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Bug#966692: stunnel4: FTBFS because of test hang

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On Thu, Aug 06, 2020 at 03:23:08AM +0200, Michał Mirosław wrote:
> On Thu, Aug 06, 2020 at 03:10:55AM +0200, Michał Mirosław wrote:
> > On Thu, Aug 06, 2020 at 02:39:42AM +0200, Michał Mirosław wrote:
> > > On Thu, Aug 06, 2020 at 03:16:35AM +0300, Peter Pentchev wrote:
> > > > On Thu, Aug 06, 2020 at 12:48:10AM +0200, Michał Mirosław wrote:
> > > > > On Thu, Aug 06, 2020 at 12:29:36AM +0300, Peter Pentchev wrote:
> > > > > > On Wed, Aug 05, 2020 at 10:52:31PM +0200, Michał Mirosław wrote:
> > > > > [...]
> > > > > > > Using print-debugging, I see that it stops at wait_for_child line just
> > > > > > > after printing the version. It seems that something is reaping the child
> > > > > > > before the main thread has a chance to wait for it.
> > > > > > 
> > > > > > OK, so the only thing that comes to my mind now is that you may be
> > > > > > hitting a crazy, crazy race between register_child() and child_reaper(),
> > > > > > and I say "a crazy, crazy race", because the test has to (apparently
> > > > > > reproducibly) receive the CHLD signal exactly between the check and
> > > > > > the creation in register_child()'s first "$children{...} //= ...cv"
> > > > > > statement.
> > > > > 
> > > > > Well, there is nothing that prevents SIGCHLD arriving between fork() and
> > > > > register_child(). You could test this with more confidence (though not
> > > > > 100%-reliably) by putting 'exit 1' just at the start of ($pid == 0) branch.
> > > > 
> > > > Nah, the problem is not just "between fork() and register_child()".
> > > > It really must arrive at a very specific moment in time, because
> > > > the //= operations for setting $children{$pid}{cv} try to make sure that
> > > > a new value is not set (that is, a new condition variable is not
> > > > created) if there already is such an element in the array. So the race
> > > > is indeed between the //= in register_child() and the //= in
> > > > child_reaper() - that is, child_reaper() must be invoked (SIGCHLD must
> > > > arrive) *during* the execution of the //= in register_child().
> > > > 
> > > > Unless I'm missing something, which is not at all out of the question :)
> > > 
> > > The assignment seems not to be at fault (see last strace). I don't know perl's
> > > internals enough to say if this statement can be interrupted visibly by a signal
> > > handler (I would guess not a perl handler, though). There are two wait4() calls
> > > even before child_reaper has a chance to run.
> > 
> > Another data point: this happens only with anyevent + libev and not with
> > anyevent + libevent. The first is preferred and installed by default with
> > libanyevent-perl, though.

Aaaaand this is why I could not reproduce it until now - I've always
(well, okay, ever since it was introduced, I'm a bit older than that)
had apt *not* automatically install recommended packages... And here
I thought I was going crazy... thanks, now it's, mm, let's say easier to
reproduce!

> AnyEvent's doc [1] mentions that the framework installs (or just might?) it's
> own SIGCHLD handler. Maybe there are just too many handlers for SIGCHLD?

Aaaaaaand this is why I should never be let near a keyboard... So how
many years have I been doing Perl programming now?... and I managed to
forget about AnyEvent installing its own SIGCHLD handler? Great. Just
great.

Thanks an *awful* lot for your perseverance, your analysis, and
basically doing my own debugging work for me! Expect another patch soon.

G'luck,
Peter

-- 
Peter Pentchev  roam@ringlet.net roam@debian.org pp@storpool.com
PGP key:        http://people.FreeBSD.org/~roam/roam.key.asc
Key fingerprint 2EE7 A7A5 17FC 124C F115  C354 651E EFB0 2527 DF13

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Bug#966692: stunnel4: FTBFS because of test hang Peter Pentchev <roam@ringlet.net> - 2020-08-05 20:40 +0200
  Bug#966692: stunnel4: FTBFS because of test hang Michał Mirosław <mirq-linux@rere.qmqm.pl> - 2020-08-05 23:00 +0200
    Bug#966692: stunnel4: FTBFS because of test hang Peter Pentchev <roam@ringlet.net> - 2020-08-05 23:40 +0200
      Bug#966692: stunnel4: FTBFS because of test hang Michał Mirosław <mirq-linux@rere.qmqm.pl> - 2020-08-06 03:20 +0200
        Bug#966692: stunnel4: FTBFS because of test hang Peter Pentchev <roam@ringlet.net> - 2020-08-06 08:10 +0200

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