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Subject: Re: Sv: 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 3/27/2020 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:

child 657
657     error:  6       No such device or address
child 658
child 659
658659  error:  child 660
parent
child 661
         error:  66606661 661 661
                 error:          661
No such device or address6No such device or address

No such device or address

[I then killed it with control-C; the parent was blocked trying to open the FIFO.]

There's a race condition in your code.  The parent is trying to open the FIFO 
for reading (without O_NONBLOCK) while the child is trying to open it for 
writing (with O_NONBLOCK).  The parent is blocked waiting for the child, and the 
child's open fails with ENXIO; see

   https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html#tag_16_357

I think you need to rearrange things so that the FIFO is open for reading before 
you try a nonblocking open for writing.

I can work around the race by using a small positive 'wait' in 
fhandler_fifo::wait(), but I'm not sure this is the right thing to do, since 
Cygwin aims to emulate Linux.  Can you find a test case that works on Linux but 
fails on Cygwin?

Ken
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