postgresql/doc/TODO.detail/win32intr

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From: "Qingqing Zhou" <zhouqq@cs.toronto.edu>
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Subject: [BUGS] FATAL:  semctl(1672698088, 12, SETVAL, 0) failed
Date: Wed, 22 Feb 2006 11:17:51 +0800
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I encountered an error when I fast shutdown 8.1.1 on Win2k:

    FATAL:  semctl(1672698088, 12, SETVAL, 0) failed:  A blocking operation
    was interrupted by a call to WSACancelBlockingCall.

A similar error on 8.1/win2003 was reported on pgsql-general (sorry, I can't
dig out the
original post from our web archives):

    From:  Niederland
    Date:  Tues, Dec 13 2005 9:49 am

    2005-12-12 20:30:00 FATAL:  semctl(50884184, 15, SETVAL, 0) failed: A
    non-blocking socket operation could not be completed immediately.

---

There are two problems here:

(1) Why a socket error?
In port/win32.h, we have

#undef EAGAIN
#undef EINTR
#define EINTR WSAEINTR
#define EAGAIN WSAEWOULDBLOCK

What's the rationale of doing so?

(2) What's happened here?
It may come from PGSemaphoreReset(), and win32 semop() looks like this:

  ret = WaitForMultipleObjectsEx(2, wh, FALSE, (sops[0].sem_flg &
IPC_NOWAIT) ? 0 : INFINITE, TRUE);
  ...
  else if (ret == WAIT_OBJECT_0 + 1 || ret == WAIT_IO_COMPLETION)
  {
   pgwin32_dispatch_queued_signals();
   errno = EINTR;
  }
  else if (ret == WAIT_TIMEOUT)
   errno = EAGAIN;

So it seems the EINTR is caused by an incoming signal, the EAGAIN is caused
by a TIMEOUT ... any ideas?

Regards,
Qingqing



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From pgsql-bugs-owner+M14482@postgresql.org Tue Feb 28 14:05:23 2006
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200602281905.k1SJ52b19021@candle.pha.pa.us>
Subject: Re: [BUGS] FATAL:  semctl(1672698088, 12, SETVAL, 0) failed
In-Reply-To: <dtglar$15tv$1@news.hub.org>
To: Qingqing Zhou <zhouqq@cs.toronto.edu>
Date: Tue, 28 Feb 2006 14:05:02 -0500 (EST)
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Status: RO

Qingqing Zhou wrote:
> I encountered an error when I fast shutdown 8.1.1 on Win2k:
> 
>     FATAL:  semctl(1672698088, 12, SETVAL, 0) failed:  A blocking operation
>     was interrupted by a call to WSACancelBlockingCall.
> 
> A similar error on 8.1/win2003 was reported on pgsql-general (sorry, I can't
> dig out the
> original post from our web archives):
> 
>     From:  Niederland
>     Date:  Tues, Dec 13 2005 9:49 am
> 
>     2005-12-12 20:30:00 FATAL:  semctl(50884184, 15, SETVAL, 0) failed: A
>     non-blocking socket operation could not be completed immediately.
> 
> ---
> 
> There are two problems here:
> 
> (1) Why a socket error?
> In port/win32.h, we have
> 
> #undef EAGAIN
> #undef EINTR
> #define EINTR WSAEINTR
> #define EAGAIN WSAEWOULDBLOCK
> 
> What's the rationale of doing so?

We did this so that our code could refer to EINTR/EAGAIN without
port-specific tests.

> (2) What's happened here?
> It may come from PGSemaphoreReset(), and win32 semop() looks like this:
> 
>   ret = WaitForMultipleObjectsEx(2, wh, FALSE, (sops[0].sem_flg &
> IPC_NOWAIT) ? 0 : INFINITE, TRUE);
>   ...
>   else if (ret == WAIT_OBJECT_0 + 1 || ret == WAIT_IO_COMPLETION)
>   {
>    pgwin32_dispatch_queued_signals();
>    errno = EINTR;
>   }
>   else if (ret == WAIT_TIMEOUT)
>    errno = EAGAIN;
> 
> So it seems the EINTR is caused by an incoming signal, the EAGAIN is caused
> by a TIMEOUT ... any ideas?

I looked at the documentation for the function:

	http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/waitformultipleobjectsex.asp

and it isn't clear what return failure values it has.  We certainly
could loop on WSAEINTR.  Can you test it?

