task: remove mutex in JoinError (#3959)

tokio/src/runtime/task/error.rs

@@ -1,7 +1,8 @@
 use std::any::Any;
 use std::fmt;
 use std::io;
-use std::sync::Mutex;
+
+use crate::util::SyncWrapper;
 
 cfg_rt! {
     /// Task failed to execute to completion.
@@ -12,7 +13,7 @@ cfg_rt! {
 
 enum Repr {
     Cancelled,
-    Panic(Mutex<Box<dyn Any + Send + 'static>>),
+    Panic(SyncWrapper<Box<dyn Any + Send + 'static>>),
 }
 
 impl JoinError {
@@ -24,7 +25,7 @@ impl JoinError {
 
     pub(crate) fn panic(err: Box<dyn Any + Send + 'static>) -> JoinError {
         JoinError {
-            repr: Repr::Panic(Mutex::new(err)),
+            repr: Repr::Panic(SyncWrapper::new(err)),
         }
     }
 
@@ -106,7 +107,7 @@ impl JoinError {
     /// ```
     pub fn try_into_panic(self) -> Result<Box<dyn Any + Send + 'static>, JoinError> {
         match self.repr {
-            Repr::Panic(p) => Ok(p.into_inner().expect("Extracting panic from mutex")),
+            Repr::Panic(p) => Ok(p.into_inner()),
             _ => Err(self),
         }
     }

tokio/src/util/mod.rs

@@ -21,6 +21,9 @@ cfg_rt! {
     mod wake;
     pub(crate) use wake::WakerRef;
     pub(crate) use wake::{waker_ref, Wake};
+
+    mod sync_wrapper;
+    pub(crate) use sync_wrapper::SyncWrapper;
 }
 
 cfg_rt_multi_thread! {

tokio/src/util/sync_wrapper.rs

@@ -0,0 +1,26 @@
+//! This module contains a type that can make `Send + !Sync` types `Sync` by
+//! disallowing all immutable access to the value.
+//!
+//! A similar primitive is provided in the `sync_wrapper` crate.
+
+pub(crate) struct SyncWrapper<T> {
+    value: T,
+}
+
+// safety: The SyncWrapper being send allows you to send the inner value across
+// thread boundaries.
+unsafe impl<T: Send> Send for SyncWrapper<T> {}
+
+// safety: An immutable reference to a SyncWrapper is useless, so moving such an
+// immutable reference across threads is safe.
+unsafe impl<T> Sync for SyncWrapper<T> {}
+
+impl<T> SyncWrapper<T> {
+    pub(crate) fn new(value: T) -> Self {
+        Self { value }
+    }
+
+    pub(crate) fn into_inner(self) -> T {
+        self.value
+    }
+}

tokio/tests/async_send_sync.rs

@@ -452,6 +452,7 @@ assert_value!(tokio::task::LocalSet: !Send & !Sync & Unpin);
 assert_value!(tokio::task::JoinHandle<YY>: Send & Sync & Unpin);
 assert_value!(tokio::task::JoinHandle<YN>: Send & Sync & Unpin);
 assert_value!(tokio::task::JoinHandle<NN>: !Send & !Sync & Unpin);
+assert_value!(tokio::task::JoinError: Send & Sync & Unpin);
 
 assert_value!(tokio::runtime::Builder: Send & Sync & Unpin);
 assert_value!(tokio::runtime::EnterGuard<'_>: Send & Sync & Unpin);