bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Perry Smith]

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Referencing these commands:
; 1
(require 'filenotify)

; 2
(defun my-callback (directory)
  (message (format "called %s" directory)))

; 3
(file-notify-add-watch "/private/tmp"  '(change attribute-change) 'my-callback)

; 4
(file-notify-add-watch "/tmp"  '(change attribute-change) 'my-callback)

Starting with a fresh emacs -Q, if I execute lines 1, 2, and 3 and then
touch a file such as /tmp/OUT, I get the notification as I should.

However if I start fresh, execute lines 1, 2, and 4, and touch /tmp/OUT,
I do not get a notification.

On a Mac, /tmp is a symbolic link to private/tmp (relative path).

I first discovered this issue using Helm's find-file and I entered a
report with Helm.  The Helm developer reports that it works in his case
with Linux.

I'm using a new M1 Mac, macOS 12.5.1 and using the newish AFS.  I built
this emacs myself so it might be pilot error with my build but that
seems less likely since file notify does generally work but not when the
watched file is a symbolic link to a directory.

Also, this is not /tmp => private/tmp specific.  I can recreate the same
issue using ~/Desktop/Dog/tmp and a symbolic link ~/Desktop/tmp that has
a relative path of Dog/tmp and I get the same issue.

In GNU Emacs 28.1 (build 1, aarch64-apple-darwin21.4.0, NS appkit-2113.40 Version 12.3.1 (Build 21E258))
of 2022-04-04 built on Peace.lan
Repository revision: bffd375b378025c8f5fd947fdac8ed710cb980d7
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bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Eli Zaretskii]

> From: Perry Smith <pedz@easesoftware.com>
> Date: Thu, 1 Sep 2022 18:34:39 -0500
> 
> Referencing these commands:
> ; 1
> (require 'filenotify)
> 
> ; 2
> (defun my-callback (directory)
>   (message (format "called %s" directory)))
> 
> ; 3
> (file-notify-add-watch "/private/tmp"  '(change attribute-change) 'my-callback)
> 
> ; 4
> (file-notify-add-watch "/tmp"  '(change attribute-change) 'my-callback)
> 
> Starting with a fresh emacs -Q, if I execute lines 1, 2, and 3 and then
> touch a file such as /tmp/OUT, I get the notification as I should.
> 
> However if I start fresh, execute lines 1, 2, and 4, and touch /tmp/OUT,
> I do not get a notification.
> 
> On a Mac, /tmp is a symbolic link to private/tmp (relative path).

I don't see any bug here.  If file-notify-add-watch would resolve
symlinks of its argument, we would be unable to watch changes to the
symlink file itself.

So if you want to watch changes for the target of a symlink, your Lisp
program needs to resolve symlinks, e.g. by calling file-truename or
something similar.

> I first discovered this issue using Helm's find-file and I entered a
> report with Helm.  The Helm developer reports that it works in his case
> with Linux.

That could be a (mis)feature of our use of inotify, which is used on
GNU/Linux for implementing file notifications.  AFAIU, inotify allows
control on whether to follow symlinks when setting a watch, and its
default is to follow symlinks.  I think we should call inotify so as
not to follow symlinks.

Michael, WDYT?

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

Hi,

>> Referencing these commands:
>> ; 1
>> (require 'filenotify)
>>
>> ; 2
>> (defun my-callback (directory)
>>   (message (format "called %s" directory)))
>>
>> ; 3
>> (file-notify-add-watch "/private/tmp"  '(change attribute-change) 'my-callback)
>>
>> ; 4
>> (file-notify-add-watch "/tmp"  '(change attribute-change) 'my-callback)
>>
>> Starting with a fresh emacs -Q, if I execute lines 1, 2, and 3 and then
>> touch a file such as /tmp/OUT, I get the notification as I should.
>>
>> However if I start fresh, execute lines 1, 2, and 4, and touch /tmp/OUT,
>> I do not get a notification.
>>
>> On a Mac, /tmp is a symbolic link to private/tmp (relative path).
>
> I don't see any bug here.  If file-notify-add-watch would resolve
> symlinks of its argument, we would be unable to watch changes to the
> symlink file itself.

I agree. Emacs' file notifications are not designed to follow
symlinks. The manual in (info "(elisp) File Notifications") is silent
about, perhaps we shall clarify.

> So if you want to watch changes for the target of a symlink, your Lisp
> program needs to resolve symlinks, e.g. by calling file-truename or
> something similar.

Yep.

>> I first discovered this issue using Helm's find-file and I entered a
>> report with Helm.  The Helm developer reports that it works in his case
>> with Linux.
>
> That could be a (mis)feature of our use of inotify, which is used on
> GNU/Linux for implementing file notifications.  AFAIU, inotify allows
> control on whether to follow symlinks when setting a watch, and its
> default is to follow symlinks.  I think we should call inotify so as
> not to follow symlinks.

