tracing: replace future names with spawn locations in task spans (#3074)

## Motivation

Currently, the per-task `tracing` spans generated by tokio's `tracing`
feature flag include the `std::any::type_name` of the future that was
spawned. When future combinators and/or libraries like Tower are in use,
these future names can get _quite_ long. Furthermore, when formatting
the `tracing` spans with their parent spans as context, any other task
spans in the span context where the future was spawned from can _also_
include extremely long future names.

In some cases, this can result in extremely high memory use just to
store the future names. For example, in Linkerd, when we enable
`tokio=trace` to enable the task spans, there's a spawned task whose
future name is _232990 characters long_. A proxy with only 14 spawned
tasks generates a task list that's over 690 KB. Enabling task spans
under load results in the process getting OOM killed very quickly.

## Solution

This branch removes future type names from the spans generated by
`spawn`. As a replacement, to allow identifying which `spawn` call a
span corresponds to, the task span now contains the source code location
where `spawn` was called, when the compiler supports the
`#[track_caller]` attribute. Since `track_caller` was stabilized in Rust
1.46.0, and our minimum supported Rust version is 1.45.0, we can't
assume that `#[track_caller]` is always available. Instead, we have a
RUSTFLAGS cfg, `tokio_track_caller`, that guards whether or not we use
it. I've also added a `build.rs` that detects the compiler minor
version, and sets the cfg flag automatically if the current compiler
version is >= 1.46. This means users shouldn't have to enable
`tokio_track_caller` manually.

Here's the trace output from the `chat` example, before this change:
![Screenshot_20201030_110157](https://user-images.githubusercontent.com/2796466/97741071-6d408800-1a9f-11eb-9ed6-b25e72f58c7b.png)
...and after:
![Screenshot_20201030_110303](https://user-images.githubusercontent.com/2796466/97741112-7e899480-1a9f-11eb-9197-c5a3f9ea1c05.png)

Closes #3073

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tokio/Cargo.toml

@@ -102,7 +102,7 @@ mio = { version = "0.7.3", optional = true }
 num_cpus = { version = "1.8.0", optional = true }
 parking_lot = { version = "0.11.0", optional = true } # Not in full
 slab = { version = "0.4.1", optional = true }
-tracing = { version = "0.1.16", default-features = false, features = ["std"], optional = true } # Not in full
+tracing = { version = "0.1.21", default-features = false, features = ["std"], optional = true } # Not in full
 
 [target.'cfg(unix)'.dependencies]
 libc = { version = "0.2.42", optional = true }
@@ -126,9 +126,12 @@ tempfile = "3.1.0"
 [target.'cfg(loom)'.dev-dependencies]
 loom = { version = "0.3.5", features = ["futures", "checkpoint"] }
 
+[build-dependencies]
+autocfg = "1" # Needed for conditionally enabling `track-caller`
+
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [package.metadata.playground]
-features = ["full"]
+features = ["full"]

tokio/build.rs

@@ -0,0 +1,22 @@
+use autocfg::AutoCfg;
+
+fn main() {
+    match AutoCfg::new() {
+        Ok(ac) => {
+            // The #[track_caller] attribute was stabilized in rustc 1.46.0.
+            if ac.probe_rustc_version(1, 46) {
+                autocfg::emit("tokio_track_caller")
+            }
+        }
+
+        Err(e) => {
+            // If we couldn't detect the compiler version and features, just
+            // print a warning. This isn't a fatal error: we can still build
+            // Tokio, we just can't enable cfgs automatically.
+            println!(
+                "cargo:warning=tokio: failed to detect compiler features: {}",
+                e
+            );
+        }
+    }
+}

tokio/src/runtime/handle.rs

@@ -39,13 +39,26 @@ impl Handle {
     //     context::enter(self.clone(), f)
     // }
 
-    /// Run the provided function on an executor dedicated to blocking operations.
+    /// Run the provided function on an executor dedicated to blocking
+    /// operations.
+    #[cfg_attr(tokio_track_caller, track_caller)]
     pub(crate) fn spawn_blocking<F, R>(&self, func: F) -> JoinHandle<R>
     where
         F: FnOnce() -> R + Send + 'static,
     {
         #[cfg(feature = "tracing")]
         let func = {
+            #[cfg(tokio_track_caller)]
+            let location = std::panic::Location::caller();
+            #[cfg(tokio_track_caller)]
+            let span = tracing::trace_span!(
+                target: "tokio::task",
+                "task",
+                kind = %"blocking",
+                function = %std::any::type_name::<F>(),
+                spawn.location = %format_args!("{}:{}:{}", location.file(), location.line(), location.column()),
+            );
+            #[cfg(not(tokio_track_caller))]
             let span = tracing::trace_span!(
                 target: "tokio::task",
                 "task",

