Span::record the parent's span field

[zohnannor]

This is the simplified version of the code I am facing the problem with. The scopes are correct, but the rest of the code is more complex (calling of #[instrument]ed trait method on a generic type).

use tracing::{metadata::LevelFilter, Span};
use tracing_subscriber::{fmt::format::FmtSpan, prelude::*};

fn main() {
    tracing_subscriber::registry()
        .with(
            tracing_subscriber::fmt::layer()
                .with_span_events(FmtSpan::FULL)
                .with_filter(LevelFilter::INFO),
        )
        .init();

    tracing::info_span!("parent", my_field = tracing::field::Empty).in_scope(|| {
        tracing::info_span!("child").in_scope(|| {
            Span::current().record("my_field", 42);
        });
    });
}

Logs

2023-08-14T10:16:36.284695Z  INFO parent: thing2: new
2023-08-14T10:16:36.284712Z  INFO parent: thing2: enter
2023-08-14T10:16:36.284719Z  INFO parent:child: thing2: new
2023-08-14T10:16:36.284725Z  INFO parent:child: thing2: enter
2023-08-14T10:16:36.284732Z  INFO parent:child: thing2: exit
2023-08-14T10:16:36.284738Z  INFO parent:child: thing2: close time.busy=7.31µs time.idle=11.6µs
2023-08-14T10:16:36.284756Z  INFO parent: thing2: exit
2023-08-14T10:16:36.284761Z  INFO parent: thing2: close time.busy=44.7µs time.idle=29.0µs

Is there any way I can somehow Span::record to the parent's field my_field? The solution without having to manually pass the parent's Span:current() span to the children would be great.

[zohnannor]

After some thinking I got to this structure:

tracing::info_span!("parent", my_field = tracing::field::Empty).in_scope(|| {
    Span::current().record("my_field", 42);
    tracing::info_span!("child").in_scope(|| {
        // rest of the code
    });
});

And in my code it looks like this:

// convert from this...
#[tracing::instrument(name = "child")]
async fn do_thing() {
    Span::current().record("my_field", 42);
	 // rest of the code
}

// ...to this
async fn do_thing() {
    Span::current().record("my_field", 42);
    async {
	     // rest of the code
	}
	.instrument(tracing::info_span!("child"))
	.await
}

Essentially, I'm just extracting the Span::record out of the scope.

Still, interesting if there's a better way of doing this. It's unfortunate I can't use #[instrument] here and have to introduce redundant code block.

[zohnannor]

Doesn't work when:

• Span::record is in the external dependency.
• recorded value is computed in the function.

[zohnannor]

For a solution, we implemented and released the crate just for that:

tracing-record-hierarchical

It allows you to record_hierarchically fields to the parent Span from anywhere in its children.