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Subject:	Re: Sv: Sv: 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>
Cc:	"'cygwin'" <cygwin AT cygwin 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.

Ken
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