let file_path = folder_path.join(hostname).with_extension("csv");

        // Can't use .with_extension here as it will not work properly with hostnames
        // that contain dots. See test::dot_filename_with_extension for an
        // example.
        let file_path = folder_path.join(format!("{}.csv", hostname));

#[cfg(test)]
mod test {
    #[test]
    fn dot_filename_with_extension() {
        let folder_path = std::path::PathBuf::from("/tmp");
        let hostname = "test.test.test";
        let expected = std::path::PathBuf::from(format!("/tmp/{}.csv", hostname));
        let bad = folder_path.join(hostname).with_extension("csv");
        let good = folder_path.join(format!("{}.csv", hostname));
        assert_ne!(bad, expected);
        assert_eq!(good, expected);
    }
}