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Watcher prototype » History » Version 17

Greg Burri, 11/11/2009 11:20 AM

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h1. Watcher prototype
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The cache file structure store all the hashes of the shared files. If a file is added or removed from a shared directory (or a subfolder) it should be added or removed from the cache.
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As far as I know there is no way to watch changes recursively of a folder tree with Qt. The Qt class _QFileSystemWatcher_ cannot watch recursively.
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The only way to synchronize is to periodically scan all the shared data and compare to the cached one. It can be a bit CPU consumer so it should be done with a low priority thread and maybe with a pause between each file hashing.
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h2. Native solution
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A better solution would be to use native platform API.
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h3. Linux : inotify
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On Linux it exists a function called "inotify":http://linux.die.net/man/7/inotify
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* The big issue is that inotify doesn't support the recursive watch of a directory.
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* There is a limitation of the number of watcher, see @/proc/sys/fs/inotify/max_user_watches@, for example : 8192 under Debian.
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* There is no problem to create thousands of watcher, it's very light. 
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h3. Windows : ReadDirectoryChangesW and WaitForMultiplesObjects
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_ReadDirectoryChangesW_ can watch recursively and asynchronously any change from a given directory. See : http://msdn.microsoft.com/en-us/library/aa365465%28VS.85%29.aspx
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Then, a thread can wait an event by calling _WaitForMultiplesObjects_, see : http://msdn.microsoft.com/en-us/library/aa365261%28VS.85%29.aspx
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h4. Events when copying :
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Beginning of the copy :
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<pre>
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waitEvent
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Action =  1 (FILE_ACTION_ADDED)
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filename =  "ACDC - Back In Black.mpg"
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waitEvent
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Action =  3 (FILE_ACTION_MODIFIED)
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filename =  "ACDC - Back In Black.mpg"
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</pre>
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End of the copy :
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<pre>
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waitEvent
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Action =  3 (FILE_ACTION_MODIFIED)
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filename =  "ACDC - Back In Black.mpg"
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<pre>