Version 2 of Widget Destruction

Widget destruction, and resource management in general, is one of the trickier things to get right in a Tk widget implementation. There are more bugs per square foot in this area of the core than anywhere else.

What I'm looking for is a recipe for how to properly handle the widget lifecycle. (When I asked about this on the chatroom, DGP said: "typically there's a "deleted" flag in the struct, and lots of Tcl_Preserve()'s around." But there are apparently no formalized guidelines anywhere...)

There are several cases to handle:

The widget instance command is deleted, in which case the command deletion proc is responsible for calling Tk_DestroyWindow (this falls through to the next case)
Someone calls Tk_DestroyWindow() on the widget or one of its ancestors, in which case the widget will receive precisely one <Destroy> event. In this case the event handler is responsible for deleting the widget command.
Some other process calls XDestroyWindow, in which case the widget will receive precisely one <Destroy> event.
A script calls [exit]
Someone deletes the Tcl_Interp in which the widget was created

There are probably more ....

more to come ...

Another complication: When the event handler destroys the widget command, thanks to command deletion traces it's possible for the interpreter to be recursively reentered.

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