Yep. We shall go through our f-n libraries and check the current
behavior, preferred by a new test case. And we shall adjust the
libraries to behave similar.

Will do next days.

Btw, there are bug#16113 and bug#18883, which report a similar problem
in auto-reverting. A possible solution could be to extend the FLAGS arg
of file-notify-add-watch by a condition 'follow', which means to
supervise the expanded symlink instead of the link file itself.

inotify knows the mask bit IN_DONT_FOLLOW (which we haven't set yet),
see inotify(7). Other libraries might offer similar possibilities, which
I haven't checked yet.

Best regards, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Cc: Perry Smith <pedz@easesoftware.com>,  57536@debbugs.gnu.org
> Date: Sun, 04 Sep 2022 13:42:16 +0200
> 
> > I don't see any bug here.  If file-notify-add-watch would resolve
> > symlinks of its argument, we would be unable to watch changes to the
> > symlink file itself.
> 
> I agree. Emacs' file notifications are not designed to follow
> symlinks. The manual in (info "(elisp) File Notifications") is silent
> about, perhaps we shall clarify.

Yes, we should clarify that, both in the manual and in the doc
strings, I think.

> Btw, there are bug#16113 and bug#18883, which report a similar problem
> in auto-reverting. A possible solution could be to extend the FLAGS arg
> of file-notify-add-watch by a condition 'follow', which means to
> supervise the expanded symlink instead of the link file itself.

I think it would be better to handle that option in Lisp, before we
call the OS-specific notification library.  That way, we can control
better what exactly "follow symlinks" means.

> inotify knows the mask bit IN_DONT_FOLLOW (which we haven't set yet),
> see inotify(7). Other libraries might offer similar possibilities, which
> I haven't checked yet.

I see that w32notify.c currently follows symlinks; that will need to
be fixed.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

Hi Eli,

>> Btw, there are bug#16113 and bug#18883, which report a similar problem
>> in auto-reverting. A possible solution could be to extend the FLAGS arg
>> of file-notify-add-watch by a condition 'follow', which means to
>> supervise the expanded symlink instead of the link file itself.
>
> I think it would be better to handle that option in Lisp, before we
> call the OS-specific notification library.  That way, we can control
> better what exactly "follow symlinks" means.

Of course, that's why I've mentioned file-notify-add-watch.

>> inotify knows the mask bit IN_DONT_FOLLOW (which we haven't set yet),
>> see inotify(7). Other libraries might offer similar possibilities, which
>> I haven't checked yet.
>
> I see that w32notify.c currently follows symlinks; that will need to
> be fixed.

Yep. I guess you'll care about w32notify.c. and I'll check inotify.c,
kqueue.c and gfilenotify.c?

Best regards, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Cc: pedz@easesoftware.com,  57536@debbugs.gnu.org
> Date: Sun, 04 Sep 2022 16:26:54 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> inotify knows the mask bit IN_DONT_FOLLOW (which we haven't set yet),
> >> see inotify(7). Other libraries might offer similar possibilities, which
> >> I haven't checked yet.
> >
> > I see that w32notify.c currently follows symlinks; that will need to
> > be fixed.
> 
> Yep. I guess you'll care about w32notify.c. and I'll check inotify.c,
> kqueue.c and gfilenotify.c?

Yes.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Eli Zaretskii]

> Cc: pedz@easesoftware.com, 57536@debbugs.gnu.org
> Date: Sun, 04 Sep 2022 17:49:45 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > From: Michael Albinus <michael.albinus@gmx.de>
> > Cc: pedz@easesoftware.com,  57536@debbugs.gnu.org
> > Date: Sun, 04 Sep 2022 16:26:54 +0200
> > 
> > Eli Zaretskii <eliz@gnu.org> writes:
> > 
> > >> inotify knows the mask bit IN_DONT_FOLLOW (which we haven't set yet),
> > >> see inotify(7). Other libraries might offer similar possibilities, which
> > >> I haven't checked yet.
> > >
> > > I see that w32notify.c currently follows symlinks; that will need to
> > > be fixed.
> > 
> > Yep. I guess you'll care about w32notify.c. and I'll check inotify.c,
> > kqueue.c and gfilenotify.c?
> 
> Yes.

Now done on the master branch.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

>> > Yep. I guess you'll care about w32notify.c. and I'll check inotify.c,
>> > kqueue.c and gfilenotify.c?
>
> Now done on the master branch.

Thanks! I hope to get done my parts next days ...

Best regrds, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

Hi Eli,

>> Yep. I guess you'll care about w32notify.c. and I'll check inotify.c,
>> kqueue.c and gfilenotify.c?
>
> Yes.

