Magic Folder on Linux: handle case when directory is removed from monitored directory

[dawuud]

We need to handle the case where a directory is removed or moved out of the monitored local directory.

Specifically this case needs to be specially handled only when the sub-directory was moved into the drop uploader directory and a inotify watch for that sub-directory was added. Further investigation is needed to determine the proper way to remove this inotify watch.

So far it seems that we will respond to either the IN_DELETE_SELF or IN_MOVE_SELF events for each of these sub-directory inotify "watches".

[daira]

It seems as though the IN_EXCL_UNLINK flag would help with this:

   IN_EXCL_UNLINK (since Linux 2.6.36)
          By default, when watching events on the children of a
          directory, events are generated for children even after
          they have been unlinked from the directory.  This can
          result in large numbers of uninteresting events for some
          applications (e.g., if watching /tmp, in which many
          applications create temporary files whose names are
          immediately unlinked).  Specifying IN_EXCL_UNLINK changes
          the default behavior, so that events are not generated for
          children after they have been unlinked from the watched
          directory.

[daira]

The constant for IN_EXCL_UNLINK isn't defined by Twisted (​https://twistedmatrix.com/trac/ticket/7789) but that's okay, we can define it ourselves, since Twisted passes through unknown flags.

[daira]

Note that this is not an issue on Windows, because ReadDirectoryChangesW only reads events that occur for paths that are still under the recursively-watched directory.

[dawuud]

Furthermore these additional tests should also demonstrate that a directory which has been moved into the drop upload directory will have the IN_EXCL_UNLINK behavior just like the upload directory itself.

[daira]

Add magic-folder keyword to all drop-upload tickets.