[AD] OSX - quit function

[Peter Hull <peter.hull90@xxxxxxxxxx>]

The main docs[1] say that attempting to quit an application by clicking 
the window's close button OR selecting the Quit menu OR pressing cmd+Q 
should call the function registered by set_close_button_callback() or 
do nothing if no function has been registered.

This is contradicted by the Allegro specifics[2], which state that Quit 
or cmd-Q closes the application. This is what happens in practice, by 
raising a SIGTERM.

This patch restores the 'general' documented behaviour.

Mac users _will_ complain if they can't cmd-Q an application. However, 
I'd rather alter the code than the documentation, because AFAICS 
there's no way to cleanly terminate an Allegro app (i.e. go via 
allegro_exit and any registered clean-up hooks.)

The reason for this is that there are 2 threads, one is the event loop 
and the other is the user code. A cmd-Q would call allegro_exit from 
the event thread. Concurrently, the second thread might call some 
allegro functions on a structure that has been or is being destroyed. 
The result is a crash or (worse) a hang.

I've found out that I cannot stop thread 2 from thread 1 - although 
there is a suitable function, pthread_cancel[3], asynchronous 
cancellation is not supported on OSX. Thread 2 is not guaranteed to hit 
a cancellation point, so deferred cancelation would not work either.

Therefore I would say to apply this patch and recommend that authors 
make use of set_close_button_callback().

Any comments?

Pete

[1] http://alleg.strangesoft.net/docs/en/set_close_button_callback.html
[2] http://www.talula.demon.co.uk/allegro/onlinedocs/en/alleg041.html
[3] http://www.hmug.org/man/3/pthread_cancel.php

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