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.