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To: "'Ken Brown'" <kbrown AT cornell DOT edu>
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Subject: Sv: Sv: Sv: Sv: Sv: Sv: Named pipes and multiple writers
Date: Wed, 1 Apr 2020 10:52:04 +0200
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From: Kristian Ivarsson via Cygwin <cygwin AT cygwin DOT com>
Reply-To: sten DOT kristian DOT ivarsson AT gmail DOT com
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> On 3/31/2020 5:10 PM, sten DOT kristian DOT ivarsson AT gmail DOT com wrote:
> >> 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.  I have a feeling that read_ready
> >>>>>>>>>> should have been created as a manual-reset event, and that
> >>>>>>>>>> more care is needed 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.  If you're able to build the git
> >>>>>>>>> repo master branch, please try the attached patch.
> >>>>>>>
> >>>>>>>> Here's a better patch.
> >>>>>>>
> >>>>>>>
> >>>>>>> I finally succeeded to build latest master (make is not my
> >>>>>>> favourite
> >>>>>>> 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.  Here's the output
> >>>>>> from one
> >>>>> run:
> >>>>>
> >>>>> You're right. That was extremely careless of me to not test this
> >>>>> in Linux first :-)
> >>>>
> >>>> No problem.
> >>>>
> >>>>> I can assure that we have a use case that works on Linux but not
> >>>>> in Cygwin, 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
> >>>>> application but in a simple way (I'll be back)
> >>>>
> >>>> OK, I'll be waiting for you.  BTW, if it's not too hard to write
> >>>> your test case in plain C, or at least less modern C++, that would
> >>>> simplify things for me.  For example, your pipe.cpp failed to
> >>>> compile on one Linux machine I wanted to test it on, presumably
> >>>> because that
> > machine had an older C++ compiler.
> >>>
> >>> Never mind.  I was able to reproduce the problem and find the cause.
> >>> What happens is that when the first subprocess exits,
> >>> fhandler_fifo::close resets read_ready.  That causes the second and
> >>> subsequent subprocesses to think that there's no reader open, so
> >>> their attempts to open a writer with O_NONBLOCK fail with ENXIO.
> >>>
> >>> 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 the attached program, which is a variant of the test case you
> > sent last week.
> >> Please test it in your use case.
> >
> >> Note: If you've previously pulled the topic/fifo branch, then you
> >> will
> > probably get a lot of conflicts when you pull again, because I did a
> > forced push a few 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
> > working on in connection with allowing multiple readers.  Even though
> > the code allows a FIFO to be *explicitly* opened for reading only
> > once, there can still be several open file descriptors for readers
> > because of dup and fork.  The existing code on git master doesn't
> > handle those situations properly.
> >
> >> The code on topic/fifo doesn't completely fix that yet, but I think
> >> it
> > should 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 attempted.
> >
> >> I'm working on removing both of these restrictions.
> >
> >> Ken
> >
> > We finally took the time to make some kind of a simplified "hack" that
> > works on Ubuntu and BSD/OSX but with latest on master newlib-cygwin gave
"ENXIO"
> > now and then but with your previous patch attached, there was no ENXIO
> > but ::read returns EAGIN (until exhausted) (with cygwin) almost every
> > run
> >
> > I will try your newest things tomorrow
> >
> > See latest attatched test-program (starts to get bloated but this time
> > more C-compatible though:-)
> 
> Thanks.  This runs fine with the current HEAD of topic/fifo.

I wrote in a previous mail in this topic that it seemed to work fine for me
as well, but when I bumped up the numbers of writers and/or the number of
messages (e.g. 25/25) it starts to fail again

The initial thought is that we're bumping into some kind of system resource
limit, but I haven't had the time to dig into details (yet) (I'm sorry for
that)

Kristian

> Ken

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