runtime(unstable): fix task hook spawn locations for tokio::spawn (#7440)

hawkw · web-flow · commit a0d5b8ab308b · 2025-07-04T09:25:40.000-07:00
## Motivation

Unfortunately, due to an oversight on my part, the capturing of spawn
locations was only tested with the `Runtime::spawn` method, and *not*
with `tokio::spawn`/`tokio::task::spawn`, which is how most tasks are
spawned in Real Life. And, it turned out that because this was not
tested...well, it was broken. Agh. My bad.

## Solution

Although the whole call chain for spawning tasks using `tokio::spawn`
was correctly annotated with `#[track_caller]`, the location wasn't
propagated correctly because of the `context::with_current(|handle| {
... })` closure that accesses the current runtime. Because the call to
spawn the task occurs inside a closure, the *closure*'s location is
captured instead of the caller. This means any task spawned by
`tokio::spawn` records its location as being in
`tokio/src/task/spawn.rs`, which is not what we'd like. This commit
fixes that by capturing the spawn location outside the `with_current`
closure and passing it in explicitly.

I've updated the tests to also spawn a task with `tokio::spawn`, so that
we ensure this works correctly.
diff --git a/tokio/src/runtime/handle.rs b/tokio/src/runtime/handle.rs
@@ -329,7 +329,7 @@ impl Handle {
     }
 
     #[track_caller]
-    pub(crate) fn spawn_named<F>(&self, future: F, _meta: SpawnMeta<'_>) -> JoinHandle<F::Output>
+    pub(crate) fn spawn_named<F>(&self, future: F, meta: SpawnMeta<'_>) -> JoinHandle<F::Output>
     where
         F: Future + Send + 'static,
         F::Output: Send + 'static,
@@ -344,16 +344,16 @@ impl Handle {
         ))]
         let future = super::task::trace::Trace::root(future);
         #[cfg(all(tokio_unstable, feature = "tracing"))]
-        let future = crate::util::trace::task(future, "task", _meta, id.as_u64());
-        self.inner.spawn(future, id)
+        let future = crate::util::trace::task(future, "task", meta, id.as_u64());
+        self.inner.spawn(future, id, meta.spawned_at)
     }
 
     #[track_caller]
     #[allow(dead_code)]
     pub(crate) unsafe fn spawn_local_named<F>(
         &self,
         future: F,
-        _meta: SpawnMeta<'_>,
+        meta: SpawnMeta<'_>,
     ) -> JoinHandle<F::Output>
     where
         F: Future + 'static,
@@ -369,8 +369,8 @@ impl Handle {
         ))]
         let future = super::task::trace::Trace::root(future);
         #[cfg(all(tokio_unstable, feature = "tracing"))]
-        let future = crate::util::trace::task(future, "task", _meta, id.as_u64());
-        self.inner.spawn_local(future, id)
+        let future = crate::util::trace::task(future, "task", meta, id.as_u64());
+        self.inner.spawn_local(future, id, meta.spawned_at)
     }
 
     /// Returns the flavor of the current `Runtime`.
diff --git a/tokio/src/runtime/scheduler/current_thread/mod.rs b/tokio/src/runtime/scheduler/current_thread/mod.rs
@@ -3,7 +3,7 @@ use crate::loom::sync::Arc;
 use crate::runtime::driver::{self, Driver};
 use crate::runtime::scheduler::{self, Defer, Inject};
 use crate::runtime::task::{
-    self, JoinHandle, OwnedTasks, Schedule, Task, TaskHarnessScheduleHooks,
+    self, JoinHandle, OwnedTasks, Schedule, SpawnLocation, Task, TaskHarnessScheduleHooks,
 };
 use crate::runtime::{
     blocking, context, Config, MetricsBatch, SchedulerMetrics, TaskHooks, TaskMeta, WorkerMetrics,
@@ -15,7 +15,6 @@ use crate::util::{waker_ref, RngSeedGenerator, Wake, WakerRef};
 use std::cell::RefCell;
 use std::collections::VecDeque;
 use std::future::{poll_fn, Future};
-use std::panic::Location;
 use std::sync::atomic::Ordering::{AcqRel, Release};
 use std::task::Poll::{Pending, Ready};
 use std::task::Waker;
@@ -451,16 +450,13 @@ impl Handle {
         me: &Arc<Self>,
         future: F,
         id: crate::runtime::task::Id,
+        spawned_at: SpawnLocation,
     ) -> JoinHandle<F::Output>
     where
         F: crate::future::Future + Send + 'static,
         F::Output: Send + 'static,
     {
-        let spawned_at = Location::caller();
-        let (handle, notified) = me
-            .shared
-            .owned
-            .bind(future, me.clone(), id, spawned_at.into());
+        let (handle, notified) = me.shared.owned.bind(future, me.clone(), id, spawned_at);
 
