Apparently java.io.File.mkdirs has a race condition that causes it to fail
if a parent directory is created by a different thread.

        if (mkdir()) {
            return true;
        }
// step one
        File canonFile = null;
        try {
            canonFile = getCanonicalFile();
        } catch (IOException e) {
            return false;
        }
        String parent = canonFile.getParent();
        return (parent != null) &&
               (new File(parent, fs.prefixLength(parent)).mkdirs() &&
// step two
                                    canonFile.mkdir());

If the parent file is created between step one and step two, the method
will fail to create the child.