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Date:	Thu, 3 Apr 2014 22:52:36 +0200
From:	Jan Kara <jack@...e.cz>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	John McCutchan <john@...nmccutchan.com>,
	Robert Love <rlove@...ve.org>, Eric Paris <eparis@...hat.com>,
	Lennart Poettering <lennart@...ttering.net>,
	radu.voicilas@...il.com, daniel@...llard.com,
	Christoph Hellwig <hch@...radead.org>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>, gamin-list@...me.org,
	lkml <linux-kernel@...r.kernel.org>,
	inotify-tools-general@...ts.sourceforge.net
Subject: Re: Things I wish I'd known about Inotify

On Thu 03-04-14 08:34:44, Michael Kerrisk (man-pages) wrote:
>    Limitations and caveats
>        The inotify API provides no information about the user or process
>        that triggered the inotify event.  In  particular,  there  is  no
>        easy  way  for a process that is monitoring events via inotify to
>        distinguish events that it triggers itself from  those  that  are
>        triggered by other processes.
> 
>        The  inotify API identifies affected files by filename.  However,
>        by the time an application processes an inotify event, the  file‐
>        name may already have been deleted or renamed.
> 
>        The  inotify  API identifies events via watch descriptors.  It is
>        the application's responsibility to cache a mapping  (if  one  is
>        needed)  between  watch descriptors and pathnames.  Be aware that
>        directory renamings may affect multiple cached pathnames.
> 
>        Inotify monitoring of directories is not  recursive:  to  monitor
>        subdirectories under a directory, additional watches must be cre‐
>        ated.  This can take a significant amount time for  large  direc‐
>        tory trees.
  And also there's a problem with the limit on the number of watches a user
can have.

>        If monitoring an entire directory subtree, and a new subdirectory
>        is created in that tree or an existing directory is renamed  into
>        that  tree,  be aware that by the time you create a watch for the
>        new subdirectory, new  files  (and  subdirectories)  may  already
>        exist  inside  the subdirectory.  Therefore, you may want to scan
>        the contents of the subdirectory  immediately  after  adding  the
>        watch (and, if desired, recursively add watches for any subdirec‐
>        tories that it contains).
> 
>        Note that the event queue can overflow.  In this case, events are
>        lost.   Robust applications should handle the possibility of lost
>        events gracefully.  For example, it may be necessary  to  rebuild
>        part  or all of the application cache.  (One simple, but possibly
>        expensive, approach is to  close  the  inotify  file  descriptor,
>        empty  the  cache, create a new inotify file descriptor, and then
>        re-create watches and cache entries for the objects to  be  moni‐
>        tored.)
> 
>    Dealing with rename() events
>        The  IN_MOVED_FROM  and  IN_MOVED_TO events that are generated by
>        rename(2) are usually available as consecutive events when  read‐
>        ing from the inotify file descriptor.  However, this is not guar‐
>        anteed.  If multiple processes are triggering  events  for  moni‐
>        tored  objects,  then  (on rare occasions) an arbitrary number of
>        other events may appear between the IN_MOVED_FROM and IN_MOVED_TO
>        events.
> 
>        Matching  up  the IN_MOVED_FROM and IN_MOVED_TO event pair gener‐
>        ated by rename(2) is thus inherently racy.  (Don't forget that if
>        an  object is renamed outside of a monitored directory, there may
>        not even be an IN_MOVED_TO event.)  Heuristic  approaches  (e.g.,
>        assume the events are always consecutive) can be used to ensure a
>        match in most cases, but will inevitably miss some cases, causing
>        the  application  to  perceive  the IN_MOVED_FROM and IN_MOVED_TO
>        events as being unrelated.  If watch  descriptors  are  destroyed
>        and  re-created as a result, then those watch descriptors will be
>        inconsistent with the watch descriptors in  any  pending  events.
>        (Re-creating the inotify file descriptor and rebuilding the cache
>        may be useful to deal with this scenario.)
  Well, but there's 'cookie' value meant exactly for matching up
IN_MOVED_FROM and IN_MOVED_TO events. And 'cookie' is guaranteed to be
unique at least within the inotify instance (in fact currently it is unique
within the whole system but I don't think we want to give that promise).
 
>        Applications should also  allow  for  the  possibility  that  the
>        IN_MOVED_FROM event was the last event that could fit in the buf‐
>        fer returned by the current call to read(2), and the accompanying
>        IN_MOVED_TO event might be fetched only on the next read(2).

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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