Reasons, why Miss Scheme should collect her garbage before shutdown.

[Sebastian H Seidel <Sebastian.H.Seidel@hno.med.uni-giessen.de>]

Dear developers of MzScheme, dear extension-programmers,
========================================================

in an earlier message I wondered why no garbage collection is done
before MzScheme exits. No special reason was given.
Because of this ill-defined behaviour of MzScheme some otherwise easy
to program tasks can only be done with the help of dirty hacks as will
be shown below.


Something that works
====================

- Often it is useful to bind the lifetime of an external object to a
lexical scope inside the Scheme-Environment, like so:

(let ((obj (make-external-object args)))
   (operation-on-external-object obj)
   ...)

For example: 
- a buffer could be created which in the end must be written to a file
or
- some kind of communication could be started which in the end must be
terminated by special commands or 
- data-objects could be created in a foreign process that in the end
must be freed again.

Finalizers are both a very useful and versatile means to put a
controlled end to each object, because they are invoked before an
object is garbage-collected and you can use them to perform any action
on the object, like:
- write the buffer to a file
- close communications with special commands
- clear data out of a foreign process


Something that should work and why it fails
===========================================

Consider the following example:

(define obj (make-external-object args))

Here, one might expect that the lifetime of the external object is
limited to that of the MzScheme-Process.
Noodles! This is not at all the case because no garbage-collection
ever happens before MzScheme is shutdown. So no finalizer will be
evoked and:
- the content of the buffer will vanish into nirvana and not at all be
written to any file
- the communication will not be ended correctly
- the foreign data will clutter the other processes namespace and
memory


What could be done?
===================

Best solution
-------------
MzScheme is changed so that she does collect her garbage before
exiting.
Well ... no well-behaved woman would leave her house in a mess before
going on holiday, would she?

Second best solution
--------------------
A proper exit-hook is provided that enables extension-programmers to
force a garbage-collection before shutdown.

Third best solution
-------------------
In MzScheme-v200 there is a function scheme_add_atexit_closer() which
is designed to operate on custodian-registered objects and might be
used to emulate an exit-hook.

Clearly not a solution
----------------------
scheme_exit is a pointer that can be set to a c-function which will be
called before MzScheme-shutdown. It is reserved for embedding programs
and if several extensions would set it - each one to its own
exit-procedure, of course - it would surely mess things up royally.


Why not go for the best and most clean solution?


Others did see the same problem
===============================

I searched the archives and discovered that - indeed - I'm not the
only person suffering from this ill-defined MzScheme-behaviour.

Following are some excerpts of a message that was sent to this
mail-list in 2000-08-08 by Mr. Fernout:

...
> I think the correct way to make an object with a protected resource
> (like a file handle) in MzScheme is to use both a finalizer and a
> custodian.
> My understanding, which may be incomplete or wrong, is:
> * When an object is garbage collected, a finalizer if any is called.
> * When Scheme shuts down, objects are not garbage collected.
> * Thus, you also need a custodian who will close the resource for 
> you.
...
> This approach seems like it would work, but is
> to me a little inelegant. Somehow I'd like to make just
> one call to MzScheme to protect a resource, rather
> than a couple which require defining a couple of functions.


Yes, messing with custodians only because there is no final
garbage-collection is not only 'inelegant' but painful and
superfluous.


One final plead
===============

Here is my plead to those who are actively developing MzScheme: please
make MzScheme collect garbage before exit!


---------------
Sincerely yours and thank you for reading till the very end,
Sebastian