         me.task_hooks.spawn(&TaskMeta {
             id,
@@ -486,16 +482,16 @@ impl Handle {
         me: &Arc<Self>,
         future: F,
         id: crate::runtime::task::Id,
+        spawned_at: SpawnLocation,
     ) -> JoinHandle<F::Output>
     where
         F: crate::future::Future + 'static,
         F::Output: 'static,
     {
-        let spawned_at = Location::caller();
-        let (handle, notified) =
-            me.shared
-                .owned
-                .bind_local(future, me.clone(), id, spawned_at.into());
+        let (handle, notified) = me
+            .shared
+            .owned
+            .bind_local(future, me.clone(), id, spawned_at);
 
         me.task_hooks.spawn(&TaskMeta {
             id,
diff --git a/tokio/src/runtime/scheduler/mod.rs b/tokio/src/runtime/scheduler/mod.rs
@@ -68,7 +68,7 @@ impl Handle {
 cfg_rt! {
     use crate::future::Future;
     use crate::loom::sync::Arc;
-    use crate::runtime::{blocking, task::Id};
+    use crate::runtime::{blocking, task::{Id, SpawnLocation}};
     use crate::runtime::context;
     use crate::task::JoinHandle;
     use crate::util::RngSeedGenerator;
@@ -117,17 +117,16 @@ cfg_rt! {
             }
         }
 
-        #[track_caller]
-        pub(crate) fn spawn<F>(&self, future: F, id: Id) -> JoinHandle<F::Output>
+        pub(crate) fn spawn<F>(&self, future: F, id: Id, spawned_at: SpawnLocation) -> JoinHandle<F::Output>
         where
             F: Future + Send + 'static,
             F::Output: Send + 'static,
         {
             match self {
-                Handle::CurrentThread(h) => current_thread::Handle::spawn(h, future, id),
+                Handle::CurrentThread(h) => current_thread::Handle::spawn(h, future, id, spawned_at),
 
                 #[cfg(feature = "rt-multi-thread")]
-                Handle::MultiThread(h) => multi_thread::Handle::spawn(h, future, id),
+                Handle::MultiThread(h) => multi_thread::Handle::spawn(h, future, id, spawned_at),
             }
         }
 
@@ -138,13 +137,13 @@ cfg_rt! {
         /// by the current thread.
         #[allow(irrefutable_let_patterns)]
         #[track_caller]
-        pub(crate) unsafe fn spawn_local<F>(&self, future: F, id: Id) -> JoinHandle<F::Output>
+        pub(crate) unsafe fn spawn_local<F>(&self, future: F, id: Id, spawned_at: SpawnLocation) -> JoinHandle<F::Output>
         where
             F: Future + 'static,
             F::Output: 'static,
         {
             if let Handle::CurrentThread(h) = self {
-                current_thread::Handle::spawn_local(h, future, id)
+                current_thread::Handle::spawn_local(h, future, id, spawned_at)
             } else {
                 panic!("Only current_thread and LocalSet have spawn_local internals implemented")
             }
diff --git a/tokio/src/runtime/scheduler/multi_thread/handle.rs b/tokio/src/runtime/scheduler/multi_thread/handle.rs
@@ -4,13 +4,12 @@ use crate::runtime::scheduler::multi_thread::worker;
 use crate::runtime::task::{Notified, Task, TaskHarnessScheduleHooks};
 use crate::runtime::{
     blocking, driver,
-    task::{self, JoinHandle},
+    task::{self, JoinHandle, SpawnLocation},
     TaskHooks, TaskMeta,
 };
 use crate::util::RngSeedGenerator;
 
 use std::fmt;
-use std::panic::Location;
 
 mod metrics;
 
@@ -38,30 +37,35 @@ pub(crate) struct Handle {
 
 impl Handle {
     /// Spawns a future onto the thread pool
-    #[track_caller]
-    pub(crate) fn spawn<F>(me: &Arc<Self>, future: F, id: task::Id) -> JoinHandle<F::Output>
+    pub(crate) fn spawn<F>(
+        me: &Arc<Self>,
+        future: F,
+        id: task::Id,
+        spawned_at: SpawnLocation,
+    ) -> JoinHandle<F::Output>
     where
         F: crate::future::Future + Send + 'static,
         F::Output: Send + 'static,
     {
-        Self::bind_new_task(me, future, id)
+        Self::bind_new_task(me, future, id, spawned_at)
     }
 
