GNU bug report logs - #51146
29.0.50; file-notify-add-watch not reporting changes of files

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

From: Carlos Pita <carlosjosepita2 <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.0.50; file-notify-add-watch not reporting changes of files
Date: Tue, 12 Oct 2021 01:16:50 -0300

Hi all,

the docstring for file-notify-add-watch states:
   
    If FILE is a directory, ‘change’ watches for file creation or
    deletion in that directory.  This does not work recursively.

OTOH the elisp documentation in [1] states:

     If file is a directory, changes for all files in that directory
     will be notified. This does not work recursively.

So the statements seem contradictory regarding the notification of
changes in the contents of the files (not their creation or
deletion). In the most lenient interpretation, the docstring may be
considered silent in this regard.

This little experiment shows that, at least in my setup, a change in the
content of a file is not notified:

    (defun my-watcher (event) (message "Not in my watch: %s" event))

    (file-notify-add-watch "/tmp/some-dir" '(change) #'my-watcher)

Then:

    touch /tmp/some-dir/x => Not in my watch: (18 created /tmp/some-dir/x)
    echo dsds > /tmp/some-dir/x => Nothing
    rm  /tmp/some-dir/x => Not in my watch: (18 deleted /tmp/some-dir/x)

If instead I track "/tmp/some-dir/x":

    echo dsds > /tmp/some-dir/x =>
        Not in my watch: (20 changed /tmp/some-dir/x)

So according to the elisp manual this is a bug, but according to the
docstring it seemingly isn't. I'm with the manual in this one.

Best regards,
Carlos

[1] https://www.gnu.org/software/emacs/manual/html_node/elisp/File-Notifications.html

---
        

In GNU Emacs 29.0.50 (build 1, x86_64-apple-darwin20.6.0, NS appkit-2022.60 Version 11.6 (Build 20G165))
 of 2021-10-07 built on Carloss-MacBook-Pro.local
Repository revision: d2611920685ca415a1d85ffb9567b968518028d6
Repository branch: master
Windowing system distributor 'Apple', version 10.3.2022
System Description:  macOS 11.6

Configured using:
 'configure --without-native-compilation'

Configured features:
ACL DBUS GLIB GMP GNUTLS JPEG JSON LCMS2 LIBXML2 MODULES NOTIFY KQUEUE
NS PDUMPER PNG RSVG THREADS TIFF TOOLKIT_SCROLL_BARS XIM ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Help

Minor modes in effect:
  shell-dirtrack-mode: t
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Message #8 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Carlos Pita <carlosjosepita2 <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: Re: 29.0.50; file-notify-add-watch not reporting changes of files
Date: Tue, 12 Oct 2021 01:30:46 -0300

May be relevant, this is macOS so no inotify:

    Does Emacs use a file notification library?             yes (kqueue)

and:

    conftest.c:143:10: fatal error: 'sys/inotify.h' file not found
    #include <sys/inotify.h>
             ^~~~~~~~~~~~~~~

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

From: Carlos Pita <carlosjosepita2 <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: Re: 29.0.50; file-notify-add-watch not reporting changes of files
Date: Tue, 12 Oct 2021 01:52:37 -0300

According to https://news.ycombinator.com/item?id=9063910 the difference
I noticed is in fact a difference between kqueue and inotify.

This is the only disclaimer the elisp manual does wrt kqueue:

     Likewise, the kqueue library does not report reliably file
     attribute changes when watching a directory.

So nothing about changes in file contents. Given that the documentation
also states:

     Since all these libraries emit different events on notified file
     changes, there is the Emacs library filenotify which provides a
     unified interface. Lisp programs that want to receive file
     notifications should always use this library in preference to the
     native ones.

I think that either this is a bug or the documentation is very
misleading in that the library provides a unified interface. If it is a
bug, it doesn't seem to be an easy cure, at least if we are to believe
what was said in the referenced ycombinator thread.

