How to setup simultaneous with features?

[TornaxO7]

Hello! I'm trying setup a simultaneous logging which can be adjusted with the feature-flags:

mod journald;

pub fn setup_logging() {
    let subscriber: Box<dyn Subscriber> = get_subscriber();
    tracing::subscriber::set_global_default(subscriber)
        .expect("Couldn't setup global subscriber (logger)");
}

fn get_subscriber() -> Box<dyn Subscriber> {
    let subscriber: Box<dyn Subscriber> = Box::new(tracing_subscriber::registry());
    let subscriber = subscriber.with(EnvFilter::from_default_env());

    #[cfg(feature = "journald")]
    let subscriber = journald::add_layer(subscriber);

    #[cfg(feature = "file")]
    // add file logging

    subscriber
}

journald.rs (from mod journald):

use tracing::Subscriber;

pub fn add_layer(subscriber: Box<dyn Subscriber>) -> Box<dyn Subscriber> {
    (*subscriber).with(tracing_journald::layer().unwrap())
}

I'm a little bit stuck at the moment because I'm getting a bunch of errors like:

error[E0277]: `dyn tracing::Subscriber` cannot be sent between threads safely
  --> leftwm/src/utils/log/mod.rs:8:45
   |
8  |     tracing::subscriber::set_global_default(subscriber)
   |     --------------------------------------- ^^^^^^^^^^ `dyn tracing::Subscriber` cannot be sent between threads safely
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Send` is not implemented for `dyn tracing::Subscriber`
   = note: required because of the requirements on the impl of `Send` for `Unique<dyn tracing::Subscriber>`
   = note: required because it appears within the type `Box<dyn tracing::Subscriber>`
note: required by a bound in `tracing::subscriber::set_global_default`
  --> /home/tornax/.cargo/registry/src/github.com-1ecc6299db9ec823/tracing-0.1.36/src/subscriber.rs:41:21
   |
41 |     S: Subscriber + Send + Sync + 'static,
   |                     ^^^^ required by this bound in `tracing::subscriber::set_global_default`

[davidbarsky]

Have you read https://docs.rs/tracing-subscriber/latest/tracing_subscriber/layer/index.html#runtime-configuration-with-layers, by any chance? I'd suggest dynamic configuration through that approach instead of using feature flags.

That being said, something like this should work:

use tracing::Subscriber;
use tracing_subscriber::{filter::EnvFilter, fmt, layer::SubscriberExt};

fn main() {
    let subscriber: Box<dyn Subscriber + Send + Sync + 'static> = get_subscriber();
    tracing::subscriber::set_global_default(subscriber)
        .expect("Couldn't setup global subscriber (logger)");
}

fn get_subscriber() -> Box<dyn Subscriber + Send + Sync + 'static> {
    let subscriber = tracing_subscriber::registry();
    let subscriber = subscriber.with(EnvFilter::from_default_env());
    Box::new(subscriber.with(fmt::Layer::default()))
}

Notably, you can't declare the registry by writing let subscriber: Box<dyn Subscriber> = Box::new(tracing_subscriber::registry()); as it skips the LookupSpan bound needed for .with() combinators to work.

[TornaxO7]

Have you read https://docs.rs/tracing-subscriber/latest/tracing_subscriber/layer/index.html#runtime-configuration-with-layers, by any chance?

Oh, no I didn't.... sorry about that... I immediately went to the structs of tracing_subscribers. Thank you for your help!

[davidbarsky]

Happy to help! Glad you got it working :)