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Re: (n-for-each-par-map 16 store download (reverse (iota (max-id))))) cr

From: Mark H Weaver
Subject: Re: (n-for-each-par-map 16 store download (reverse (iota (max-id))))) crash
Date: Thu, 05 Jul 2018 14:30:28 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)


Amirouche Boubekki <address@hidden> writes:

> I have a program that try to download hackernews locally.
> What it does is simple, it fetch the max identifier and
> http-get each json value starting with the most recent
> item. I use n-for-each-par-map with 16 threads I have
> 8 cores.
> Here is the full program:


> (define (store pair)
>   (if (null? pair)
>       (format #t "X\n")
>       (let ((port (open-file "hn.scm" "a")))
>         (format #t "~a\n" (car pair))

These calls to (format #t ...), which write to a port that is shared by
multiple threads, should be performed while holding a mutex to prevent
concurrent writes to the same port.

I/O operations in Guile do not include built-in thread synchronization,
at least not in the fast path cases.  However, an effort was made to
avoid _crashes_ on common architectures in the event of concurrent use
of the same port.  Our hope was that the worst that would typically
happen is garbled I/O.  Perhaps we failed to realize that hope.

> How can I debug this?

It would be helpful to see a GDB backtrace from a crash in this program.
Does it help to protect the shared port with a mutex?


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