     pub(crate) fn shutdown(&self) {
         self.close();
     }
 
     #[track_caller]
-    pub(super) fn bind_new_task<T>(me: &Arc<Self>, future: T, id: task::Id) -> JoinHandle<T::Output>
+    pub(super) fn bind_new_task<T>(
+        me: &Arc<Self>,
+        future: T,
+        id: task::Id,
+        spawned_at: SpawnLocation,
+    ) -> JoinHandle<T::Output>
     where
         T: Future + Send + 'static,
         T::Output: Send + 'static,
     {
-        let spawned_at = Location::caller();
-        let (handle, notified) = me
-            .shared
-            .owned
-            .bind(future, me.clone(), id, spawned_at.into());
+        let (handle, notified) = me.shared.owned.bind(future, me.clone(), id, spawned_at);
 
         me.task_hooks.spawn(&TaskMeta {
             id,
diff --git a/tokio/src/runtime/task/harness.rs b/tokio/src/runtime/task/harness.rs
@@ -373,7 +373,7 @@ where
             let _ = panic::catch_unwind(panic::AssertUnwindSafe(|| {
                 f(&TaskMeta {
                     id: self.core().task_id,
-                    spawned_at: self.core().spawned_at,
+                    spawned_at: self.core().spawned_at.into(),
                     _phantom: Default::default(),
                 })
             }));
diff --git a/tokio/src/runtime/task/mod.rs b/tokio/src/runtime/task/mod.rs
@@ -444,7 +444,7 @@ impl<S: 'static> Task<S> {
     pub(crate) fn task_meta<'task, 'meta>(&'task self) -> crate::runtime::TaskMeta<'meta> {
         crate::runtime::TaskMeta {
             id: self.id(),
-            spawned_at: self.spawned_at(),
+            spawned_at: self.spawned_at().into(),
             _phantom: PhantomData,
         }
     }
diff --git a/tokio/src/runtime/task_hooks.rs b/tokio/src/runtime/task_hooks.rs
@@ -1,7 +1,5 @@
-use std::marker::PhantomData;
-use std::panic::Location;
-
 use super::Config;
+use std::marker::PhantomData;
 
 impl TaskHooks {
     pub(crate) fn spawn(&self, meta: &TaskMeta<'_>) {
@@ -62,7 +60,8 @@ pub struct TaskMeta<'a> {
     /// The opaque ID of the task.
     pub(crate) id: super::task::Id,
     /// The location where the task was spawned.
-    pub(crate) spawned_at: &'static Location<'static>,
+    #[cfg_attr(not(tokio_unstable), allow(unreachable_pub, dead_code))]
+    pub(crate) spawned_at: crate::runtime::task::SpawnLocation,
     pub(crate) _phantom: PhantomData<&'a ()>,
 }
 
@@ -74,9 +73,9 @@ impl<'a> TaskMeta<'a> {
     }
 
     /// Return the source code location where the task was spawned.
-    #[cfg_attr(not(tokio_unstable), allow(unreachable_pub, dead_code))]
-    pub fn spawned_at(&self) -> &'static Location<'static> {
-        self.spawned_at
+    #[cfg(tokio_unstable)]
+    pub fn spawned_at(&self) -> &'static std::panic::Location<'static> {
+        self.spawned_at.0
     }
 }
 
diff --git a/tokio/src/task/local.rs b/tokio/src/task/local.rs
@@ -414,7 +414,7 @@ cfg_rt! {
                 let task = crate::util::trace::task(future, "task", meta, id.as_u64());
 
                 // safety: we have verified that this is a `LocalRuntime` owned by the current thread
-                unsafe { handle.spawn_local(task, id) }
+                unsafe { handle.spawn_local(task, id, meta.spawned_at) }
             } else {
                 match CURRENT.with(|LocalData { ctx, .. }| ctx.get()) {
                     None => panic!("`spawn_local` called from outside of a `task::LocalSet` or LocalRuntime"),
diff --git a/tokio/src/task/spawn.rs b/tokio/src/task/spawn.rs
@@ -199,7 +199,7 @@ cfg_rt! {
         let id = task::Id::next();
         let task = crate::util::trace::task(future, "task", meta, id.as_u64());
 
-        match context::with_current(|handle| handle.spawn(task, id)) {
+        match context::with_current(|handle| handle.spawn(task, id, meta.spawned_at)) {
             Ok(join_handle) => join_handle,
             Err(e) => panic!("{}", e),
         }
diff --git a/tokio/src/util/trace.rs b/tokio/src/util/trace.rs
@@ -9,21 +9,29 @@ cfg_rt! {
         /// The original size of the future or function being spawned
         #[cfg(all(tokio_unstable, feature = "tracing"))]
         pub(crate) original_size: usize,
+        /// The source code location where the task was spawned.
+        ///
+        /// This is wrapped in a type that may be empty when `tokio_unstable` is
+        /// not enabled.
+        pub(crate) spawned_at: crate::runtime::task::SpawnLocation,
         _pd: PhantomData<&'a ()>,
     }
 
     impl<'a> SpawnMeta<'a> {
         /// Create new spawn meta with a name and original size (before possible auto-boxing)
         #[cfg(all(tokio_unstable, feature = "tracing"))]
+        #[track_caller]
         pub(crate) fn new(name: Option<&'a str>, original_size: usize) -> Self {
             Self {
                 name,
                 original_size,
+                spawned_at: crate::runtime::task::SpawnLocation::capture(),
                 _pd: PhantomData,
             }
         }
 
