Multiple attempts at opening same pipe for writing fail

Multiple attempts at opening same pipe for writing fail

[João Eiras]

Howdy.

I've prepared a testcase for a bug or inconsistency with linux that is
affecting one of my projects.

Basically I have a background process that keeps some state always
running with a pipe open for reading requests.

Foreground processes that try to open that pipe for writting (to send
a request) then block/fail on the second and subsequent tries.

The whole setup can be simplified with a small testcase that runs in a
single process.

https://github.com/joaoe/cygwin-pipe-testcase

I left a README that explains things in a bit more detail.

To summarize here, consider the following order of events:
* open pipe for reading with NONBLOCK into FD1 -> OK
* poll() FD1 -> empty event list, OK
* read from FD1 -> zero length read, OK

* open pipe for writting into FD2, write something -> OK
* poll() FD1 -> POLLIN and POLLPRI, POLLPRI is unexpected here, FAIL
* read from FD1 -> got stuff that was written

* close FD2
* poll() FD1 -> POLLIN and POLLPRI, wrong, should report POLLHUP, FAIL
* read from FD1 -> zero length read, OK

* open pipe for writting into FD2 again -> got error ENXIO, FAIL

So, known bug ? Do you confirm my observations ?

Any pointers so I could perhaps look into the relevant code ? It seems
like Cygwin still keeps some dirty state somewhere that tells the pipe
is still open by a writer somewhere.

Thank you !

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Re: Multiple attempts at opening same pipe for writing fail

[Takashi Yano]

Hi João,

On Wed, 6 Jun 2018 18:23:08 +0200
João Eiras wrote:
> Howdy.
> 
> I've prepared a testcase for a bug or inconsistency with linux that is
> affecting one of my projects.
> 
> Basically I have a background process that keeps some state always
> running with a pipe open for reading requests.
> 
> Foreground processes that try to open that pipe for writting (to send
> a request) then block/fail on the second and subsequent tries.
> 
> The whole setup can be simplified with a small testcase that runs in a
> single process.
> 
> https://github.com/joaoe/cygwin-pipe-testcase
> 
> I left a README that explains things in a bit more detail.
> 
> To summarize here, consider the following order of events:
> * open pipe for reading with NONBLOCK into FD1 -> OK
> * poll() FD1 -> empty event list, OK
> * read from FD1 -> zero length read, OK
> 
> * open pipe for writting into FD2, write something -> OK
> * poll() FD1 -> POLLIN and POLLPRI, POLLPRI is unexpected here, FAIL
> * read from FD1 -> got stuff that was written
> 
> * close FD2
> * poll() FD1 -> POLLIN and POLLPRI, wrong, should report POLLHUP, FAIL
> * read from FD1 -> zero length read, OK
> 
> * open pipe for writting into FD2 again -> got error ENXIO, FAIL
> 
> So, known bug ? Do you confirm my observations ?
> 
> Any pointers so I could perhaps look into the relevant code ? It seems
> like Cygwin still keeps some dirty state somewhere that tells the pipe
> is still open by a writer somewhere.
> 
> Thank you !

Is this related to https://cygwin.com/ml/cygwin/2017-03/msg00188.html ?

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

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Re: Multiple attempts at opening same pipe for writing fail

[João Eiras]

On 13 June 2018 at 17:44, Takashi Yano <takashi.yano@nifty.ne.jp> wrote:
> Hi João,
>
> Is this related to https://cygwin.com/ml/cygwin/2017-03/msg00188.html ?
>

Hi.

That seems quite likely.

In my test case, I do open+close+open and the second open fails with
errno.ENXIO.

If I skip the close and do open+open I get the same error.

Next on my TODO list I'll make another testcase just using the Win32
API and see if that is a Windows issue.

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