Re: File watch support in autorevert.el

[Eli Zaretskii <eliz@gnu.org>]

> From: Michael Albinus <michael.albinus@gmx.de>
> Date: Thu, 10 Jan 2013 15:28:27 +0100
> 
> As a proof of concept, I have installed a patch in autorevert.el
> implementing file watches.  This shall work for `auto-revert-mode' and
> `global-auto-revert-mode' for buffers visiting files.
> `auto-revert-tail-mode' is not supported (yet).

Thanks.

> The implementation uses `inotify-*-watch' functions. I have also added
> the calls for `w32-*-watch' functions, but this is untested.

The functions' names are w32notify-*-watch.  I fixed that in the trunk
revision 111482.  I also did some simple testing, and it seems to
generally work (but see the problems described below).

> Any feedback is welcome.

My feedback is as follows:

 . The code as written is too naive: it blindly assumes that every
   single notification reported by the filesystem for a given watch is
   necessarily the one requested in the auto-revert-notify-add-watch
   call.  But that assumption is false, at least on Windows, where the
   implementation actually watches events to the entire parent
   directory of the file we are interested in.  So Emacs reverts the
   file whenever _any_ file in the same directory was changed.  I
   believe similar problems can happen with inotify, albeit much more
   rarely.  For that reason, I think auto-revert-notify-handler should
   filter events by ASPECTS/ACTION member, and on Windows also by FILE
   member of the event.

 . It isn't clear to me that using IN_CLOSE_WRITE with inotify is TRT:
   AFAIU, that would mean we only revert a file when the application
   writing to it closes its descriptor.  IOW, if the application makes
   several changes to the file during a prolonged operation, and
   doesn't close and reopen the file in between, we will only see the
   changes at the end, but not during the operation.  Wouldn't it be
   better to use IN_MODIFY instead?

 . At least on Windows, turning on auto-revert-mode and then modifying
   and saving the file announces that it was auto-reverted.  This
   didn't happen with the auto-revert method that doesn't use file
   notifications.  Is this a bug?

 . I believe some of the features added to autorevert.el, such as a
   hash list of watch descriptors, should be in some infrastructure
   with appropriate APIs.