Re: Things I wish I'd known about Inotify

[Michael Kerrisk (man-pages)]

Late follow up on this thread..., since another question occurred in
discussions with Jake.

On Fri, Apr 4, 2014 at 2:43 PM, Jan Kara <jack@xxxxxxx> wrote:
> On Fri 04-04-14 09:35:50, Michael Kerrisk (man-pages) wrote:
>> On 04/03/2014 10:52 PM, Jan Kara wrote:
>> > On Thu 03-04-14 08:34:44, Michael Kerrisk (man-pages) wrote:
[...]
>> >>    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  readâ
>> >>        ing from the inotify file descriptor.  However, this is not guarâ
>> >>        anteed.  If multiple processes are triggering  events  for  moniâ
>> >>        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 generâ
>> >>        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).
>>
>> Yes, that's already assumed by my discussion above (its described elsewhere
>> in the page). But your comment makes me think I should add a few words to
>> remind the reader of that fact. I'll do that.
>   Yes, that would be good.
>
>> But, the point is that even with the cookie, matching the events is
>> nontrivial, since:
>>
>> * There may not even be an IN_MOVED_FROM event
>> * There may be an arbitrary number of other events in between the
>>   IN_MOVED_FROM and the IN_MOVED_TO.
>>
>> Therefore, one has to use heuristic approaches such as "allow at least
>> N millisconds" or "check the next N events" to see if there is an
>> IN_MOVED_FROM that matches the IN_MOVED_TO. I can't see any way around
>> that being inherently racy. (It's unfortunate that the kernel can't
>> provide a guarantee that the two events are always consecutive, since
>> that would simply user space's life considerably.)
>   Yeah, it's unpleasant but doing that would be quite costly/complex at the
> kernel side. And the race would in the worst case lead to application
> thinking there's been file moved outside of watched area & a file moved
> somewhere else inside the watched area. So the application will have to
> possibly inspect that file. That doesn't seem too bad.

One further question. The IN_MOVED_FROM+IN_MOVED_TO pair may not be
guaranteed to be contiguous in the read buffer, but is their insertion
in the event queue guaranteed to be atomic from a user-space point of
view? That is to say: having read an IN_MOVED_FROM event, does user
space have the guarantee that if there is an IN_MOVED_TO event, then
it will already be in the queue? The reason I ask is that this would
affect how user space might try to read the IN_MOVED_TO event. If
there is no such guarantee, then a read() (or select()/poll()) with
(small) timeout is needed. If such a guarantee is provided, then a
nonblocking read() would suffice.

Cheers,

Michael

PS I just now found this code by John McCutchan
https://git.gnome.org/browse/gnome-vfs/tree/modules/inotify-kernel.c#n570
which suggests that the insertion of the event pair is not atomic
w.r.t. user space. Still, I wonder if there is any definitive
statement about this.

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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