Re: crashes with Fibers

["Clément Lassieur" <clement@lassieur.org>]

Clément Lassieur <clement@lassieur.org> writes:

> Ludovic Courtès <ludo@gnu.org> writes:
>
>> Hello Clément,
>>
>> Clément Lassieur <clement@lassieur.org> skribis:
>>
>>>     ;; bad
>>>     (define (test4)
>>>       (run-fibers
>>>        (lambda ()
>>>          (spawn-fiber
>>>           (lambda ()
>>>             (let ((channel (make-channel)))
>>>               (call-with-new-thread
>>>                (lambda ()
>>>                  (put-message channel "hello world")))))))
>>>        #:drain? #t))
>>>     ⊣ scheme@(guile-user)> In /home/clement/.guix-profile/share/guile/site/2.2/fibers/internal.scm:
>>>           402:6  1 (suspend-current-fiber _)
>>>       In unknown file:
>>>                  0 (scm-error misc-error #f "~A" ("Attempt to suspend fiber within continuation barrier") #f)
>>>       ERROR: In procedure scm-error:
>>>       Attempt to suspend fiber within continuation barrier
>>
>> I think the problem here is that the new thread inherit the dynamic
>> environment of the spawning thread.  Thus, ‘put-message’, called in that
>> new thread, thinks it’s running within a Fiber, but it’s not.
>>
>> Because of that, ‘put-message’ tries to suspend itself, but it cannot:
>> ‘call-with-new-thread’ is written in C, so it’s a “continuation barrier”
>> (meaning that it’s a continuation that cannot be captured and resumed
>> later.)
>>
>> So I think if you really want that, you can perhaps do something like
>> (untested):
>>
>>   (call-with-new-thread
>>     (lambda ()
>>       (parameterize ((current-fiber #f))
>>         (put-message channel "hello world"))))
>
> It works, but only with (@@ (fibers internal) current-fiber) because the
> parameter isn't exported.
>
> Thank you Ludo!

Hi Andy,

I'm adding you to this conversation to notify you about this Fibers
issue.  (I don't have a Github account to report the bug there.)

Thanks,
Clément