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From Jan Kaluža <jkal...@redhat.com>
Subject Re: svn commit: r1676417 - /perl/modperl/trunk/src/modules/perl/modperl_interp.c
Date Fri, 15 May 2015 09:57:50 GMT
On 05/15/2015 10:01 AM, Steve Hay wrote:
> On 15 May 2015 at 08:56, Steve Hay <steve.m.hay@googlemail.com> wrote:
>> On 15 May 2015 at 07:14, Jan Kaluža <jkaluza@redhat.com> wrote:
>>> On 05/14/2015 07:42 PM, Steve Hay wrote:
>>>>
>>>> On 14 May 2015 at 12:48, Jan Kaluža <jkaluza@redhat.com> wrote:
>>>>>
>>>>> On 05/14/2015 11:24 AM, Niko Tyni wrote:
>>>>>>
>>>>>>
>>>>>> On Sun, May 10, 2015 at 01:47:19PM +0100, Steve Hay wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 28 April 2015 at 07:51,  <jkaluza@apache.org> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> Author: jkaluza
>>>>>>>> Date: Tue Apr 28 06:51:12 2015
>>>>>>>> New Revision: 1676417
>>>>>>>>
>>>>>>>> URL: http://svn.apache.org/r1676417
>>>>>>>> Log:
>>>>>>>> Initialize interp->refcnt to 1 in modperl_interp_select.
>>>>>>
>>>>>>
>>>>>>
>>>>>>> I cannot understand why, but since this patch was applied I find
that
>>>>>>> t\modules\proxy.t fails every time when I run the full "nmake
test",
>>>>>>> but it always succeeds when I run it in isolation so I'm at a
loss to
>>>>>>> find out what is going wrong. All other tests (apart from those
known
>>>>>>> Win32-specific failures documented in README) still pass. Reverting
>>>>>>> the patch "fixes" the proxy.t problem, but probably isn't the
right
>>>>>>> solution.
>>>>>
>>>>>
>>>>>
>>>>> It's caused by Perl_croak/modperl_croak.
>>>>>
>>>>> Lets take modperl_run_filter as an example. When following code-path
is
>>>>> executed ...
>>>>>
>>>>>                   modperl_croak(aTHX_ MODPERL_FILTER_ERROR,
>>>>>                                 "a filter calling $f->read "
>>>>>                                 "must return OK and not DECLINED");
>>>>>
>>>>> ... the MP_INTERP_PUTBACK is not reached for some reason (I presume it's
>>>>> because of Perl_croak, but I don't understand why it stops the execution
>>>>> of
>>>>> the rest of modperl_run_filter method).
>>>>>
>>>>> Because of that, the interp->refcnt is not decreased, and the interp
is
>>>>> not
>>>>> freed.
>>>>>
>>>>> I has been able to "fix" it by attached patch, but I would like to
>>>>> discuss
>>>>> more generic way how to fix that problem...
>>>>>
>>>>> Any ideas?
>>>>>
>>>>
>>>> modperl_croak() calls Perl_croak(), which is an XS interface to Perl's
>>>> die() function, so surely you wouldn't expect anything immediately
>>>> after it to be run?
>>>>
>>>> I'm not sure exactly where it does end up, though. It must be getting
>>>> caught by some eval somewhere since we aren't exiting the process, but
>>>> presumably it wouldn't be possible to do appropriate clean-up wherever
>>>> it lands up unless there is some mechanism for registering required
>>>> clean-up behaviour? Otherwise maybe we need to pass interp into
>>>> modperl_croak(), or into a new version of that if not all cases
>>>> require it, so that it can do the MP_INTERP_PUTBACK(interp, aTHX)
>>>> call?
>>>>
>>>
>>> What worries me here a bit is that we would have to MP_INTEPR_PUTBACK the
>>> PerlInterp which is later used for PerlCroak, if I understand it right.
>>>
>>> I have found out that usually when modperl_croak is called, the refcnt of
>>> the interp is above 1, so it wouldn't get freed prematurely, but still.
>>>
>>> I think for now we should putback the interp only when interp->refcnt >
1,
>>> it wouldn't fully fix all bugs, but lot of them would be fixed by that.
>>>
>>> If someone knows how Perl_croak works and if it's possible to cleanup the
>>> interp after that, it would be great to share that info .
>>>
>>
>> My understanding of Perl_croak() is that it either exits the process
>> (if not inside an eval()) or else calls the system's longjmp(), which
>> resumes execution from immediately after where the corresponding
>> setjmp() was called, having restored the process environment to the
>> original state at that point too.
>>
>> In the perl source, the setjmp()/longjmp() of eval()/die() are done by
>> the JMPENV_PUSH in Perl_eval_sv() (maybe called from Perl_eval_pv())
>> and the JMPENV_JUMP in Perl_die_unwind(), called from Perl_vcroak().
>> The JMPENV* macros are in cop.h, and call
>> PerlProc_setjmp()/PerlProc_longjmp(), which are typically
>> setjmp()/longjmp(), or maybe sigsetjmp()/siglongjmp() if you have
>> them.
>>
>> I think you're right that we should probably check that interp->refcnt
>>> 1 if we go ahead and pass interp into modperl_croak(). There aren't
>> too many calls, so this may be workable; we also have a few call
>> MP_RUN_CROAK()/MP_RUN_CROAK_RESET() calls to look at too. What worries
>> me is the (much larger number of!) calls to Perl_croak(). They will
>> also not return, so we presumably need to do cleanup before each one
>> of those too? Maybe we need a little wrapper function/macro to do
>> clean up and then call Perl_croak() and use that everywhere instead of
>> Perl_croak() (including the call inside modperl_croak(), of course)?
>
> The other approach I mentioned earlier was to try to do the cleanup in
> the eval() where the die() has landed. If that's possible then it
> might be a cleaner approach.
>
> In this case, I think we're inside the eval_pv done in
> modperl_filter_resolve_init_handler(). I only see three other eval*()s
> (one more eval_pv() and two eval_sv()s) around the mod_perl C source
> code so this could be worth pursuing.

Hm, maybe stupid idea, but could we store the refcnt before the eval and 
reset it after the eval? I presume that perlinterp is valid for the 
evaluated code only when we are in the eval(), so if something increases 
the refcnt in the eval() and forgets the decrease it later, we could 
just decrease it ourselves using the set-eval-reset aproach.

Otherwise we would need a way to find out that:

a) Perl_croak has been executed - could be easy, I bet this is possible 
somehow with Perl API.
b) There is actually lost reference - Not sure how to do that, we don't 
know if PUTBACK has been called before the Perl_croak or not...

Regards,
Jan Kaluza


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