tokio/src/runtime/mod.rs

@@ -357,11 +357,14 @@ cfg_rt! {
         /// });
         /// # }
         /// ```
+        #[cfg_attr(tokio_track_caller, track_caller)]
         pub fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
         where
             F: Future + Send + 'static,
             F::Output: Send + 'static,
         {
+            #[cfg(feature = "tracing")]
+            let future = crate::util::trace::task(future, "task");
             match &self.kind {
                 #[cfg(feature = "rt-multi-thread")]
                 Kind::ThreadPool(exec) => exec.spawn(future),
@@ -385,6 +388,7 @@ cfg_rt! {
         ///     println!("now running on a worker thread");
         /// });
         /// # }
+        #[cfg_attr(tokio_track_caller, track_caller)]
         pub fn spawn_blocking<F, R>(&self, func: F) -> JoinHandle<R>
         where
             F: FnOnce() -> R + Send + 'static,

tokio/src/task/blocking.rs

@@ -104,6 +104,7 @@ cfg_rt_multi_thread! {
 /// # Ok(())
 /// # }
 /// ```
+#[cfg_attr(tokio_track_caller, track_caller)]
 pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R>
 where
     F: FnOnce() -> R + Send + 'static,

tokio/src/task/local.rs

@@ -190,6 +190,7 @@ cfg_rt! {
     ///     }).await;
     /// }
     /// ```
+    #[cfg_attr(tokio_track_caller, track_caller)]
     pub fn spawn_local<F>(future: F) -> JoinHandle<F::Output>
     where
         F: Future + 'static,
@@ -273,6 +274,7 @@ impl LocalSet {
     /// }
     /// ```
     /// [`spawn_local`]: fn@spawn_local
+    #[cfg_attr(tokio_track_caller, track_caller)]
     pub fn spawn_local<F>(&self, future: F) -> JoinHandle<F::Output>
     where
         F: Future + 'static,

tokio/src/task/spawn.rs

@@ -122,6 +122,7 @@ cfg_rt! {
     /// ```text
     /// error[E0391]: cycle detected when processing `main`
     /// ```
+    #[cfg_attr(tokio_track_caller, track_caller)]
     pub fn spawn<T>(task: T) -> JoinHandle<T::Output>
     where
         T: Future + Send + 'static,

tokio/src/util/trace.rs

@@ -1,47 +1,27 @@
 cfg_trace! {
     cfg_rt! {
-        use std::future::Future;
-        use std::pin::Pin;
-        use std::task::{Context, Poll};
-        use pin_project_lite::pin_project;
-
-        use tracing::Span;
-
-        pin_project! {
-            /// A future that has been instrumented with a `tracing` span.
-            #[derive(Debug, Clone)]
-            pub(crate) struct Instrumented<T> {
-                #[pin]
-                inner: T,
-                span: Span,
-            }
-        }
-
-        impl<T: Future> Future for Instrumented<T> {
-            type Output = T::Output;
-
-            fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-                let this = self.project();
-                let _enter = this.span.enter();
-                this.inner.poll(cx)
-            }
-        }
-
-        impl<T> Instrumented<T> {
-            pub(crate) fn new(inner: T, span: Span) -> Self {
-                Self { inner, span }
-            }
-        }
+        pub(crate) use tracing::instrument::Instrumented;
 
         #[inline]
+        #[cfg_attr(tokio_track_caller, track_caller)]
         pub(crate) fn task<F>(task: F, kind: &'static str) -> Instrumented<F> {
+            use tracing::instrument::Instrument;
+            #[cfg(tokio_track_caller)]
+            let location = std::panic::Location::caller();
+            #[cfg(tokio_track_caller)]
+            let span = tracing::trace_span!(
+                target: "tokio::task",
+                "task",
+                %kind,
+                spawn.location = %format_args!("{}:{}:{}", location.file(), location.line(), location.column()),
+            );
+            #[cfg(not(tokio_track_caller))]
             let span = tracing::trace_span!(
                 target: "tokio::task",
                 "task",
                 %kind,
-                future = %std::any::type_name::<F>(),
             );
-            Instrumented::new(task, span)
+            task.instrument(span)
         }
     }
 }