo then the buffers are swapped and the
content is shown on screen.

Notice that the window_surface is never destroyed explicitly because it is
owned by the window widget and so it will be destroyed when the window is
destroyed by gtk.

The problem comes when I enable the multithreading support by uncommenting
the line:


//(void)g_timeout_add(33, (GSourceFunc)timer_exe, window);

It triggers the timer_exe function 33 times per second and then it creates
threads that will call do_draw each time.

Once enabled it crashes at the first painting function from Cairo.

I believe that the problem is that the window_surface belongs to the window
widget when I created it and then I can't modify it outside the main gtk
thread.

How can I solve this? If I enclose the Cairo painting operations inside a
gdk_threads_enter and
gdk_threads_leave I would add a lot of time consuming operations to the
main gtk thread, which is precisely what I want to avoid using
multithreading.

Any help? I would like to know at last, if my assumptions of the
window_surface ownership are true or not.

Thanks!

[1]
https://github.com/genete/my-test-area/blob/direct_draw/src/cairo_sample.c
[2] http://cairographics.org/threaded_animation_with_cairo/

-- 
Carlos
http://synfig.org

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