Re: Things I wish I'd known about Inotify

[ebiederm@xmission.com (Eric W. Biederman)]

"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:

> (To: == [the set of people I believe know a lot about inotify])
>
> Hello all,
>
> Lately, I've been studying the inotify API fairly thoroughly and
> realized that there's a very big gap between knowing what the system
> calls do versus using them to reliably and efficiently monitor the
> state of a set of filesystem objects.
>
> With that in mind, I've drafted some substantial additions to the
> inotify(7) man page. I would be very happy if folk on the "To:" list
> could comment on the text below, since I believe you all have a lot of
> practical experience with Inotify. (Of course, I also welcome comments
> from anyone else.) In particular, I would like comments on the
> accuracy of the various technical points (especially those relating to
> matching up related IN_MOVED_FROM and IN_MOVED_TO events), as well as
> pointers on any other pitfalls that the programmers should be wary of
> that should be added to the page.


Other pitfalls.

Inotify only report events that a user space program triggers through
the filesystem API.  Which means inotify is limited for remote
filesystems, and filesystems like proc and sys have no monitorable
events.

Eric


> Thanks,
>
> Michael
>
>    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.
>
>        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.)
>
>        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).