Description Ansgar Radermacher 2011-09-20 09:06:57 EDT

A combo box has (among other handles) a button handle. When the operation pack() is called, the button handle is freed by an underlying GTK function - in this case gtk_widget_size_request() which is called by computeSize() which is turn is called by pack().

I create a test class inheriting from Combo. The function test() within this class has the following code;

public void test() {
  int index = (int)/*64*/ OS.g_object_get_qdata (buttonHandle, display.SWT_OBJECT_INDEX) - 1;
  System.err.println (0 <= index && index < display.widgetTable.length ?
    "widget found (before)" : "widget not found (before)");
  GtkRequisition requisition = new GtkRequisition ();
  gtk_widget_size_request (handle, requisition);
  index = (int)/*64*/ OS.g_object_get_qdata (buttonHandle, display.SWT_OBJECT_INDEX) - 1;
  System.err.println (0 <= index && index < display.widgetTable.length ?
    "widget found (after)" : "widget not found (after)");
}

The code will produce the output on my machine (x86_64, with GTK 2.22.1 and 2.24.6, Window manager is KDE 4.6.0)
  widget found (before)
  widget not found (after)
[The combo has been created within a ToolBar with the option SWT.DROP_DOWN]

The problem is that the buttonHandle attribute within the Combo remains assigned (non zero) and that the removeWidget operation of the combo tries to free this handle. In the meantime, other widgets might have reclaimed the handle and thus the release operation of the combo might dispose a totally unrelated widget. This happens with Papyrus, typically an arbitrary menuitem is disposed when the combo is released, causing random null pointer exception.
creation of a combo box within a toolbar (as a control)