docs: def allocateFile(self, sharename, peer):
          """
          return a 2-tuple of incompletePath, fullPath
          """
          peerDir = self.basepath.child(str(peer))
          if not peerDir.isdir():
              peerDir.makedirs()
          return (peerDir.child(sharename+'.incomplete'),
                  peerDir.child(sharename))