MainWindow: Mark inconsistent and non-deterministic aspects of closin…

…g process

src/crystal/browser/__init__.py

@@ -376,9 +376,16 @@ def __exit__(self, exc_type, exc_value, exc_traceback) -> None:
     # === File Menu: Events ===
 
     def _on_close_project(self, event: wx.CommandEvent) -> None:
+        # TODO: Move these dispose operations inside _on_close_frame().
+        #       But be careful! The last time this movement was attempted,
+        #       various shutdown-related race conditions were triggered.
+        #       
+        #       Wait to perform movement until enough time to formalize
+        #       a deterministic shutdown procedure.
         self.entity_tree.dispose()
         self.task_tree.dispose()
-        self._frame.Close()
+
+        self._frame.Close()  # will trigger call to _on_close_frame()
 
     def _on_quit(self, event: wx.CommandEvent) -> None:
         if event.Id == wx.ID_EXIT:

src/crystal/model.py

@@ -697,7 +697,11 @@ def _resource_group_did_instantiate(self, group: ResourceGroup) -> None:
     # === Close ===
 
     def close(self) -> None:
-        self.root_task.close()
+        # Stop scheduler thread soon
+        self.root_task.interrupt()
+        # (TODO: Actually wait for the scheduler thread to exit
+        #        in a deterministic fashion, rather than relying on 
+        #        garbage collection to clean up objects.)
 
         # Disable Write Ahead Log (WAL) mode when closing database
         # in case the user decides to burn the project to read-only media,

src/crystal/task.py

@@ -1299,8 +1299,6 @@ class RootTask(Task):
     
     Access to this task's children list must be synchronized with the
     foreground thread.
-    
-    This task never completes.
     """
     icon_name = None
     scheduling_style = SCHEDULING_STYLE_ROUND_ROBIN
@@ -1365,8 +1363,11 @@ def fg_task() -> None:
         # NOTE: Must synchronize access to RootTask.children with foreground thread
         fg_call_and_wait(fg_task)
 
-    def close(self) -> None:
-        """Stop all descendent tasks, asynchronously."""
+    def interrupt(self) -> None:
+        """
+        Stop all descendent tasks, asynchronously,
+        by interrupting the scheduler thread.
+        """
         self.finish()
 
     def __repr__(self) -> str:

src/crystal/util/xthreading.py

@@ -232,9 +232,7 @@ def bg_call_later(callable: Callable[..., None], daemon: bool=False, *args) -> N
         if True, forces the background thread to be a daemon,
         and not prevent program termination while it is running.
     """
-    thread = threading.Thread(target=callable, args=args)
-    if daemon:
-        thread.daemon = True
+    thread = threading.Thread(target=callable, args=args, daemon=daemon)
     thread.start()