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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:	Sat, 28 Mar 2020 22:19:09 -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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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.

Ken
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