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Subject: Re: Cygwin outputting message to stderr on dofork EAGAIN failure even
 when Python exception is caught and handled
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Date: Tue, 18 Jun 2024 17:22:10 -0500
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From: Nicholas Williams via Cygwin <cygwin AT cygwin DOT com>
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Dan,

> On Jun 18, 2024, at 09:44, Dan Shelton via Cygwin <cygwin AT cygwin DOT com> wrote:
> 
> On Mon, 17 Jun 2024 at 18:03, Dale Lobb (Sys Admin) via Cygwin
> <cygwin AT cygwin DOT com> wrote:
>> 
>> 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]
> 
> Does python require fork(), or can it use posix_spawn() in this case?

I imagine this would be question for the CPython development team. While I have a basic understanding of C programming in Python (for the purposes of writing extensions), I do not know why subprocess.Popen is written in this way, and I do not know enough to question their choice here. It uses fork, and for now I have to live with that.

Thanks,

Nick


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