-- 
  Bruce Momjian   http://candle.pha.pa.us
  SRA OSS, Inc.   http://www.sraoss.com

  + If your life is a hard drive, Christ can be your backup. +

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From pgsql-bugs-owner+M14483@postgresql.org Tue Feb 28 21:12:39 2006
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From: "Qingqing Zhou" <zhouqq@cs.toronto.edu>
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Subject: Re: [BUGS] FATAL:  semctl(1672698088, 12, SETVAL, 0) failed
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"Bruce Momjian" <pgman@candle.pha.pa.us> wrote
> > In port/win32.h, we have
> >
> > #undef EAGAIN
> > #undef EINTR
> > #define EINTR WSAEINTR
> > #define EAGAIN WSAEWOULDBLOCK
> >
> > What's the rationale of doing so?
>
> We did this so that our code could refer to EINTR/EAGAIN without
> port-specific tests.
>

AFAICS, by doing so, the EINTR/EAGAIN will be translated into
WSAINTR/WSAEWOULDBLOCK through *all* the backend code. That's seems not
appropriate for the code not involving any socket stuff ... I think we need
a fix here.

> > (2) What's happened here?
> > It may come from PGSemaphoreReset(), and win32 semop() looks like this:
> >
> >   ret = WaitForMultipleObjectsEx(2, wh, FALSE, (sops[0].sem_flg &
> > IPC_NOWAIT) ? 0 : INFINITE, TRUE);
> >   ...
> >   else if (ret == WAIT_OBJECT_0 + 1 || ret == WAIT_IO_COMPLETION)
> >   {
> >    pgwin32_dispatch_queued_signals();
> >    errno = EINTR;
> >   }
> >   else if (ret == WAIT_TIMEOUT)
> >    errno = EAGAIN;
> >
> > So it seems the EINTR is caused by an incoming signal, the EAGAIN is
caused
> > by a TIMEOUT ... any ideas?
>
> I looked at the documentation for the function:
>
>
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/waitformultipleobjectsex.asp
>
> and it isn't clear what return failure values it has.  We certainly
> could loop on WSAEINTR.  Can you test it?
>

Yeah, looking at other code of using semop(), we could plug in a loop in the
win32 semctl():

   /* Quickly lock/unlock the semaphore (if we can) */
+ do
+ {
+    errStatus = semop(semId, &sops, 1);
+ } while (errStatus < 0 && errno == EINTR);

   if (semop(semId, &sops, 1) < 0)
    return -1;

But:
(1) The EINTR problem happens rather rare, so testing it is difficult;
(2)  I would rather not doing the above changes before we understand what's
happened here, especially when we have seen a EAGAIN reported here.

Regards,
Qingqing



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From pgsql-bugs-owner+M14485@postgresql.org Tue Feb 28 22:08:57 2006
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200603010308.k2138bO09022@candle.pha.pa.us>
Subject: Re: [BUGS] FATAL:  semctl(1672698088, 12, SETVAL, 0) failed
In-Reply-To: <du2vtq$eu3$1@news.hub.org>
To: Qingqing Zhou <zhouqq@cs.toronto.edu>
Date: Tue, 28 Feb 2006 22:08:37 -0500 (EST)
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Qingqing Zhou wrote:
> 
> "Bruce Momjian" <pgman@candle.pha.pa.us> wrote
> > > In port/win32.h, we have
> > >
> > > #undef EAGAIN
> > > #undef EINTR
> > > #define EINTR WSAEINTR
> > > #define EAGAIN WSAEWOULDBLOCK
> > >
> > > What's the rationale of doing so?
> >
> > We did this so that our code could refer to EINTR/EAGAIN without
> > port-specific tests.
> >
> 
> AFAICS, by doing so, the EINTR/EAGAIN will be translated into
> WSAINTR/WSAEWOULDBLOCK through *all* the backend code. That's seems not
> appropriate for the code not involving any socket stuff ... I think we need
> a fix here.

Uh, how do we handle it now?  I thought we did just that.

> http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/waitformultipleobjectsex.asp
> >
> > and it isn't clear what return failure values it has.  We certainly
> > could loop on WSAEINTR.  Can you test it?
> >
> 
> Yeah, looking at other code of using semop(), we could plug in a loop in the
> win32 semctl():
> 
>    /* Quickly lock/unlock the semaphore (if we can) */
> + do
> + {
> +    errStatus = semop(semId, &sops, 1);
> + } while (errStatus < 0 && errno == EINTR);
> 
>    if (semop(semId, &sops, 1) < 0)
>     return -1;
> 
> But:
> (1) The EINTR problem happens rather rare, so testing it is difficult;
> (2)  I would rather not doing the above changes before we understand what's
> happened here, especially when we have seen a EAGAIN reported here.

OK, so how do we find the answer?

-- 
  Bruce Momjian   http://candle.pha.pa.us
  SRA OSS, Inc.   http://www.sraoss.com

  + If your life is a hard drive, Christ can be your backup. +

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