Re: Need help embedding Guile

[Olivier Dion via General Guile related discussions <guile-user@gnu.org>]

On Wed, 22 Dec 2021, Dimitris Papavasiliou <dpapavas@protonmail.ch> wrote:

> Foreign objects currently come in two categories:
>
> 1. Complete geometric operations such as `cube' and `difference'
>    above.  These are allocated on the C++ side and a so-called "smart
>    pointer" (shared_ptr) is exported to Scheme.  Failure to finalize
>    this retains a reference on the C++ side, which would prevent
>    destroying the operation.  Since these can get quite large in terms
>    of memory, destroying them after they're no longer needed can be
>    essential.

Since you have a graph of all the primitives in the second phase, you're
basicaly doing garbage collection there.  But from:

> Creating the complete graph before evaluation begins in the second
> phase is probably not necessary (nodes could be evaluated as they're
> created), but it creates the opportunity for certain optimizations
> (like dead code elimination for instance).  This makes some sort of
> forcing/ensuring that Guile has terminated desirable.

If I understood, objects can be garbage before phase 2, thus not
appearing in the final graph of operations.

> One idea would be to simply call `scm_gc()' and `scm_run_finalizers()'
> until the latter returns 0.  As far as I can see, this should ensure
> all finalizers are called, assumming no references to any foreign
> objects remain, but I see no way of ensuring the latter short of
> process termination...

One way I think you could do this is to evaluate all the user operations
in a sandbox environment.

Example:
--------------------------------------------------------------------------------
(use-modules (ice-9 sandbox))
;; ...
(let ([mod (make-sandbox-module (cons '((my-app primitives))
                                       all-pure-bindings))])
   (eval-in-sandbox '(eval-user-file "...") #:module mod))
--------------------------------------------------------------------------------

From `eval-in-sandbox` documentation:

 If SEVER-MODULE? is true (the default), the module will be unlinked
 from the global module tree after the evaluation returns, to allow MOD
 to be garbage-collected.

So I _think_ you're guarantee here that all references in your module
will be garbage collected.  You can then do a single `gc/finalizer`.

-- 
Olivier Dion
Polymtl