drop-upload on Windows: can't tolerate creation of a directory under the watched directory

[daira]

[ERROR]
Traceback (most recent call last):
  File "c:\tahoe\git\tahoe-magic\src\allmydata\frontends\drop_upload.py", line 93, in _add_file
    u = FileName(path.path, self._convergence)
  File "c:\tahoe\git\tahoe-magic\src\allmydata\immutable\upload.py", line 1482, in __init__
    FileHandle.__init__(self, open(filename, "rb"), convergence=convergence)
exceptions.IOError: [Errno 13] Permission denied: u'\\\\?\\C:\\tahoe\\git\\tahoe-magic\\_trial_temp-
1\\drop_upload.RealTest.test_drop_upload\\loc\u0101l_dir\\directory'

allmydata.test.test_drop_upload.RealTest.test_drop_upload

This failure happens only if ​this code is included in DropUploadTestMixin._test:

    # Test that we tolerate creation of a subdirectory.
    d.addCallback(lambda ign: os.mkdir(os.path.join(self.local_dir, u"directory")))

The problem is that on Windows, DropUploader._process is called for changes to directories as well as files (because ReadDirectoryChangesW makes no distinction between them). So, in ​this code:

    u = FileName(path.path, self._convergence)
    return self._parent.add_file(name, u)

path.path will be the path of a directory, which causes a permission error when it is opened for reading as though it were a file.