Sv: Sv: Sv: Sv: Sv: Named pipes and multiple writers
Tue Mar 31 21:10:19 GMT 2020

>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, wrote:
>>>>> On 3/27/2020 10:53 AM, 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, 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
>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

>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.


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:-)


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