         /// Create a new unnamed spawn meta with the original size (before possible auto-boxing)
+        #[track_caller]
         pub(crate) fn new_unnamed(original_size: usize) -> Self {
             #[cfg(not(all(tokio_unstable, feature = "tracing")))]
             let _original_size = original_size;
@@ -33,6 +41,7 @@ cfg_rt! {
                 name: None,
                 #[cfg(all(tokio_unstable, feature = "tracing"))]
                 original_size,
+                spawned_at: crate::runtime::task::SpawnLocation::capture(),
                 _pd: PhantomData,
             }
         }
@@ -50,11 +59,8 @@ cfg_rt! {
         pub(crate) use tracing::instrument::Instrumented;
 
         #[inline]
-        #[track_caller]
         pub(crate) fn task<F>(task: F, kind: &'static str, meta: SpawnMeta<'_>, id: u64) -> Instrumented<F> {
-            #[track_caller]
             fn get_span(kind: &'static str, spawn_meta: SpawnMeta<'_>, id: u64, task_size: usize) -> tracing::Span {
-                let location = std::panic::Location::caller();
                 let original_size = if spawn_meta.original_size != task_size {
                     Some(spawn_meta.original_size)
                 } else {
@@ -69,9 +75,9 @@ cfg_rt! {
                     task.id = id,
                     original_size.bytes = original_size,
                     size.bytes = task_size,
-                    loc.file = location.file(),
-                    loc.line = location.line(),
-                    loc.col = location.column(),
+                    loc.file = spawn_meta.spawned_at.0.file(),
+                    loc.line = spawn_meta.spawned_at.0.line(),
+                    loc.col = spawn_meta.spawned_at.0.column(),
                 )
             }
             use tracing::instrument::Instrument;
@@ -80,10 +86,7 @@ cfg_rt! {
         }
 
         #[inline]
-        #[track_caller]
         pub(crate) fn blocking_task<Fn, Fut>(task: Fut, spawn_meta: SpawnMeta<'_>, id: u64) -> Instrumented<Fut> {
-            let location = std::panic::Location::caller();
-
             let fn_size = mem::size_of::<Fn>();
             let original_size = if spawn_meta.original_size != fn_size {
                 Some(spawn_meta.original_size)
@@ -100,9 +103,9 @@ cfg_rt! {
                 "fn" = %std::any::type_name::<Fn>(),
                 original_size.bytes = original_size,
                 size.bytes = fn_size,
-                loc.file = location.file(),
-                loc.line = location.line(),
-                loc.col = location.column(),
+                loc.file = spawn_meta.spawned_at.0.file(),
+                loc.line = spawn_meta.spawned_at.0.line(),
+                loc.col = spawn_meta.spawned_at.0.column(),
             );
             task.instrument(span)
 
diff --git a/tokio/tests/task_hooks.rs b/tokio/tests/task_hooks.rs

Original file line number	Diff line number	Diff line change
`@@ -444,7 +444,7 @@ impl<S: 'static> Task<S> {`
`444`	`444`	`pub(crate) fn task_meta<'task, 'meta>(&'task self) -> crate::runtime::TaskMeta<'meta> {`
`445`	`445`	`crate::runtime::TaskMeta {`
`446`	`446`	`id: self.id(),`
`447`		`- spawned_at: self.spawned_at(),`
	`447`	`+ spawned_at: self.spawned_at().into(),`
`448`	`448`	`_phantom: PhantomData,`
`449`	`449`	`}`
`450`	`450`	`}`
Original file line number	Diff line number	Diff line change
`@@ -199,7 +199,7 @@ cfg_rt! {`
`199`	`199`	`let id = task::Id::next();`
`200`	`200`	`let task = crate::util::trace::task(future, "task", meta, id.as_u64());`
`201`	`201`
`202`		`- match context::with_current(\|handle\| handle.spawn(task, id)) {`
	`202`	`+ match context::with_current(\|handle\| handle.spawn(task, id, meta.spawned_at)) {`
`203`	`203`	`Ok(join_handle) => join_handle,`
`204`	`204`	`Err(e) => panic!("{}", e),`
`205`	`205`	`}`