feat(rt): change TokioExecutor to not trace, add rt-tracing-exec-force feature

.github/workflows/CI.yml

@@ -53,7 +53,11 @@ jobs:
         uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{ matrix.rust }}
+      # All features includes rt-tracing-exec-force, testing span propagation.
       - run: cargo test --all-features
+      # full excludes that opt-in, so also check that TokioExecutor does not
+      # propagate spans by default. Limit this extra run to runtime tests.
+      - run: "cargo test --features full --lib rt::"
 
   msrv:
     name: Check MSRV (${{ matrix.rust }}) on ${{ matrix.os }}

Cargo.toml

@@ -45,6 +45,7 @@ tokio = { version = "1", features = ["macros", "test-util", "signal", "net", "io
 tokio-test = "0.4"
 tower-test = "0.4"
 pretty_env_logger = "0.5"
+tracing-subscriber = { version = "0.3", default-features = false, features = ["registry", "std"] }
 
 [target.'cfg(any(target_os = "linux", target_os = "macos"))'.dev-dependencies]
 pnet_datalink = "0.35.0"
@@ -58,7 +59,7 @@ windows-registry = { version = ">=0.3, <0.7", optional = true }
 [features]
 default = []
 
-# Shorthand to enable everything
+# Shorthand to enable all standard features (excluding temporary opt-ins).
 full = [
     "client",
     "client-legacy",
@@ -94,6 +95,9 @@ tokio = ["dep:tokio", "tokio/rt", "tokio/time"]
 
 tracing = ["dep:tracing"]
 
+# Temporary compatibility opt-in, deliberately excluded from `full`.
+rt-tracing-exec-force = ["tokio", "tracing"]
+
 # internal features used in CI
 __internal_happy_eyeballs_tests = []
 

src/rt/tokio.rs

@@ -60,7 +60,7 @@ use std::{
 use hyper::rt::{Executor, Sleep, Timer};
 use pin_project_lite::pin_project;
 
-#[cfg(feature = "tracing")]
+#[cfg(feature = "rt-tracing-exec-force")]
 use tracing::instrument::Instrument;
 
 pub use self::{with_hyper_io::WithHyperIo, with_tokio_io::WithTokioIo};
@@ -69,6 +69,13 @@ mod with_hyper_io;
 mod with_tokio_io;
 
 /// Future executor that utilises `tokio` threads.
+///
+/// Spawned futures do not inherit the current tracing span, even when the
+/// `tracing` feature is enabled.
+///
+/// The temporary `rt-tracing-exec-force` feature restores propagation of the
+/// current span for libraries that do not allow customizing their executor.
+/// It is excluded from `full` and may be removed in a future breaking release.
 #[non_exhaustive]
 #[derive(Default, Debug, Clone)]
 pub struct TokioExecutor {}
@@ -107,10 +114,10 @@ where
     Fut::Output: Send + 'static,
 {
     fn execute(&self, fut: Fut) {
-        #[cfg(feature = "tracing")]
+        #[cfg(feature = "rt-tracing-exec-force")]
         tokio::spawn(fut.in_current_span());
 
-        #[cfg(not(feature = "tracing"))]
+        #[cfg(not(feature = "rt-tracing-exec-force"))]
         tokio::spawn(fut);
     }
 }
@@ -338,4 +345,29 @@ mod tests {
         });
         rx.await.map_err(Into::into)
     }
+
+    #[cfg(feature = "tracing")]
+    #[tokio::test]
+    async fn execute_tracing_span() {
+        // The current-thread runtime keeps the subscriber active while the
+        // spawned future is polled, after the caller has exited its span.
+        let _subscriber = tracing::subscriber::set_default(tracing_subscriber::registry());
+        let span = tracing::info_span!("caller");
+        assert!(span.id().is_some());
+        let (tx, rx) = oneshot::channel();
+
+        {
+            let _entered = span.enter();
+            TokioExecutor::new().execute(async move {
+                tx.send(tracing::Span::current().id()).unwrap();
+            });
+        }
+
+        let spawned_span = rx.await.unwrap();
+        if cfg!(feature = "rt-tracing-exec-force") {
+            assert_eq!(spawned_span, span.id());
+        } else {
+            assert_eq!(spawned_span, None);
+        }
+    }
 }