Multiple ownership of tracing_subscriber::Registry not possible

[jacobmarble]

Bug Report

Happy to learn that I'm doing it wrong. :)

Version

tracing 0.1.24
tracing-subscriber 0.2.17

Platform

Darwin Orient.local 20.3.0 Darwin Kernel Version 20.3.0: Thu Jan 21 00:07:06 PST 2021; root:xnu-7195.81.3~1/RELEASE_X86_64 x86_64

Crates

tracing-subscriber

Description

I need tracing to send some events to stdout logs, and some events to Jaeger. These two sets of events may intersect, or may be disjoint.

My solution is to write a ForkingLayer, which wraps (1) a stdout/logs layer and (2) an OpenTelemetry/Jaeger layer. These two layers need to share a Registry (or some other Subscriber + LookupSpan).

I tried multiple ownership like this:

    let subscriber = Rc::new(tracing_subscriber::Registry::default());
    let log_layer = log_layer.with_subscriber(Rc::clone(&subscriber));
    let traces_layer = traces_layer.with_subscriber(Rc::clone(&subscriber));

I expect the trait implementations of Registry to be exposed by Rc<Registry>.

However the traits of Registry are not available. Furthermore, the orphan rule prevents me from writing wrapper implementations for Subscriber and LookupSpan:

error[E0277]: the trait bound `Rc<observability_deps::tracing_subscriber::Registry>: observability_deps::tracing::Subscriber` is not satisfied
  --> src/commands/tracing.rs:63:47
   |
63 |     let log_layer = log_layer.with_subscriber(Rc::clone(&subscriber));
   |                                               ^^^^^^^^^^^^^^^^^^^^^^ the trait `observability_deps::tracing::Subscriber` is not implemented for `Rc<observability_deps::tracing_subscriber::Registry>`

error[E0277]: the trait bound `for<'a> Rc<observability_deps::tracing_subscriber::Registry>: LookupSpan<'a>` is not satisfied
  --> src/commands/tracing.rs:63:47
   |
63 |     let log_layer = log_layer.with_subscriber(Rc::clone(&subscriber));
   |                                               ^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'a> LookupSpan<'a>` is not implemented for `Rc<observability_deps::tracing_subscriber::Registry>`
   |
   = note: required because of the requirements on the impl of `__tracing_subscriber_Layer<Rc<observability_deps::tracing_subscriber::Registry>>` for `observability_deps::tracing_subscriber::fmt::Layer<Rc<observability_deps::tracing_subscriber::Registry>, JsonFields, observability_deps::tracing_subscriber::fmt::format::Format<Json>>`
   = note: required because of the requirements on the impl of `__tracing_subscriber_Layer<Rc<observability_deps::tracing_subscriber::Registry>>` for `std::option::Option<observability_deps::tracing_subscriber::fmt::Layer<Rc<observability_deps::tracing_subscriber::Registry>, JsonFields, observability_deps::tracing_subscriber::fmt::format::Format<Json>>>`
   = note: required because of the requirements on the impl of `__tracing_subscriber_Layer<Rc<observability_deps::tracing_subscriber::Registry>>` for `Layered<std::option::Option<observability_deps::tracing_subscriber::fmt::Layer<Rc<observability_deps::tracing_subscriber::Registry>, JsonFields, observability_deps::tracing_subscriber::fmt::format::Format<Json>>>, Layered<std::option::Option<observability_deps::tracing_subscriber::fmt::Layer<Rc<observability_deps::tracing_subscriber::Registry>, observability_deps::tracing_subscriber::fmt::format::Pretty, observability_deps::tracing_subscriber::fmt::format::Format<observability_deps::tracing_subscriber::fmt::format::Pretty>>>, Layered<std::option::Option<observability_deps::tracing_subscriber::fmt::Layer<Rc<observability_deps::tracing_subscriber::Registry>>>, observability_deps::tracing_subscriber::EnvFilter, Rc<observability_deps::tracing_subscriber::Registry>>, Rc<observability_deps::tracing_subscriber::Registry>>, Rc<observability_deps::tracing_subscriber::Registry>>`

[jacobmarble]

Closing as duplicate of #1348

[hawkw]

As a workaround, you could write a newtype around Arc<Registry>, and implement the appropriate traits for that. For example,

#[derive(Clone, Debug)]
pub struct SharedRegistry(Arc<Registry>);

impl<'a> LookupSpan<'a> for SharedRegistry {
    // ... forward implementation to the inner `Registry`'s methods...
}

impl Subscriber for SharedRegistry {
    // ... forward implementation to the inner `Registry`'s methods...
}

Note that you will need to use Arc rather than Rc, because Subscriber requires Send + Sync, as it may be shared across threads. Also, it's important to ensure your implementation forwards all of the Subscriber trait methods, even the ones with default implementations.

We could also add an implementation of Subscriber for Arc<T> where T: Subscriber in tracing-core, to make this easier in the future.