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PODetecting foreign IN_MOVED_FROM renames with inotify
 

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  1. PODetecting foreign IN_MOVED_FROM renames with inotify
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    <p>Assume a C program which mirrors the changes in a watched directory to a mirror directory. You recursively watch all subdirectories and use a structure indexed on <code>watch descriptors</code> containing the <code>pathnames</code> of the watched directories to be able to reconstruct the full pathname of the file in case of an event. This is all you need to process all events but <code>IN_MOVED_FROM</code> and <code>IN_MOVED_TO</code>. In the case of these two, you, in addition to the aforementioned, seem to also need the following:</p> <ul> <li>A structure indexed on <code>cookies</code> allowing you to store and retrieve the <code>watch descriptor</code> and the <code>filename</code> of a particular <code>IN_MOVED_FROM</code> event once a matching <code>IN_MOVED_TO</code> event appears.</li> <li>A <code>timestamp</code>-ordered queue (redundant) containing cookies which have not yet been paired, so that they can be reinterpreted as <code>IN_DELETE</code> events and removed from both of the structures once an arbitrary period of time has passed.</li> <li>A <code>pathname/filename</code> -> <code>watch descriptor</code> relation (redundant), which allows you to effectively find out the <code>watch descriptor</code> of a timeouted <code>cookie</code> in case the (re)moved filesystem object was a directory, which you now need to unwatch.</li> </ul> <p>Quite some overhead for supporting rename operations. Is there really no other way to detect a foreign rename other than waiting for an unpaired cookie to time out? Given the fact that there <a href="http://www.spinics.net/lists/linux-fsdevel/msg46040.html" rel="nofollow">appears to be a way of telling between a local and foreign rename</a> in <a href="http://lxr.free-electrons.com/source/include/linux/fsnotify.h" rel="nofollow">fsnotify</a>, this seems as a rather odd design choice.</p>
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    1. COI would consider the mirrordir command. Either use it directly or peruse the source. http://linux.die.net/man/1/mirrordir and http://mirrordir.sourcearchive.com/documentation/0.10.49-intl/files.html
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    2. COThe command doesn't use inotify to detect filesystem events. In fact, it doesn't really seem to be detecting filesystem events at all — it just mirrors the directory and exits instead of monitoring it for changes.
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    3. COIMO - that is because a true mirror operates at driver level in the kernel. rsync also mirrors changes to local/non-local directories. It also runs and exits. So, other than for learning, I can't see trying to develop a userland mirror.
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