improve interpreter shutdown process (bug #56952) This change is a further attempt to avoid segfaults when shutting down the interpreter and exiting the GUI event loop. The latest approach is to have the interpreter signal that it is finished with "normal" command execution (REPL, command line script, or --eval option code), then let the GUI thread process any remaining functions in its event loop(s) then signal back to the interpreter that it is OK to shutdown. Once the shutdown has happened (which may involve further calls to the GUI thread while executing atexit functions or finish.m or other shutdown code, the interpreter signals back to the GUI that shutdown is complete. At that point, the GUI can delete the interpreter object and exit. * ObjectProxy.h, ObjectProxy.cc (ObjectProxy::sendFinalize): New signal. (ObjectProxy::ObjectProxy): Connect/disconnect sendFinalize signal. (ObjectProxy::update, ObjectProxy::finalize): Use normal signal/slot connection. * interpreter-qobject.h, interpreter-qobject.cc (interpreter_qobject::ready): Rename from octave_ready_signal. Change all uses. (interpreter_qobject::execution_finished): Rename from octave_finished_singal. Change all uses. (interpreter_qobject::shutdown_finished): New signal. (interpreter_qobject::shutdown): New slot. (interpreter_qobject::execute): After interpreter finishes with normal execution, simply signal that execution has finished. Don't attempt to disable connecton to GUI or cleanup/delete interpreter. * octave-qobject.h, octave-qobject.cc (base_qobject::handle_interpreter_execution_finished): Rename from handle_octave_finished): Simply emit a signal requesting an orderly shutdown of the interpreter. Change all uses. (base_qobject::handle_interpreter_shutdown_finished): New slot. (base_qobject::request_interpreter_shutdown): New signal. (base_qobject::base_qobject): Connect request_interpreter_shutdown to interpreter_qobject::shutdown slot. * interpreter.h, interpreter.cc (interpreter::shutdown): Rename from cleanup and make public. (interpreter::~interpreter): Don't call cleanup here.

--- a/libgui/src/interpreter-qobject.h	Thu May 21 11:23:24 2020 -0400
+++ b/libgui/src/interpreter-qobject.h	Fri May 22 12:54:13 2020 -0400
@@ -54,9 +54,11 @@
 
   signals:
 
-    void octave_ready_signal (void);
+    void ready (void);
 
-    void octave_finished_signal (int);
+    void execution_finished (int);
+
+    void shutdown_finished (int);
 
   public slots:
 
@@ -82,6 +84,8 @@
 
     void execute (void);
 
+    void shutdown (int);
+
   private:
 
     base_qobject& m_octave_qobj;