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>

1 files changed, 14 insertions, 1 deletions

diff --git a/tokio/src/runtime/handle.rs b/tokio/src/runtime/handle.rs
index b1e8d8f1..c9ffd5cd 100644
--- a/tokio/src/runtime/handle.rs
+++ b/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",