I've pushed the changes for inotify and the remote inotifywait. There's
also a new testcase file-notify-test11-symlinks. Do you mind to check it
on w32?

The other cases will follow.

Best regards, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Cc: pedz@easesoftware.com,  57536@debbugs.gnu.org
> Date: Fri, 16 Sep 2022 17:27:34 +0200
> 
> I've pushed the changes for inotify and the remote inotifywait. There's
> also a new testcase file-notify-test11-symlinks. Do you mind to check it
> on w32?

I don't think I can: I have XP here, where symlinks aren't supported.

Would someone else please see if the new test works on MS-Windows?

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

Hi Eli,

>> I've pushed the changes for inotify and the remote inotifywait. There's
>> also a new testcase file-notify-test11-symlinks. Do you mind to check it
>> on w32?
>
> I don't think I can: I have XP here, where symlinks aren't supported.
>
> Would someone else please see if the new test works on MS-Windows?

Hmm. Somewhere, I have a Windows 10 VM lying around. Perhaps I can
convince it to support symlinks in Emacs, and test then.

Disclaimer: I don't know what I'm doing.

Best regards, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

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

Hi Eli,

>>> I've pushed the changes for inotify and the remote inotifywait. There's
>>> also a new testcase file-notify-test11-symlinks. Do you mind to check it
>>> on w32?
>>
>> I don't think I can: I have XP here, where symlinks aren't supported.
>>
>> Would someone else please see if the new test works on MS-Windows?
>
> Hmm. Somewhere, I have a Windows 10 VM lying around. Perhaps I can
> convince it to support symlinks in Emacs, and test then.

Well, the test fails with

--8<---------------cut here---------------start------------->8---
Test file-notify-test11-symlinks condition:
    (file-error "Making symbolic link" "Operation not permitted" "c:/Users/albinus/AppData/Local/Temp/file-notify-testQ4fWLh/file-notify-testoFYuvn" "c:/Users/albinus/AppData/Local/Temp/file-notify-testQ4fWLh/file-notify-testLaRL78")
--8<---------------cut here---------------end--------------->8---

So a simple `make-symbolic-link' doesn't seem to work on MS
Windows. What shall I do instead?

Best regards, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Cc: pedz@easesoftware.com,  57536@debbugs.gnu.org
> Date: Sat, 17 Sep 2022 11:02:49 +0200
> 
> >> Would someone else please see if the new test works on MS-Windows?
> >
> > Hmm. Somewhere, I have a Windows 10 VM lying around. Perhaps I can
> > convince it to support symlinks in Emacs, and test then.
> 
> Well, the test fails with
> 
> --8<---------------cut here---------------start------------->8---
> Test file-notify-test11-symlinks condition:
>     (file-error "Making symbolic link" "Operation not permitted" "c:/Users/albinus/AppData/Local/Temp/file-notify-testQ4fWLh/file-notify-testoFYuvn" "c:/Users/albinus/AppData/Local/Temp/file-notify-testQ4fWLh/file-notify-testLaRL78")
> --8<---------------cut here---------------end--------------->8---
> 
> So a simple `make-symbolic-link' doesn't seem to work on MS
> Windows.

It should work if you give your user the privilege of creating
symlinks.

Alternatively, try running Emacs from a cmd.exe window that is "Run as
Administrator".

Creating symlinks is a privileged operation on MS-Windows (at least
before Windows 11, where I hear that restriction was lifted?)

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

Hi Eli,

>> So a simple `make-symbolic-link' doesn't seem to work on MS
>> Windows.
>
> It should work if you give your user the privilege of creating
> symlinks.

But the damn User Account Settings don't give me this possibility :-(

> Alternatively, try running Emacs from a cmd.exe window that is "Run as
> Administrator".

That worked, thanks.

Your changes in w32notify.c work as expected, I simply needed to adapt
the test case a little bit. I've added also to skip this test, when
make-symbolic-link doesn't work.

Pushed to master.

Best regards, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

Hi Eli,

>> Yep. I guess you'll care about w32notify.c. and I'll check inotify.c,
>> kqueue.c and gfilenotify.c?
>
> Yes.

So everything is tested, the cases mentioned above as well as the remote
inotifywait and gio variants. Everything works as expected except
kqueue, where I get an EMLINK error when running file-notify-add-watch
on a symlink file. Needs further investigation, but unfortunately, while
testing, my FreeBSD 12 VM has crashed with an unrecoverable error. So I
need to install a new VM first.

Besides this, do we want to extend file notifications to follow symlinks
when indicated? As said already, we could add a new symbol `follow' to
be used in the FLAGS argument for that function. Implementation shall be
simple for inotify and w32notify; for the other backends it needs to be
investigated.

WDYT?

