bug#21435: 25.0.50; file-notify has problems after renames

[Eli Zaretskii]

> From: Tassilo Horn <tsdh@gnu.org>
> Date: Tue, 08 Sep 2015 10:47:00 +0200
> 
> file-notify using the inotify backend reports wrong events after
> directory renames.  For example, with
> 
> (let* ((flags '(change attribute-change))
>        (handler (lambda (event)
>                 (message "WATCH: %S" event)))
>        (x "/tmp/x")
>        (y "/tmp/x/y"))
>   (make-directory x)
>   (make-directory y)
>   (file-notify-add-watch x flags handler)
>   (file-notify-add-watch y flags handler))
> 
> the following events are reported after these shell operations:
> 
>   1. $ touch /tmp/x/y/test1
>      WATCH: ((12) created "/tmp/x/y/test1")
>      WATCH: ((12) attribute-changed "/tmp/x/y/test1")
>   2. $ mv /tmp/x/y /tmp/x/z
>      WATCH: ((11) renamed "/tmp/x/y" "/tmp/x/z") [2 times]
>   3. $ touch /tmp/x/z/test2
>      WATCH: ((12) created "/tmp/x/y/test2")
>      WATCH: ((12) attribute-changed "/tmp/x/y/test2")
> 
> In 3, the file created is actually /tmp/x/z/test2, not /tmp/x/y/test2.

With w32notify, I see something very similar:

  1. $ touch /tmp/x/y/test1
     WATCH: (133902432 created "d:/tmp/x/y/test1")
     WATCH: (133902432 changed "d:/tmp/x/y/test1")
     WATCH: (159542912 changed "d:/tmp/x/y")
  2. $ mv /tmp/x/y /tmp/x/z
     WATCH: (159542912 renamed "d:/tmp/x/y" "d:/tmp/x/z")

(Here the watching thread exist with abnormal status.)

  3. $ touch /tmp/x/z/test2
     WATCH: (133902432 created "d:/tmp/x/y/test2")
     WATCH: (133902432 changed "d:/tmp/x/y/test2")
     WATCH: (159542912 changed "d:/tmp/x/z")

> There is also a problem with single-file watches:
> 
> (let* ((flags '(change attribute-change))
>        (handler (lambda (event)
>                 (message "WATCH: %S" event)))
>        (x "/tmp/x"))
>   (with-temp-buffer
>     (insert "TEST\n")
>     (write-file x))
>   (file-notify-add-watch x flags handler))
> 
>   1. $ mv /tmp/x /tmp/y
>      WATCH: ((1 . "x") renamed "/tmp/x" "/tmp/y") [2 times]
>   2. $ echo "more" >> /tmp/y
>      ;; no notification
>   3. $ touch /tmp/x
>      WATCH: ((1 . "x") created "/tmp/x")
>      WATCH: ((1 . "x") changed "/tmp/x")

On Windows:

  1. $ mv /tmp/x /tmp/y
     WATCH: (159487136 renamed "d:/tmp/x" "d:/tmp/x")
  2. $ echo "more" >> /tmp/y
     ;; no notification
  3. $ touch /tmp/x
     WATCH: (159487136 created "d:/tmp/x")
     WATCH: (159487136 changed "d:/tmp/x")

> So in the case of single-file watches, the watch seems to stick to a
> filename rather than a file.  Well, I think that's ok but then I would
> expect it to be similar with directories, e.g., in the first example
> step 3 should not report a notification because I intended to watch
> /tmp/x/y and not /tmp/x/z.

I don't see any problems here.  filenotify.el just reports to us what
the notification back-end receives.  It doesn't (and shouldn't, IMO)
include any non-trivial logic to "fix" the back-ends in these tricky
cases, because it has no idea what the application will want to do
with that.  Its role is to present a platform-independent API that
uses one of the available back-ends, that's all.

IOW, technically, the file/directory you watched disappeared from the
filesystem, so what is reported afterwards is IMO entirely resonable
at this low level.

One application we have on top of filenotify.el is auto-revert.el.
Now, if you see some user-level problems there when files are
renamed/deleted, please report them.

> Another thing is that renames always seem to be reported twice (although
> the example in the manual lists only one event but that example has
> probably gathered with the gfilenotify backend).

This doesn't happen with w32notify.  filenotify.el includes some code
to produce a single notification out of the 2 reported by back-ends in
this case; perhaps something doesn't work there with inotify.  Can you
step through the code and see why?