Jesse Welton <jwelton at pacific.mps.ohio-state.edu> wrote:
> Date: Wed, 28 Jan 2004 14:22:28 -0500 (EST)
> From: Jesse Welton <jwelton at pacific.mps.ohio-state.edu>
> Subject: Re: finalisation
> To: squeak-dev at lists.squeakfoundation.org
> reply-to: The general-purpose Squeak developers list <squeak-dev at lists.squeakfoundation.org>
> 
> Andreas Raab wrote:
> > 
> > > Does this not catch all the possibilities ?
> > 
> > No, of course not. That's why I find the style of putting the resource
> > creation inside the ensured block so terribly misleading. It puts you under
> > the impression that you caught all the possibilities, where it is easy to
> > see from an example like
> > 
> >     [file := (FileStream newFileNamed: 'foo.bar') halt.
> >     ] ensure:[file close].
> > 
> > that you do not catch anything that happens on the right hand side of the
> > assignment. IOW, execute the above and close the resulting debugger - it
> > will end up with a DNU for file close which illustrates that - although the
> > file has been created - it is not being cleaned up after encountering the
> > halt.
> 
> Andreas, you correctly point out that this failure mode cannot be
> caught and handled.  If I understood Craig, the failure mode he was
> referring to would be simulated by
> 
>     (file := (FileStream newFileNamed: 'foo.bar')) halt.
>     [ file doStuff ] ensure: [ file close ].
> 
> Here, the failure can be caught by moving the file creation into the
> block.  You argue that this is statistically unlikely, but as Craig
> and Richard point out, this kind of thing can and does happen
> occasionally. 

Why would this failure mode happen?  The only time I can see is if you
are sending #terminate to your processes from code, and you should not
do that.

If you are talking about people pressing alt-., then I think you just
have to live with an occasional stray resource.  That seems perfectly
reasonable; run a garbage collection if need be.

If you are doing #terminate, then you cannot win.  Even if you do this
version:

    [ file := "open a file" ]
    ensure: [ "close the file if non-nil" ]

then you *still* do not catch it if the file gets opened, and the
file-open method is returning, but the interrupt happens before the
return statement completes.



-Lex