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Date: Thu, 15 Aug 2019 17:09:08 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
Cc: cygwin AT cygwin DOT com
Subject: Re: [ANNOUNCEMENT] TEST: Cygwin 3.1.0-0.1
Message-ID: <20190815150908.GQ11632@calimero.vinschen.de>
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Mail-Followup-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>,	cygwin AT cygwin DOT com
References: <20190812224403 DOT 37e4ccfdcfb7046ed70fef16 AT nifty DOT ne DOT jp> <20190812153613 DOT GN11632 AT calimero DOT vinschen DOT de> <20190813104753 DOT GU11632 AT calimero DOT vinschen DOT de> <20190814204100 DOT 659fe40d928eae15338198a7 AT nifty DOT ne DOT jp> <20190814204717 DOT caf6884b1216bbeee2f586d6 AT nifty DOT ne DOT jp> <20190814134900 DOT GY11632 AT calimero DOT vinschen DOT de> <20190815042126 DOT 7c2f0baf57b4a82f7d013f74 AT nifty DOT ne DOT jp> <20190815074930 DOT GF11632 AT calimero DOT vinschen DOT de> <20190815103638 DOT GO11632 AT calimero DOT vinschen DOT de> <20190815150436 DOT GP11632 AT calimero DOT vinschen DOT de>
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On Aug 15 17:04, Corinna Vinschen wrote:
> On Aug 15 12:36, Corinna Vinschen wrote:
> > On Aug 15 09:49, Corinna Vinschen wrote:
> > > On Aug 15 04:21, Takashi Yano wrote:
> > > > On Wed, 14 Aug 2019 15:49:00 +0200
> > > > Corinna Vinschen wrote:
> > > > > The only reason I can see is if sigwait_common() returns EINTR be=
cause
> > > > > it was interrupted by an unrelated signal.  This in turn lets the=
 read()
> > > > > call fail with EINTR and that should be expected by the callers, =
in
> > > > > theory.
> > > >=20
> > > > Strangely, this problem also disappears with this patch.
> > > >=20
> > > > diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
> > > > index 9cf892801..82ac0674f 100644
> > > > --- a/winsup/cygwin/select.cc
> > > > +++ b/winsup/cygwin/select.cc
> > > > @@ -1869,7 +1869,7 @@ thread_signalfd (void *arg)
> > > >        switch (WaitForSingleObject (si->evt, INFINITE))
> > > >         {
> > > >         case WAIT_OBJECT_0:
> > > > -         tls->signalfd_select_wait =3D NULL;
> > > > +         //tls->signalfd_select_wait =3D NULL;
> > > >           event =3D true;
> > > >           break;
> > > >         default:
> > >=20
> > > The problem with not setting signalfd_select_wait to NULL here is that
> > > only a subsequent read or sigwaitinfo will do, so there's a time
> > > post-select which will reroute the signal wrongly.
> >=20
> > Worse, thread_signalfd() closes the handle on exit, so keeping
> > signalfd_select_wait set may result in strange behaviour after select
> > returns.
> >=20
> > > Any ideas greatly appreciated.
>=20
> Here's a vague idea:
>=20
> Right now, signalfd_select_wait is not only used to signal select/poll,
> but also to keep the signal in the queue for the next call to read.
> This read call then calls sigwait_common under the hood.
>=20
> Afaics, the problem here is that the signal is still in the queue
> even after it has been, basically, assigned to the signalfd.  Because
> of that, any subsequent signal dispatch trigger will fire, in the above
> case select's own signal handling.
>=20
> My (really still vague) idea would be to remove the signalfd_select_wait
> code and call sigwait_common from select instead.  If it catched a
> signal, the signal will have been dequeued, as usual.  However, the
> select thread function thread_signalfd() would just call sigwait_common,
> too, then create a signalfd_siginfo record which gets assigned to the
> signalfd fhandler.  The read function would check if the record is
> valid and return that as first record in the read buffer, and only
> then it would fall back to sigwait_common calls itself.
>=20
> Does that sound feasible?

Reply to self: Not really because calling sigwait_common from the
thread_signalfd() function will run in the wrong thread with the wrong
tls.  Bummer again.


Corinna

--=20
Corinna Vinschen
Cygwin Maintainer

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