Best regards,
Carlos

Message #14 received at 51146 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Carlos Pita <carlosjosepita2 <at> gmail.com>
Cc: 51146 <at> debbugs.gnu.org
Subject: Re: bug#51146: 29.0.50;
 file-notify-add-watch not reporting changes of files
Date: Tue, 12 Oct 2021 16:54:17 +0300

> From: Carlos Pita <carlosjosepita2 <at> gmail.com>
> Date: Tue, 12 Oct 2021 01:52:37 -0300
> 
> I think that either this is a bug or the documentation is very
> misleading in that the library provides a unified interface. If it is a
> bug, it doesn't seem to be an easy cure, at least if we are to believe
> what was said in the referenced ycombinator thread.

IMO, this is a bug in macOS: the kqueue feature is incomplete and
unreliable.  I suggest to file a bug report with kqueue developers
about that.  I don't see how Emacs can do anything to improve the
situation on macOS regarding file notifications, as long as macOS
doesn't get their act together and fix this.

Message #17 received at 51146 <at> debbugs.gnu.org (full text, mbox):

From: Carlos Pita <carlosjosepita2 <at> gmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 51146 <at> debbugs.gnu.org
Subject: Re: bug#51146: 29.0.50; file-notify-add-watch not reporting changes
 of files
Date: Tue, 12 Oct 2021 11:41:36 -0300

Hi Eli,

> IMO, this is a bug in macOS: the kqueue feature is incomplete and
> unreliable.  I suggest to file a bug report with kqueue developers
> about that.  I don't see how Emacs can do anything to improve the
> situation on macOS regarding file notifications, as long as macOS
> doesn't get their act together and fix this.

I agree there's not a lot that Emacs can do here. I disagree in than
it's a bug in macOS. Although I'm not an expert in the matter from what
I have been reading this is a fact about watchers implemented on top of
kqueue. See for example this remark in the documentation of fswatch [1]:

    The kqueue monitor requires a file descriptor to be opened for every
    file being watched. As a result, this monitor scales badly with the
    number of files being observed and may begin to misbehave as soon as
    the fswatch process runs out of file descriptors. In this case,
    fswatch dumps one error on standard error for every file that cannot
    be opened so that users are notified and can take action, including
    terminating the fswatch session. Beware that on some systems the
    maximum number of file descriptors that can be opened by a process
    is set to a very low value (values as low as 256 are not uncommon),
    even if the operating system may allow a much larger value.

They also present a number of alternatives:

    - Consider raising the number of maximum open file descriptors.
    - Consider using the (--directories, -d) option.
    - Consider using another monitor.

The --directories option:

    Request the monitor to watch directories only during a recursive
    scan. This feature helps reducing the number of open file
    descriptors if a generic change event for a directory is acceptable
    instead of events on directory children.

So it's not a strategy that fully addresses the problem and, moreover,
it will miss directory changes that are part of the contract of
file-notify-add-watch anyway.

Besides the aforementioned problems of scalability, watching each file
would lead to a more complex implementation that updates the list of
watched files each time an add/remove event is detected.

Is there is no other option I sympathize with the idea of leaving emacs
alone, but the documentation is still lacking and should be very clear
about what to expect in the case of kqueue (as the documentation quoted
above is). Also the documentation in the manual and in the docstring
should converge.

Best regards,
Carlos

---

[1] http://emcrisostomo.github.io/fswatch/doc/1.16.0/fswatch.html/Monitors.html#Peculiarities-1

Message #20 received at 51146 <at> debbugs.gnu.org (full text, mbox):

From: Carlos Pita <carlosjosepita2 <at> gmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 51146 <at> debbugs.gnu.org
Subject: Re: bug#51146: 29.0.50; file-notify-add-watch not reporting changes
 of files
Date: Tue, 12 Oct 2021 11:52:20 -0300

That was about kqueue, but in macOS there is the FSEvents monitor:

   The FSEvents monitor, available only on Apple macOS, has no known
   limitations and scales very well with the number of files being
   observed. In fact, I observed no performance degradation when testing
   fswatch observing changes on a filesystem of 500 GB over long periods
   of time. This is the default monitor on Apple macOS.

