Re: Trouble joining with threads from C

[Andy Wingo]

Hi Ludovic,

On Wed 13 Apr 2011 23:34, address@hidden (Ludovic Courtès) writes:

> After reviewing f60a7648d5926555c7760364a6fbb7dc0cf60720 (which
> addressed the same issue), I lean towards reverting it and instead
> applying the patch I just sent.
>
> The problem I see with f60a7648d5926555c7760364a6fbb7dc0cf60720 is that
> it re-introduces a pthread_key, even when using TLS, and make things a
> bit complex IMO.

The issue is that threads in Guile are not always spawned by Guile.
It's true that of the two cases in which Guile spawned a thread, one of
them wasn't getting the cleanup handlers called, and your patch fixes
that; but that ignores the case of threads that are spawned by a user's
program.

For example in the following program:

    void* thread (void*)
    {
      scm_with_guile (do_something, NULL);
      scm_with_guile (do_something_else, NULL);
      return NULL;
    }

    int main ()
    {
      pthread_t thr;
      pthread_create (&thr, NULL, thread, NULL);
      pthread_join (thr, NULL);
      return 0;
    }

When do you propose that the cleanup handlers for the thread be called?

As far as I understand things, reliably cleaning up after the thread
*requires* the use of pthread_key with a destructor.  It is the only way
to attach a cleanup callback to a thread.  It's true that the key is not
used when TLS is available, except for its destructor capabilities, but
it is not more complicated than before.

I think they context that you are missing here is bug 32436.

It does appear that I have introduced some bugs here: Mark's
after-gc-hook not being called, and a failure to build
--without-threads.  For that I apologize.  But these bugs are fixable.
Consider bug 32436 for a moment: you can't fix that with
pthread_cleanup_push.

Does this explanation help?

Let me know,

Andy
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