Elegant conditional appender?

[tumbl3w33d]

I want my app to always log to stdout and if the user sets an environment variable, I want to log to a file in addition. I got the desired behavior with the following code and was wondering, if there's a prettier way because I need to duplicate the subscriber creation code due to layer typing issues when trying it differently. Here's what I do:

macro_rules! json_layer {
    ($writer:expr) => {
        fmt::layer()
            .json()
            .with_writer($writer)
            .with_timer(time::ChronoUtc::default())
            .flatten_event(true)
    };
}

pub(crate) fn init() -> io::Result<Vec<WorkerGuard>> {
    LogTracer::init().expect("Unable to setup log tracer!");

    // collect and hold on to the guards to keep loggers alive
    let mut guards: Vec<WorkerGuard> = Vec::new();

    // always log to stdout
    let (stdout_writer, stdout_writer_guard) = tracing_appender::non_blocking(std::io::stdout());
    guards.push(stdout_writer_guard);
    let stdout_layer = json_layer!(stdout_writer);

    // when LOG_PATH is specified and valid, write logs to a file as well
    let log_path = match std::env::var("LOG_PATH") {
        Ok(env) => {
            let path = Path::new(&env);
            if path.is_dir() && path.writable() {
                let file_appender = tracing_appender::rolling::daily(path, DEFAULT_LOG_FILE_PREFIX);
                let (file_writer, file_writer_guard) =
                    tracing_appender::non_blocking(file_appender);
                let file_writer_layer = json_layer!(file_writer);
                guards.push(file_writer_guard);
                Some(file_writer_layer)
            } else {
                None
            }
        }
        Err(e) => {
            println!("Apparently you tried to specify a LOG_PATH, but it could not be validated as path to a writable directory - {:?}", e);
            None
        }
    };

    match log_path {
        Some(file_appender_layer) => {
            // log to stdout and file
            let subscriber = Registry::default()
                .with(EnvFilter::from_default_env())
                .with(stdout_layer)
                .with(file_appender_layer);

            tracing::subscriber::set_global_default(subscriber)
                .map_err(|e| io::Error::new(ErrorKind::Other, e.to_string()))?;
        }
        None => {
            // only log to stdout
            let subscriber = Registry::default()
                .with(EnvFilter::from_default_env())
                .with(stdout_layer);

            tracing::subscriber::set_global_default(subscriber)
                .map_err(|e| io::Error::new(ErrorKind::Other, e.to_string()))?;
        }
    }

    Ok(guards)
}

You would think one could simply put let subscriber = before the match log_path but this results in a horrible type error message along these lines:

`match` arms have incompatible types
expected type `Layered<tracing_subscriber::fmt::Layer<Layered<tracing_subscriber::fmt::Layer<Layered<EnvFilter, Registry>, JsonFields, Format<tracing_subscriber::fmt::format::Json, ChronoUtc>, NonBlocking>, Layered<EnvFilter, Registry>, Layered<EnvFilter, Registry>>, _, _, _>, Layered<tracing_subscriber::fmt::Layer<Layered<EnvFilter, Registry>, JsonFields, Format<tracing_subscriber::fmt::format::Json, ChronoUtc>, NonBlocking>, Layered<EnvFilter, Registry>, Layered<EnvFilter, Registry>>, Layered<tracing_subscriber::fmt::Layer<Layered<EnvFilter, Registry>, JsonFields, Format<tracing_subscriber::fmt::format::Json, ChronoUtc>, NonBlocking>, Layered<EnvFilter, Registry>, Layered<EnvFilter, Registry>>>`
 found struct `Layered<tracing_subscriber::fmt::Layer<Layered<EnvFilter, Registry, Registry>, _, _, _>, Layered<EnvFilter, Registry, Registry>, Layered<EnvFilter, Registry, Registry>>`

[hawkw]

Try using the implementation of Layer for Option<L: Layer<S>>, like this:

pub(crate) fn init() -> io::Result<Vec<WorkerGuard>> {
    LogTracer::init().expect("Unable to setup log tracer!");

    // collect and hold on to the guards to keep loggers alive
    let mut guards: Vec<WorkerGuard> = Vec::new();

    // always log to stdout
    let (stdout_writer, stdout_writer_guard) = tracing_appender::non_blocking(std::io::stdout());
    guards.push(stdout_writer_guard);
    let stdout_layer = json_layer!(stdout_writer);

    // when LOG_PATH is specified and valid, write logs to a file as well
    let log_path = match std::env::var("LOG_PATH") {
         // ...
    };
 
    let subscriber = Registry::default()
        .with(EnvFilter::from_default_env())
        .with(stdout_layer)
        // `file_appender_layer` is an `Option<fmt::Layer<...>>`. if the file is enabled, it will
        // be `Some`. otherwise, it will be `None`, and adding it to the subscriber will do 
        // nothing.
        .with(file_appender_layer);

    tracing::subscriber::set_global_default(subscriber)
        .map_err(|e| io::Error::new(ErrorKind::Other, e.to_string()))?;

    Ok(guards)
}

[tumbl3w33d]

That's exactly what I was looking for. Thank you so much! :)

[hawkw]

Glad I could help! :)