bug#59021: Unbounded heap growth when combining dynamic states & delimited continuation

[Ludovic Courtès]

Hi,

Ludovic Courtès <ludo@gnu.org> skribis:

> Ludovic Courtès <ludo@gnu.org> skribis:
>
>> Consider this code:
>>
>> ;; https://issues.guix.gnu.org/58631
>> ;; https://github.com/wingo/fibers/issues/65
>>
>> (define loss
>>   (make-vector 1000000))
>>
>> (let ((tag (make-prompt-tag "my prompt")))
>>   (define handler
>>     (lambda (k i)
>>       (when (zero? (modulo i 2000000))
>>         (pk 'heap-size (assoc-ref (gc-stats) 'heap-size)))
>>
>>       (call-with-prompt tag
>>         (lambda ()
>>           (k (modulo (+ 1 i) 10000000)))
>>         handler)))
>>
>>   (call-with-prompt tag
>>     (let ((state (current-dynamic-state)))
>>       (lambda ()
>>         ;; (define (with-dynamic-state state thunk)
>>         ;;   (let ((previous #f))
>>         ;;     (dynamic-wind
>>         ;;       (lambda () (set! previous (set-current-dynamic-state 
>> state)))
>>         ;;       thunk
>>         ;;       (lambda () (set-current-dynamic-state previous)))))
>>         (with-dynamic-state state
>>                             (lambda ()
>>                               (let loop ((i 0))
>>                                 (loop (abort-to-prompt tag i)))))))
>>     handler))
>>
>> On Guile 3.0.8, this program exhibits seemingly unbounded heap growth.
>
> This is fixed by the patch below (tested against the test case above and
> the Fibers and Shepherd test cases mentioned before):

Pushed as e47a153317c046ea5d335940412999e7dc604c33.

> Using a simple heap profiler (more on that later), I noticed that the
> stacks allocated at ‘p->stack_bottom’ would be partly retained,
> explaining the heap growth.
>
> I couldn’t pinpoint what exactly is keeping a pointer to the stack, but
> what I can tell is that the trick above makes that impossible (because
> we disable interior pointer tracing), hence the difference.
>
> Also, why changing the SCM_DYNSTACK_TYPE_DYNAMIC_STATE entry to an
> SCM_DYNSTACK_TYPE_UNWINDER entry would make a difference remains a
> mystery to me.
>
> I’m interested in theories that would explain all this in more detail!
> I’ll go ahead with the fix above if there are no objections.

I still am.  :-)

Ludo’.