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Fix io_loop span #445

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Merged
merged 1 commit into from
Oct 12, 2025
Merged

Fix io_loop span #445

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30 changes: 27 additions & 3 deletions src/io_loop.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ use std::{
thread::Builder as ThreadBuilder,
time::Duration,
};
use tracing::{error, trace};
use tracing::{error, trace, Level};

const FRAMES_STORAGE: usize = 32;

Expand Down Expand Up @@ -159,12 +159,13 @@ impl IoLoop {
pub fn start(mut self) -> Result<()> {
let waker = self.socket_state.handle();
let handle = self.connection_io_loop_handle.clone();
let current_span = tracing::Span::current();
let connect_span = tracing::Span::current();
handle.register(
ThreadBuilder::new()
.name("lapin-io-loop".to_owned())
.spawn(move || {
let _enter = current_span.enter();
let loop_span = io_loop_span(connect_span);
let _enter = loop_span.enter();
let readable_waker = self.readable_waker();
let mut readable_context = Context::from_waker(&readable_waker);
let writable_waker = self.writable_waker();
Expand Down Expand Up @@ -457,3 +458,26 @@ impl IoLoop {
}
}
}

/// Create a new span for the io_loop thread that follows from the connect span, and has the same level.
///
/// Importantly, we drop the `connect_span` so it closes properly.
fn io_loop_span(connect_span: tracing::Span) -> tracing::Span {
let span_level = connect_span.metadata().unwrap().level();
let span = if *span_level == Level::ERROR {
tracing::span!(Level::ERROR, "io_loop")
} else if *span_level == Level::WARN {
tracing::span!(Level::WARN, "io_loop")
} else if *span_level == Level::INFO {
tracing::span!(Level::INFO, "io_loop")
} else if *span_level == Level::DEBUG {
tracing::span!(Level::DEBUG, "io_loop")
} else {
tracing::span!(Level::TRACE, "io_loop")
};

// This span doesn't contribute to the duration of the connect span, but it is caused by the
// connect operation, so we set it as a follows_from relationship.
span.follows_from(&connect_span);
span
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@Keruspe Keruspe Oct 7, 2025

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Maybe all of this could be replaced with something like

let span_level = connect_span.metadata().map_or(Level::TRACE, |metadata| metadata.level());
tracing::span!(parent: connect_span, span_level, "io_loop")

Also, if you target the lapin-2.x branch instead, I'll cut a release for it, then merge in newer branch and cut 3.x and 4.x releases

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Contributor Author

@ThomWright ThomWright Oct 8, 2025 •
edited
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A few things:

  1. Span level – This can't be dynamic, otherwise yeah that approach would be better! You get this error:

    attempt to use a non-constant value in a constant non-constant value

    Which has serious "how much wood would a woodchuck chuck..." vibes.

    Yeah, this is a kind of annoying limitation of tracing.

  2. Relationship with connect span – I decided not to set the connect span as the parent. See the docs from tracing:

    In addition to having zero or one parent, a span may also follow from any number of other spans. This indicates a causal relationship between the span and the spans that it follows from, but a follower is not typically considered part of the duration of the span it follows.

    Which seems to match what we're trying to represent here.

  3. Branch target – Grand, I can change that 👍

}
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