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To: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>,
        Nicholas Williams
 <nicholas DOT williams AT zaxiom DOT com>
Subject: Re: Cygwin outputting message to stderr on dofork EAGAIN failure even
 when Python exception is caught and handled
Thread-Topic: Cygwin outputting message to stderr on dofork EAGAIN failure
 even when Python exception is caught and handled
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Date: Mon, 17 Jun 2024 16:03:08 +0000
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From: "Dale Lobb \(Sys Admin\) via Cygwin" <cygwin AT cygwin DOT com>
Reply-To: "Dale Lobb \(Sys Admin\)" <Dale DOT Lobb AT bryanhealth DOT org>
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Greetings, Nicholas;

> From: Cygwin <cygwin-bounces+dale.lobb=bryanhealth DOT org AT cygwin DOT com> On Behalf Of Andrey Repin via Cygwin
> Sent: Monday, June 17, 2024 2:58 AM
> To: Nicholas Williams <nicholas DOT williams AT zaxiom DOT com>; cygwin AT cygwin DOT com
> Cc: Andrey Repin <anrdaemon AT yandex DOT ru>
> Subject: EXTERNAL SENDER: Re: Cygwin outputting message to stderr on dofork EAGAIN failure even when Python exception is caught and handled
>
> Greetings, Nicholas Williams! > We have a Python (installed and run through Cygwin) process running on > Windows Server 2022 that was very, very occasionally failing when subprocess. check_output was called: > 0 [main] python3 28481
> 
> Greetings, Nicholas Williams!
>
> > We have a Python (installed and run through Cygwin) process running on
> > Windows Server 2022 that was very, very occasionally failing when subprocess.check_output was called:
>
> > 0 [main] python3 28481 dofork: child -1 - forked process 16856 died
> > unexpectedly, retry 0, exit code 0xC0000142, errno 11
> > …
> >     subprocess.check_output(["cygpath", "-w", directory.name], encoding="utf-8").strip()
> > File "/usr/lib/python3.9/subprocess.py", line 424, in check_output <>
> >      <>return run(*popenargs, stdout=PIPE, timeout=timeout, check=True, <>
> > File "/usr/lib/python3.9/subprocess.py", line 505, in run <>
> >      <>with Popen(*popenargs, **kwargs) as process: <>
> > File "/usr/lib/python3.9/subprocess.py", line 951, in __init__ <>
> >      <>self._execute_child(args, executable, preexec_fn, close_fds, <>
> > File "/usr/lib/python3.9/subprocess.py", line 1754, in _execute_child
> >     self.pid = _posixsubprocess.fork_exec(
> > BlockingIOError: [Errno 11] Resource temporarily unavailable
>
> > Setting aside for a minute the various reasons this might be happening
> > occasionally, which we cannot solve for at this moment, the error number
> > (EAGAIN) indicates that you should “try again.” So that’s exactly what we
> > did. We added a try/catch to the Python code to catch the BlockingIOError
> > and, if and only if the error number is EAGAIN, we try up to two more times.
> > This fixed the problem and caused the application to stop quitting. We
> > output a warning to our log so that we don’t forget about the problem, but
> > the warning only ever appears once, so retrying a single time seems to help.
>
> > However … even though Python handles the dofork error, turns it into a
> > Python exception, and our code catches the Python exception and handles it
> > properly, Cygwin (not Python … Cygwin) still outputs a message to stderr
> > right before our warning message. This Cygwin error message shows up as an error in our log tracking:
>
> > 0 [main] python3 15042 dofork: child -1 - forked process 6780 died
> > unexpectedly, retry 0, exit code 0xC0000142, errno 11
> > 06/16 13:57:53. 87520: WARNING: Retrying command in 2 seconds due to EAGAIN: [the command we’re running]
>
> > I’m sure there could be any number of things I might be missing, but IMO,
> > if the process calling dofork properly handles the error raised by dofork,
> > Cygwin should not be outputting an error message to stderr.
>
> > Thoughts?
>
> My inexperienced and uneducated thought would be that forking code is fragile
> and some parts of it prone to misbehavior. When an unforeseen error is
> detected, it's better to report it sooner, than to get bitten by it later.
>
> Regarding your specific issue, if you create a STC[1] (a minimally enough
> version of your code that, say, fork a process thousands of times, which
> reliable reproduce the issue) somebody else could run to test the cause, that
> would be wonderful.
>
> (If, however, you could find and fix the cause, that would be even more wonderful!)
>

  I have seen this exact issue on every Windows  2019 or 2022
server where I have installed new versions of Cygwin since fall of 2023.
Admittedly, that has only been 3 or 4 machines, but it sure seems like
a pattern.  I have resisted upgrading old Cygwin installations for fear
that they also would start to exhibit this fork problem.

https://cygwin.com/pipermail/cygwin/2023-September/254417.html

  The only thing I have found that decreases the frequency of the
errors is to increase the amount of RAM assigned to the machine.
It does not eliminate the issue.  I've tried a ton of different
options with re-basing the Cygwin executables, to no avail.


Best Regards,

Dale


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