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Subject:	Re: Sv: Sv: Sv: Sv: Named pipes and multiple writers
To:	sten DOT kristian DOT ivarsson AT gmail DOT com
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Date:	Mon, 30 Mar 2020 13:44:24 -0400
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From:	Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Ken Brown <kbrown AT cornell DOT edu>
Cc:	"'cygwin'" <cygwin AT cygwin DOT com>
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--------------3490EC22AEDD2AA7601603DD
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On 3/28/2020 10:19 PM, Ken Brown via Cygwin wrote:
> On 3/28/2020 11:43 AM, Ken Brown via Cygwin wrote:
>> On 3/28/2020 8:10 AM, sten DOT kristian DOT ivarsson AT gmail DOT com wrote:
>>>> On 3/27/2020 10:53 AM, sten DOT kristian DOT ivarsson AT gmail DOT com wrote:
>>>>>> 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
>>>>>>>> fhandler_fifo::wait.=A0 I have a feeling that read_ready should =
have
>>>>>>>> been created as a manual-reset event, and that more care is need=
ed
>>>>>>>> to make sure it's set when it should be.
>>>>>>>>
>>>>>>>>> I could provide a code-snippet
>>>>>>>>> to reproduce it if wanted ?
>>>>>>>>
>>>>>>>> Yes, please!
>>>>>>>
>>>>>>> That might not be necessary.=A0 If you're able to build the git r=
epo
>>>>>>> master branch, please try the attached patch.
>>>>>
>>>>>> Here's a better patch.
>>>>>
>>>>>
>>>>> I finally succeeded to build latest master (make is not my favourit=
e
>>>>> tool) and added the patch, but still no success in my little
>>>>> test-program (see
>>>>> attachment) when creating a write-file-descriptor with O_NONBLOCK
>>>
>>>> Your test program fails for me on Linux too.=A0 Here's the output fr=
om one
>>> run:
>>>
>>> You're right. That was extremely careless of me to not test this in L=
inux
>>> first :-)
>>
>> No problem.
>>
>>> I can assure that we have a use case that works on Linux but not in C=
ygwin,
>>> but it seems like I failed to narrow it down in the wrong way
>>>
>>> I'll try to rearrange my code (that works in Linux) to mimic our appl=
ication
>>> but in a simple way (I'll be back)
>>
>> OK, I'll be waiting for you.=A0 BTW, if it's not too hard to write you=
r test=20
>> case in plain C, or at least less modern C++, that would simplify thin=
gs for=20
>> me.=A0 For example, your pipe.cpp failed to compile on one Linux machi=
ne I=20
>> wanted to test it on, presumably because that machine had an older C++=
 compiler.
>=20
> Never mind.=A0 I was able to reproduce the problem and find the cause.=A0=
 What=20
> happens is that when the first subprocess exits, fhandler_fifo::close r=
esets=20
> read_ready.=A0 That causes the second and subsequent subprocesses to th=
ink that=20
> there's no reader open, so their attempts to open a writer with O_NONBL=
OCK fail=20
> with ENXIO.
>=20
> I should be able to fix this tomorrow.

I've pushed what I think is a fix to the topic/fifo branch.  I tested it =
with=20
the attached program, which is a variant of the test case you sent last w=
eek.=20
Please test it in your use case.

Note: If you've previously pulled the topic/fifo branch, then you will pr=
obably=20
get a lot of conflicts when you pull again, because I did a forced push a=
 few=20
days ago.  If that happens, just do

   git reset --hard origin/topic/fifo

It turned out that the fix required some of the ideas that I've been work=
ing on=20
in connection with allowing multiple readers.  Even though the code allow=
s a=20
FIFO to be *explicitly* opened for reading only once, there can still be =
several=20
open file descriptors for readers because of dup and fork.  The existing =
code on=20
git master doesn't handle those situations properly.

The code on topic/fifo doesn't completely fix that yet, but I think it sh=
ould=20
work under the following assumptions:

1. The FIFO is opened only once for reading.

2. The file descriptor obtained from this is the only one on which a read=
 is=20
attempted.

I'm working on removing both of these restrictions.

Ken

--------------3490EC22AEDD2AA7601603DD
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 filename="fifo_fork_nonblocking_writers.c"

/* Adapted from
   https://sourceware.org/pipermail/cygwin/2020-March/244219.html */

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/wait.h>
#include <string.h>

#define FIFO "/tmp/myfifo"
#define nsubproc  5
#define nmessages 4
#define pid_len   4

int
error (const int n, const char *name)
{
  fprintf (stderr, "\n%d\t%s:\t%d\t%s\n", getpid (), name, n, strerror (n));
  return n;
}

int
main ()
{
  if (mkfifo (FIFO, S_IRUSR | S_IWUSR | S_IWGRP) < 0
      && errno != EEXIST)
    return error (errno, "mkfifo");

  int rfd = open (FIFO, O_RDWR);
  if (rfd < 0)
    return error (errno, "open reader");

  for (int i = 0; i < nsubproc; i++)
    {
      pid_t pid = fork ();

      if (pid < 0)
	return error (errno, "fork");

      if (pid == 0)
	{
	  printf ("child %d\n", getpid ());
	  for (int j = 0; j < nmessages; j++)
	    {
	      char buf[pid_len + 2]; /* +1 for newline, +1 for nul */

	      int wfd = open (FIFO, O_WRONLY | O_NONBLOCK);
	      if (wfd < 0)
		_exit (error (errno, "open writer"));
	      sprintf (buf, "%d\n", getpid ());
	      ssize_t nwritten = write (wfd, buf, strlen (buf));
	      if (nwritten < 0)
		error (errno, "write");
	      /* printf ("i = %d, j = %d, nwritten = %zd\n", i, j, nwritten); */
	      close (wfd);
	    }
	  _exit (0);
	}
    }

  printf ("parent\n");
  char buf[pid_len + 2];
  for (int i = 0; i < nsubproc; i++)
    for (int j = 0; j < nmessages; j++)
      {
	if (read (rfd, buf, nsubproc) < 0)
	  error (errno, "read");
	buf[pid_len + 1] = '\0';
	printf ("%s", buf);
      }
  close (rfd);
}

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