gtk_widget_set_size_request delayed until next allocation change

[Sandro Mani <manisandro gmail com>]

Hello,

I have a GtkDrawingArea inside a GtkViewport, and I have a callback 
connected to the "size-allocate" signal of the GtkViewport which resizes 
the GtkDrawingArea by calling gtk_widget_set_size_request on the latter, 
see sample source code below.

I however notice that the GtkDrawingArea allocates the requested size 
only the _next_ time the allocation of the GtkViewport changes, and 
hence the allocation of the GtkDrawingArea is the one requested for the 
previous allocation of the GtkViewport.

The issue is best visible when maximizing and unmaximizing the window: 
when maximizing, the size of the GtkDrawingArea is still the one for the 
unmaximized state, and when unmaximizing again, the size of the 
GtkDrawingArea is the one for the maximized state.

Question: Is this a mistake in my code or a Gtk bug? I'm using 
gtk3-3.8.1-1.fc20.x86_64

Thanks for any inputs.

Sandro

------ Sample code: ------
// gcc alloc.c -o alloc $(pkg-config --cflags --libs gtk+-3.0)
#include <gtk/gtk.h>
#include <stdio.h>

void resize_drawarea (GtkWidget* viewport, GdkRectangle* rect, gpointer 
data)
{
  gtk_widget_set_size_request (GTK_WIDGET (data), rect->width / 2, 
rect->height / 2);
  gtk_widget_queue_resize (GTK_WIDGET (data));
  printf("set_size_request(%d, %d)\n", rect->width /2, rect->height / 2);
  // Wait for size_request
  while (gtk_events_pending ()) {
    gtk_main_iteration ();
  }
  int w = gtk_widget_get_allocated_width (GTK_WIDGET (data));
  int h = gtk_widget_get_allocated_height (GTK_WIDGET (data));
  printf("allocation = %d x %d\n\n", w, h);
}

int main(int argc, char* argv[])
{
  gtk_init (&argc, &argv);
  GtkWidget* window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  GtkWidget* viewport = gtk_viewport_new (NULL, NULL);
  GtkWidget* drawarea = gtk_drawing_area_new ();

  gtk_container_add (GTK_CONTAINER (window), viewport);
  gtk_container_add (GTK_CONTAINER (viewport), drawarea);

  gtk_widget_set_halign (drawarea, GTK_ALIGN_CENTER);
  gtk_widget_set_valign (drawarea, GTK_ALIGN_CENTER);
  GdkRGBA black = {0., 0., 0., 1.};
  gtk_widget_override_background_color (drawarea, 
GTK_STATE_FLAG_NORMAL, &black);
  gtk_window_set_default_size (GTK_WINDOW (window), 640, 480);

  g_signal_connect (window, "destroy", G_CALLBACK (gtk_main_quit), NULL);
  g_signal_connect (viewport, "size-allocate", G_CALLBACK 
(resize_drawarea), drawarea);

  gtk_widget_show_all (window);
  gtk_main ();
  return 0;
}