GNU bug report logs - #53191
29.0.50; file-notify-add-watch fails for socket files on MacOS

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: "Troy de Freitas" <me <at> ntdef.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.0.50; file-notify-add-watch fails for socket files on MacOS
Date: Tue, 11 Jan 2022 10:22:01 -0600

This bug affects MacOS (and most likely FreeBSD and OpenBSD as well).
On MacOS running `file-notify-add-watch' throws an error when attached to a UNIX socket
file but works fine on Linux. To reproduce, using 'emacs -Q', evaluate the following elisp:

    (let* ((sock "/tmp/netproc.sock")
           (p (make-network-process
               :name "test"
               :family 'local
               :local sock
               :server t)))
      (unwind-protect
          (file-notify-add-watch sock '(change)
                                 (lambda (_) (message (format "%s changed." sock))))
        (delete-process p)
        (delete-file sock)))

It throws the error:

   edebug-signal: File cannot be opened: Operation not supported on
   socket, /tmp/netproc.sock

The issue appears to be in the underlying call to `kqueue-add-watch',
which uses `openat()' (by way of `emacs_open()`) to get a
file-descriptor. According to the BSD docs for `openat()', calling
`openat()' on socket files is not supported.

In GNU Emacs 29.0.50 (build 1, aarch64-apple-darwin20.6.0, NS appkit-2022.60 Version 11.5.1 (Build 20G80))
 of 2021-10-26 built on C02FW2L5Q05P
Windowing system distributor 'Apple', version 10.3.2113
System Description:  macOS 12.1

Configured using:
 'configure --disable-dependency-tracking --disable-silent-rules
 --enable-locallisppath=/opt/homebrew/share/emacs/site-lisp
 --infodir=/opt/homebrew/Cellar/emacs-plus <at> 29/29.0.50/share/info/emacs
 --prefix=/opt/homebrew/Cellar/emacs-plus <at> 29/29.0.50 --with-xml2
 --with-gnutls --without-dbus --with-imagemagick --with-modules
 --with-rsvg --with-ns --disable-ns-self-contained'

Configured features:
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Memory information:
((conses 16 1524406 530432)
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- Troy
me <at> ntdef.com

Message #8 received at 53191 <at> debbugs.gnu.org (full text, mbox):

From: Michael Albinus <michael.albinus <at> gmx.de>
To: "Troy de Freitas" <me <at> ntdef.com>
Cc: 53191 <at> debbugs.gnu.org
Subject: Re: bug#53191: 29.0.50; file-notify-add-watch fails for socket
 files on MacOS
Date: Wed, 12 Jan 2022 10:01:55 +0100

"Troy de Freitas" <me <at> ntdef.com> writes:

Hi Troy,

> This bug affects MacOS (and most likely FreeBSD and OpenBSD as well).
> On MacOS running `file-notify-add-watch' throws an error when attached to a UNIX socket
> file but works fine on Linux. To reproduce, using 'emacs -Q', evaluate the following elisp:
>
>     (let* ((sock "/tmp/netproc.sock")
>            (p (make-network-process
>                :name "test"
>                :family 'local
>                :local sock
>                :server t)))
>       (unwind-protect
>           (file-notify-add-watch sock '(change)
>                                  (lambda (_) (message (format "%s changed." sock))))
>         (delete-process p)
>         (delete-file sock)))
>
> It throws the error:
>
>    edebug-signal: File cannot be opened: Operation not supported on
>    socket, /tmp/netproc.sock
>
> The issue appears to be in the underlying call to `kqueue-add-watch',
> which uses `openat()' (by way of `emacs_open()`) to get a
> file-descriptor. According to the BSD docs for `openat()', calling
> `openat()' on socket files is not supported.

Thanks for the report. The incrimed behavior is due to unconditional
using of the EVFILT_VNODE filter, calling EV_SET in
Fkqueue_add_watch. This requires a file descriptor.

I'll investigate, whether another filter, like EVFILT_READ, is better
suited for sockets, using a sockfd. This might take some days, though.

See kqueue(2) for details, if you're interested in.

> - Troy

Best regards, Michael.

Message #11 received at 53191 <at> debbugs.gnu.org (full text, mbox):

From: Michael Albinus <michael.albinus <at> gmx.de>
To: "Troy de Freitas" <me <at> ntdef.com>
Cc: 53191 <at> debbugs.gnu.org
Subject: Re: bug#53191: 29.0.50; file-notify-add-watch fails for socket
 files on MacOS
Date: Sat, 15 Jan 2022 15:45:23 +0100

Michael Albinus <michael.albinus <at> gmx.de> writes:

Hi Troy,

> Thanks for the report. The incrimed behavior is due to unconditional
> using of the EVFILT_VNODE filter, calling EV_SET in
> Fkqueue_add_watch. This requires a file descriptor.
>
> I'll investigate, whether another filter, like EVFILT_READ, is better
> suited for sockets, using a sockfd. This might take some days, though.

I'm sorry, but it seems that EV_SET requires a file descriptor. I
couldn't find a way to pass the socket file otherwise but via open(),
which doesn't work on *BSD-like systems, as you have analyzed.

What is your use case to supervise a socket file? You can't get more
information except that the file is deleted. And this you get also if
you monitor the parent directory.

>> - Troy

Best regards, Michael.

Message #16 received at 53191-done <at> debbugs.gnu.org (full text, mbox):

From: Michael Albinus <michael.albinus <at> gmx.de>
To: Troy de Freitas <me <at> ntdef.com>
Cc: 53191-done <at> debbugs.gnu.org
Subject: Re: bug#53191: 29.0.50; file-notify-add-watch fails for socket
 files on MacOS
Date: Sat, 15 Jan 2022 18:37:45 +0100

Troy de Freitas <me <at> ntdef.com> writes:

> Hi Michael,

Hi Troy,

> Thanks for looking into this. Actually, all I wanted was to be able to see when the socket file was deleted.

So watching the parent directory would suffice, indeed. There's nothing
left to do; I'm closing the bug.

> Regards,
> Troy de Freitas

Best regards, Michael.

Message #21 received at 53191 <at> debbugs.gnu.org (full text, mbox):

From: Troy de Freitas <me <at> ntdef.com>
To: Michael Albinus <michael.albinus <at> gmx.de>
Cc: 53191 <at> debbugs.gnu.org
Subject: Re: bug#53191: 29.0.50;
 file-notify-add-watch fails for socket files on MacOS
Date: Sat, 15 Jan 2022 10:49:27 -0600

Hi Michael, 

Thanks for looking into this. Actually, all I wanted was to be able to see when the socket file was deleted. 

Regards,
Troy de Freitas

> On Jan 15, 2022, at 8:45 AM, Michael Albinus <michael.albinus <at> gmx.de> wrote:
> 
> Michael Albinus <michael.albinus <at> gmx.de> writes:
> 
> Hi Troy,
> 
>> Thanks for the report. The incrimed behavior is due to unconditional
>> using of the EVFILT_VNODE filter, calling EV_SET in
>> Fkqueue_add_watch. This requires a file descriptor.
>> 
>> I'll investigate, whether another filter, like EVFILT_READ, is better
>> suited for sockets, using a sockfd. This might take some days, though.
> 
> I'm sorry, but it seems that EV_SET requires a file descriptor. I
> couldn't find a way to pass the socket file otherwise but via open(),
> which doesn't work on *BSD-like systems, as you have analyzed.
> 
> What is your use case to supervise a socket file? You can't get more
> information except that the file is deleted. And this you get also if
> you monitor the parent directory.
> 
>>> - Troy
> 
> Best regards, Michael.

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