So this seems to be an option, either directly or through fswatch.

Message #23 received at 51146 <at> debbugs.gnu.org (full text, mbox):

From: Michael Albinus <michael.albinus <at> gmx.de>
To: Carlos Pita <carlosjosepita2 <at> gmail.com>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 51146 <at> debbugs.gnu.org
Subject: Re: bug#51146: 29.0.50; file-notify-add-watch not reporting changes
 of files
Date: Wed, 13 Oct 2021 09:45:23 +0200

Carlos Pita <carlosjosepita2 <at> gmail.com> writes:

> That was about kqueue, but in macOS there is the FSEvents monitor:
>
>    The FSEvents monitor, available only on Apple macOS, has no known
>    limitations and scales very well with the number of files being
>    observed. In fact, I observed no performance degradation when testing
>    fswatch observing changes on a filesystem of 500 GB over long periods
>    of time. This is the default monitor on Apple macOS.
>
> So this seems to be an option, either directly or through fswatch.

Of course. I wrote the Emacs kqueue support on top of FreeBSD. Likely, I
won't write something macOS specific, since I have no respective
machine, and I'm not interested in. Somebody else must do the job, if
required.

(And I will analyze your bug report in detail next days, as time permits).

Best regards, Michael.

Message #26 received at 51146 <at> debbugs.gnu.org (full text, mbox):

From: Michael Albinus <michael.albinus <at> gmx.de>
To: Carlos Pita <carlosjosepita2 <at> gmail.com>
Cc: 51146 <at> debbugs.gnu.org
Subject: Re: bug#51146: 29.0.50; file-notify-add-watch not reporting changes
 of files
Date: Fri, 15 Oct 2021 14:06:51 +0200

Carlos Pita <carlosjosepita2 <at> gmail.com> writes:

> Hi all,

Hi Carlos,

> the docstring for file-notify-add-watch states:
>    
>     If FILE is a directory, ‘change’ watches for file creation or
>     deletion in that directory.  This does not work recursively.
>
> OTOH the elisp documentation in [1] states:
>
>      If file is a directory, changes for all files in that directory
>      will be notified. This does not work recursively.
>
> So the statements seem contradictory regarding the notification of
> changes in the contents of the files (not their creation or
> deletion). In the most lenient interpretation, the docstring may be
> considered silent in this regard.

Well, the situation is complex. We have 6 (in words: "six") different
file notification libraries: inotify, kqueue, gfilenotify, w32notify,
inotifywatch, and gio. The two latter libraries are for remote file
systems.

gfilenotify / gio know different implementations, called monitors. Until
now I could test 5 (in words: "five") of them: GFamFileMonitor,
GFamDirectoryMonitor, GInotifyFileMonitor, GKqueueFileMonitor, and
GPollFileMonitor. Likely there are more.

All of them behave slightly different. filenotify.el is the attempt to
harmonize them, especially to let them fire the same events. This has
restrictions.

If you want to get an impressions, I recommend you to read
test/lisp/filenotify-tests.el.

> This little experiment shows that, at least in my setup, a change in the
> content of a file is not notified:
>
>     (defun my-watcher (event) (message "Not in my watch: %s" event))
>
>     (file-notify-add-watch "/tmp/some-dir" '(change) #'my-watcher)
>
> Then:
>
>     touch /tmp/some-dir/x => Not in my watch: (18 created /tmp/some-dir/x)
>     echo dsds > /tmp/some-dir/x => Nothing
>     rm  /tmp/some-dir/x => Not in my watch: (18 deleted /tmp/some-dir/x)

