Re: [PATCH 1/3] usage: refactor die-recursion checks

[Jeff King <peff@peff.net>]

On Mon, Apr 15, 2013 at 07:34:07PM -0700, Brandon Casey wrote:

> > Right. My assumption was that we are primarily interested in protecting
> > against the die_routine. Compat functions should never be calling die.
> 
> I think the rule we have been enforcing is less strict than that.  We
> have only said that any compat function in a die handler path should
> never call die.  But maybe that's what you meant.

No, I assumed we were following the stronger rule. If you are a compat
function for a C library function, then you should never need to die.
You should be conforming to the existing interface, which will have some
mechanism for passing back an error.

> The primary motivation was that Hannes Sixt had to step in and point
> out yet again that the high-level memory allocators should not be
> called in anything that could be in a die handler code path.  I was on
> the thread, but I don't remember the topic (ah, Jonathan has stepped
> in with the answer).  I do remember that I was not the only one who
> had forgotten about that rule though.

Yeah, it is subtle enough that it may be worth protecting against.

> To implement this check correctly/completely (i.e. detect recursion in
> the main thread as well as in any child threads), I think you really
> do need to use thread-local storage as you mentioned in 3/3 which
> could look something like:
> 
>    static pthread_key_t dying;
>    static pthread_once_t dying_once = PTHREAD_ONCE_INIT;
> 
>    void setup_die_counter(void)
>    {
>            pthread_key_create(&dying, NULL);
>    }
> 
>    check_die_recursion(void)
>    {
>            pthread_once(&dying_once, setup_die_counter);
>            if (pthread(getspecific(dying)) {
>                    puts("BUG: recursion...");
>                    exit(128);
>            }
> 
>            pthread_setspecific(dying, &dying);
>    }

Yeah, that seems sane; my biggest worry was that it would create
headaches for Windows folks, who would have to emulate pthread_key. But
it seems like we already added support in 9ba604a.

I'll try to re-work the series with thread-local storage, and I'll leave
off the extra printing. This _should_ never happen, so if we are going
to put in the check, it is probably better to be more thorough than to
worry about what the error message looks like.

Thanks for looking it over.

-Peff