Best regards, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Cc: pedz@easesoftware.com,  57536@debbugs.gnu.org
> Date: Sat, 17 Sep 2022 15:27:45 +0200
> 
> Besides this, do we want to extend file notifications to follow symlinks
> when indicated? As said already, we could add a new symbol `follow' to
> be used in the FLAGS argument for that function. Implementation shall be
> simple for inotify and w32notify; for the other backends it needs to be
> investigated.

I'm not sure this is needed: clients can always resolve the symlinks
themselves, no?

I'd start by documenting that we no longer follow symlinks when
watching: that's a kind-of incompatible change.  Then I'd go by
complaints, if any.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Cc: pedz@easesoftware.com,  57536@debbugs.gnu.org
> Date: Sat, 17 Sep 2022 15:16:37 +0200
> 
> > It should work if you give your user the privilege of creating
> > symlinks.
> 
> But the damn User Account Settings don't give me this possibility :-(

AFAIK, it is not under User Account Settings.  It is under

  Control Panel->Administrative Tools->Local Security Policy->Local Policies->User Rights Assignment

Look for "Create symbolic links" policy.

The above will have no effect if your user is in the Administrators
group: those must use the "Run as Administrator" method.

> > Alternatively, try running Emacs from a cmd.exe window that is "Run as
> > Administrator".
> 
> That worked, thanks.
> 
> Your changes in w32notify.c work as expected, I simply needed to adapt
> the test case a little bit. I've added also to skip this test, when
> make-symbolic-link doesn't work.

Thanks.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

Hi Eli,

>> But the damn User Account Settings don't give me this possibility :-(
>
> AFAIK, it is not under User Account Settings.  It is under
>
>   Control Panel->Administrative Tools->Local Security Policy->Local Policies->User Rights Assignment
>
> Look for "Create symbolic links" policy.

Indeed. Thanks.

Best regards, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Michael Albinus]

Eli Zaretskii <eliz@gnu.org> writes:

Hi Eli,

>> Besides this, do we want to extend file notifications to follow symlinks
>> when indicated? As said already, we could add a new symbol `follow' to
>> be used in the FLAGS argument for that function. Implementation shall be
>> simple for inotify and w32notify; for the other backends it needs to be
>> investigated.
>
> I'm not sure this is needed: clients can always resolve the symlinks
> themselves, no?

OK.

> I'd start by documenting that we no longer follow symlinks when
> watching: that's a kind-of incompatible change.  Then I'd go by
> complaints, if any.

I've extended already the doc yesterday, see (info "(elisp) File Notifications")

--8<---------------cut here---------------start------------->8---
     If FILE is a symlink, it doesn’t follow that link.  Just FILE
     itself will be watched.
--8<---------------cut here---------------end--------------->8---

And it isn't a visible incompatibility. Yes, inotify and w32notify did
follow the link, and they have raised events for the link target. But in
file-notify-handle-event this event must be adapted in order to keep the
unified action names we have introduced in filenotify.el. And the event
will be propagated only in case the full file name in that event is
supervised via file-notify-add-watch. This didn't happen for the
followed file names (the symlink targets), such events weren't
propagated, and the effect for the users was the same as when inotify
and w32notify didn't follow the link (as we have now). So I believe we
have nothing to explain but just the clarification I have done
yesterday.

From my pov, this bug can be closed. The problem with kqueue I will work
on once I have a new FreeBSD VM.

Best regards, Michael.

bug#57536: 28.1; filenotify problems on macOS with symbolic links to directories

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Cc: pedz@easesoftware.com,  57536@debbugs.gnu.org
> Date: Sat, 17 Sep 2022 17:15:04 +0200
> 
> > I'd start by documenting that we no longer follow symlinks when
> > watching: that's a kind-of incompatible change.  Then I'd go by
> > complaints, if any.
> 
> I've extended already the doc yesterday, see (info "(elisp) File Notifications")
> 
> --8<---------------cut here---------------start------------->8---
>      If FILE is a symlink, it doesn’t follow that link.  Just FILE
>      itself will be watched.
> --8<---------------cut here---------------end--------------->8---
> 
> And it isn't a visible incompatibility. Yes, inotify and w32notify did
> follow the link, and they have raised events for the link target. But in
> file-notify-handle-event this event must be adapted in order to keep the
> unified action names we have introduced in filenotify.el. And the event
> will be propagated only in case the full file name in that event is
> supervised via file-notify-add-watch. This didn't happen for the
> followed file names (the symlink targets), such events weren't
> propagated, and the effect for the users was the same as when inotify
> and w32notify didn't follow the link (as we have now). So I believe we
> have nothing to explain but just the clarification I have done
> yesterday.

Yes, you are right.

> >From my pov, this bug can be closed. The problem with kqueue I will work
> on once I have a new FreeBSD VM.

Fine by me, closing.

Thanks.