Running your test with some of them shows:

inotify
=======
touch /tmp/x => Not in my watch: ((1 . 0) created /tmp/x)
echo dsds > /tmp/x => Not in my watch: ((1 . 0) changed /tmp/x)
rm  /tmp/x => Not in my watch: ((1 . 0) deleted /tmp/x)

kqueue
======
touch /tmp/x => Not in my watch: (9 created /tmp/x)
echo dsds > /tmp/x => Nothing
rm  /tmp/x => Not in my watch: (9 deleted /tmp/x)

gfilenotify (GFamFileMonitor)
===========
touch /tmp/x => Not in my watch: (8590168344 created /tmp/x)
echo dsds > /tmp/x => Not in my watch: (8590168344 changed /tmp/x)
rm  /tmp/x => Not in my watch: (8590168344 deleted /tmp/x)

w32notify
=========
touch c:/Users/albinus/AppData/Roaming/x => Not in my watch: (649583674484 created c:/Users/albinus/AppData/Roaming/x)
echo dsds > c:/Users/albinus/AppData/Roaming/x => Not in my watch: (649583674484 changed c:/Users/albinus/AppData/Roaming/x)
rm  c:/Users/albinus/AppData/Roaming/x => Not in my watch: (649583674484 deleted c:/Users/albinus/AppData/Roaming/x)

inotifywait
===========
touch /tmp/x => Not in my watch: (inotifywait created /ssh:gandalf:/tmp/x)
echo dsds > /tmp/x => Not in my watch: (inotifywait changed /ssh:gandalf:/tmp/x)
rm  /tmp/x => Not in my watch: (inotifywait deleted /ssh:gandalf:/tmp/x)

gio (GKqueueFileMonitor)
===
touch /tmp/x => Not in my watch: (gio created /ssh:freebsd12:/tmp/x)
echo dsds > /tmp/x => Nothing
rm  /tmp/x => Not in my watch: (gio deleted /ssh:freebsd12:/tmp/x)

gio (GPollFileMonitor)
===
touch /tmp/x => Not in my watch: (gio created /mock:win10:/tmp/x)
echo dsds > /tmp/x => Not in my watch: (gio changed /mock:win10:/tmp/x)
rm  /tmp/x => Not in my watch: (gio deleted /mock:win10:/tmp/x)

> So according to the elisp manual this is a bug, but according to the
> docstring it seemingly isn't. I'm with the manual in this one.

In case a directory is watched, creation and deletion of a file are
notified. Changes to a file are notified depending on the underlying
library, in short everything which depends on kqueue does not notify
such a change.

I will adapt the docstring of file-notify-add-watch as well as the elisp
manual accordingly.

> Best regards,
> Carlos

Best regards, Michael.

Message #29 received at 51146 <at> debbugs.gnu.org (full text, mbox):

From: Carlos Pita <carlosjosepita2 <at> gmail.com>
To: Michael Albinus <michael.albinus <at> gmx.de>
Cc: 51146 <at> debbugs.gnu.org
Subject: Re: bug#51146: 29.0.50; file-notify-add-watch not reporting changes
 of files
Date: Fri, 15 Oct 2021 15:43:23 -0300

Hi Michael,

I highly appreciate your detailed explanation.

> I will adapt the docstring of file-notify-add-watch as well as the elisp
> manual accordingly.

Thanks, this should be enough, it's clear from the above that there's
only so much emacs can do in order to pull off the illusion of a
common interface.

Best regards,
Carlos

Message #34 received at 51146-done <at> debbugs.gnu.org (full text, mbox):

From: Michael Albinus <michael.albinus <at> gmx.de>
To: Carlos Pita <carlosjosepita2 <at> gmail.com>
Cc: 51146-done <at> debbugs.gnu.org
Subject: Re: bug#51146: 29.0.50; file-notify-add-watch not reporting changes
 of files
Date: Sat, 16 Oct 2021 08:53:11 +0200

Version: 28.1

Carlos Pita <carlosjosepita2 <at> gmail.com> writes:

> Hi Michael,

Hi Carlos,

>> I will adapt the docstring of file-notify-add-watch as well as the elisp
>> manual accordingly.
>
> Thanks, this should be enough, it's clear from the above that there's
> only so much emacs can do in order to pull off the illusion of a
> common interface.

Thanks for the feedback, I'm closing the bug.

> Best regards,
> Carlos

Best regards, Michael.

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