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Re: master reporting "creating pipe: too many open files"

From: Stephen Leake
Subject: Re: master reporting "creating pipe: too many open files"
Date: Sat, 16 Jul 2022 14:43:34 -0700
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (windows-nt)

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Stephen Leake <stephen_leake@stephe-leake.org>
>> Date: Sat, 16 Jul 2022 09:58:18 -0700
>> I have a custom git front-end that works fine in emacs 28, but in
>> current emacs master it always reports "creating pipe: too many open
>> files" after a commands. Then any subsequent command in emacs that
>> requires a file (such as saving a buffer) also fails.
>> This is on Windows 8.1, emacs built with mingw64.
>> I'm guessing there is some file/process handle that is opened and not
>> closed.
>> I update my local copy of emacs master rarely, so this change happened
>> sometime since Dec 2021; I have not tried to bisect.
>> Attached is some simplified code that shows the problem; it runs a
>> subprocess the same way the git front-end does. To run, load the file,
>> then run M-x dd-test. On my system, master is ok with 10 processes,
>> fails with 100 (I did not bisect further). Emacs 28 is ok forever.
>> Before I submit a bug report, I want to check that there is nothing
>> simple wrong with the code; is it relying on undocumented behaviour in
>> emacs 28, or something similar?
> You code starts subprocesses in a loop.

Yes, but it waits for each to complete before starting the next one.
That's why it works fine in Emacs 28.

> Emacs on MS-Windows cannot support more than 30 subprocesses (assuming
> no other process objects, like network connection, exist).  That's
> because the pselect emulation is limited to 64 handles, and we use 2
> handles per subprocess.
> Is what you see consistent with this limitation?

No. That has not changed between 28 and master.

I did test on Debian; no problems there.

I'll create a bug report.

-- Stephe

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