fix(subscriber): fix compilation on targets without 64-bit atomics (#282

)

## Motivation

Currently, the `console-subscriber` crate does not compile on targets
without 64-bit atomic operations, such as `mipsel-unknown-linux-musl`.
This is because a 64-bit atomic is used to store the last task ID that
polled a resource.

## Solution

This branch changes this code to use the `AtomicCell` type from
`crossbeam-utils`, instead. `AtomicCell` will use atomic operations on
targets that support 64-bit atomics, and fall back to using a mutex on
platforms that don't have 64-bit atomic operations. This should fix the
build on those platforms.

Fixes #279

console-subscriber/Cargo.toml

@@ -30,6 +30,7 @@ parking_lot = ["parking_lot_crate", "tracing-subscriber/parking_lot"]
 
 [dependencies]
 
+crossbeam-utils = "0.8.7"
 tokio = { version = "^1.15", features = ["sync", "time", "macros", "tracing"] }
 tokio-stream = "0.1"
 thread_local = "1.1.3"

console-subscriber/src/stats.rs

@@ -1,11 +1,12 @@
 use crate::{attribute, sync::Mutex, ToProto};
+use crossbeam_utils::atomic::AtomicCell;
 use hdrhistogram::{
     serialization::{Serializer, V2Serializer},
     Histogram,
 };
 use std::cmp;
 use std::sync::{
-    atomic::{AtomicBool, AtomicU64, AtomicUsize, Ordering::*},
+    atomic::{AtomicBool, AtomicUsize, Ordering::*},
     Arc,
 };
 use std::time::{Duration, SystemTime};
@@ -70,7 +71,7 @@ pub(crate) struct AsyncOpStats {
     ///
     /// This is set every time the async op is polled, in case a future is
     /// passed between tasks.
-    task_id: AtomicU64,
+    task_id: AtomicCell<u64>,
 
     /// Fields shared with `ResourceStats`.
     pub(crate) stats: ResourceStats,
@@ -265,14 +266,14 @@ impl AsyncOpStats {
         parent_id: Option<Id>,
     ) -> Self {
         Self {
-            task_id: AtomicU64::new(0),
+            task_id: AtomicCell::new(0),
             stats: ResourceStats::new(created_at, inherit_child_attributes, parent_id),
             poll_stats: PollStats::default(),
         }
     }
 
     pub(crate) fn task_id(&self) -> Option<u64> {
-        let id = self.task_id.load(Acquire);
+        let id = self.task_id.load();
         if id > 0 {
             Some(id as u64)
         } else {
@@ -281,7 +282,7 @@ impl AsyncOpStats {
     }
 
     pub(crate) fn set_task_id(&self, id: &tracing::span::Id) {
-        self.task_id.store(id.into_u64(), Release);
+        self.task_id.store(id.into_u64());
         self.make_dirty();
     }