From aef2d64b0a519ff6726f8c139ee1d3e6b1959b0b Mon Sep 17 00:00:00 2001
From: Alice Ryhl <alice@ryhl.io>
Date: Fri, 16 Jul 2021 09:50:03 +0200
Subject: [PATCH] task: remove mutex in JoinError (#3959)

---
 tokio/src/runtime/task/error.rs |  9 +++++----
 tokio/src/util/mod.rs           |  3 +++
 tokio/src/util/sync_wrapper.rs  | 26 ++++++++++++++++++++++++++
 tokio/tests/async_send_sync.rs  |  1 +
 4 files changed, 35 insertions(+), 4 deletions(-)
 create mode 100644 tokio/src/util/sync_wrapper.rs

diff --git a/tokio/src/runtime/task/error.rs b/tokio/src/runtime/task/error.rs
index 177fe65e9b9..17fb09390ae 100644
--- a/tokio/src/runtime/task/error.rs
+++ b/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),
         }
     }
diff --git a/tokio/src/util/mod.rs b/tokio/src/util/mod.rs
index b267125b15b..bcc73ed68de 100644
--- a/tokio/src/util/mod.rs
+++ b/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! {
diff --git a/tokio/src/util/sync_wrapper.rs b/tokio/src/util/sync_wrapper.rs
new file mode 100644
index 00000000000..5ffc8f96b16
--- /dev/null
+++ b/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
+    }
+}
diff --git a/tokio/tests/async_send_sync.rs b/tokio/tests/async_send_sync.rs
index 12d13239387..97118ce66dc 100644
--- a/tokio/tests/async_send_sync.rs
+++ b/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);