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Subject: Re: Sv: Sv: Named pipes and multiple writers
To: sten DOT kristian DOT ivarsson AT gmail DOT com
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From: Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Ken Brown <kbrown AT cornell DOT edu>
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On 3/26/2020 7:19 PM, Ken Brown via Cygwin wrote:
> On 3/26/2020 6:39 PM, Ken Brown via Cygwin wrote:
>> On 3/26/2020 6:01 PM, sten DOT kristian DOT ivarsson AT gmail DOT com wrote:
>>> The ENIXIO occurs when parallel child-processes simultaneously using
>>> O_NONBLOCK opening the descriptor.
>>
>> This is consistent with my guess that the error is generated by=20
>> fhandler_fifo::wait.=A0 I have a feeling that read_ready should have b=
een=20
>> created as a manual-reset event, and that more care is needed to make =
sure=20
>> it's set when it should be.
>>
>>> I could provide a code-snippet
>>> to reproduce it if wanted ?
>>
>> Yes, please!
>=20
> That might not be necessary.=A0 If you're able to build the git repo ma=
ster=20
> branch, please try the attached patch.

Here's a better patch.

Ken

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From 3efd5a8cbff8d48b8cf9807070134bb79f591b7d Mon Sep 17 00:00:00 2001
From: Ken Brown <kbrown AT cornell DOT edu>
Date: Thu, 26 Mar 2020 19:02:16 -0400
Subject: [PATCH] Cygwin: FIFO: fix a problem opening nonblocking writers

Make read_ready a manual-reset event.  Previously, when it was an
auto-reset event, there was a brief period when read_ready was not set
after a writer opened.  An attempt to open a second writer during this
period would fail with ENXIO if O_NONBLOCK was set, even if a reader
was open.

For the same reason, move ResetEvent(read_ready) from
listen_client_thread() to close().

Addresses: https://sourceware.org/pipermail/cygwin/2020-March/244201.html
---
 winsup/cygwin/fhandler_fifo.cc | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/fhandler_fifo.cc b/winsup/cygwin/fhandler_fifo.cc
index 19cd0e507..c7e27e883 100644
--- a/winsup/cygwin/fhandler_fifo.cc
+++ b/winsup/cygwin/fhandler_fifo.cc
@@ -463,7 +463,6 @@ fhandler_fifo::listen_client_thread ()
 out:
   if (evt)
     CloseHandle (evt);
-  ResetEvent (read_ready);
   if (ret < 0)
     debug_printf ("exiting with error, %E");
   else
@@ -516,7 +515,7 @@ fhandler_fifo::open (int flags, mode_t)
 
   char npbuf[MAX_PATH];
   __small_sprintf (npbuf, "r-event.%08x.%016X", get_dev (), get_ino ());
-  if (!(read_ready = CreateEvent (sa_buf, false, false, npbuf)))
+  if (!(read_ready = CreateEvent (sa_buf, true, false, npbuf)))
     {
       debug_printf ("CreateEvent for %s failed, %E", npbuf);
       res = error_set_errno;
@@ -1016,6 +1015,8 @@ fhandler_fifo::close ()
      handler or another thread. */
   fifo_client_unlock ();
   int ret = stop_listen_client ();
+  if (reader && read_ready)
+    ResetEvent (read_ready);
   if (read_ready)
     CloseHandle (read_ready);
   if (write_ready)
-- 
2.21.0


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