bug#18880: 25.0.50; inotify-based file notifications missing sometimes

[Dima Kogan <dima@secretsauce.net>]

This bug is about an issue on the emacs-devel-mailing list:

 http://lists.gnu.org/archive/html/emacs-devel/2014-10/msg00911.html

The main message is copied below:

I ran into an issue with inotify-base file notifications when looking at
auto-revert stuff from an earlier thread. To show the issue, I build
the latest emacs from source using

 ./configure --with-file-notification=inotify

I then run

 ./emacs --eval "`cat  /tmp/tstnotify.el`" -Q -nw

with tstnotify.el being

 (progn
   (require 'filenotify)

   (dolist (fil '("/tmp/tst1" "/tmp/tst2"))
     (file-notify-add-watch fil  '(change attribute-change)
                            (lambda (event)
                              (message "notify event %s" event)))
     (find-file fil))
   (switch-to-buffer "*Messages*"))


Here I ask for notifications for two files, and print out the events as
they come in. While emacs is running this way, I modify those two files
using an external tool. I would expect to see modification events for
both of these files, but I only see them for one of the files.

The issue is that emacs doesn't ask inotify to look at the two files, it
asks it to look at the containing directory (/tmp in both cases),
filtering out all the files we don't care about later. inotify is being
intelligent, notices that we're asking to monitor the same object twice
(/tmp), and simply returns the same descriptor a second time. Emacs is
assuming a unique descriptor, and the result is that we only get
notifications for one file.