std: allow after-main use of synchronization primitives

By creating an unnamed thread handle when the actual one has already been destroyed, synchronization primitives using thread parking can be used even outside the Rust runtime.

This also fixes an inefficiency in the queue-based `RwLock`: if `thread::current` was not initialized yet, it will create a new handle on every parking attempt without initializing `thread::current`. The private `current_or_unnamed` function introduced here fixes this.

4 files changed, 19 insertions, 9 deletions

diff --git a/library/std/src/sync/mpmc/array.rs b/library/std/src/sync/mpmc/array.rs
index 2c8ba411f30..a467237fef1 100644
--- a/library/std/src/sync/mpmc/array.rs
+++ b/library/std/src/sync/mpmc/array.rs
@@ -346,7 +346,8 @@ impl<T> Channel<T> {
                 }
 
                 // Block the current thread.
-                let sel = cx.wait_until(deadline);
+                // SAFETY: the context belongs to the current thread.
+                let sel = unsafe { cx.wait_until(deadline) };
 
                 match sel {
                     Selected::Waiting => unreachable!(),
@@ -397,7 +398,8 @@ impl<T> Channel<T> {
                 }
 
                 // Block the current thread.
-                let sel = cx.wait_until(deadline);
+                // SAFETY: the context belongs to the current thread.
+                let sel = unsafe { cx.wait_until(deadline) };
 
                 match sel {
                     Selected::Waiting => unreachable!(),
diff --git a/library/std/src/sync/mpmc/context.rs b/library/std/src/sync/mpmc/context.rs
index 2371d32d4ea..51aa7e82e78 100644
--- a/library/std/src/sync/mpmc/context.rs
+++ b/library/std/src/sync/mpmc/context.rs
@@ -69,7 +69,7 @@ impl Context {
             inner: Arc::new(Inner {
                 select: AtomicUsize::new(Selected::Waiting.into()),
                 packet: AtomicPtr::new(ptr::null_mut()),
-                thread: thread::current(),
+                thread: thread::current_or_unnamed(),
                 thread_id: current_thread_id(),
             }),
         }
@@ -112,8 +112,11 @@ impl Context {
     /// Waits until an operation is selected and returns it.
     ///
     /// If the deadline is reached, `Selected::Aborted` will be selected.
+    ///
+    /// # Safety
+    /// This may only be called from the thread this `Context` belongs to.
     #[inline]
-    pub fn wait_until(&self, deadline: Option<Instant>) -> Selected {
+    pub unsafe fn wait_until(&self, deadline: Option<Instant>) -> Selected {
         loop {
             // Check whether an operation has been selected.
             let sel = Selected::from(self.inner.select.load(Ordering::Acquire));
@@ -126,7 +129,8 @@ impl Context {
                 let now = Instant::now();
 
                 if now < end {
-                    thread::park_timeout(end - now);
+                    // SAFETY: guaranteed by caller.
+                    unsafe { self.inner.thread.park_timeout(end - now) };
                 } else {
                     // The deadline has been reached. Try aborting select.
                     return match self.try_select(Selected::Aborted) {
@@ -135,7 +139,8 @@ impl Context {
                     };
                 }
             } else {
-                thread::park();
+                // SAFETY: guaranteed by caller.
+                unsafe { self.inner.thread.park() };
             }
         }
     }
diff --git a/library/std/src/sync/mpmc/list.rs b/library/std/src/sync/mpmc/list.rs
index 523e6d2f3bb..d88914f5291 100644
--- a/library/std/src/sync/mpmc/list.rs
+++ b/library/std/src/sync/mpmc/list.rs
@@ -444,7 +444,8 @@ impl<T> Channel<T> {
                 }
 
                 // Block the current thread.
-                let sel = cx.wait_until(deadline);
+                // SAFETY: the context belongs to the current thread.
+                let sel = unsafe { cx.wait_until(deadline) };
 
                 match sel {
                     Selected::Waiting => unreachable!(),
diff --git a/library/std/src/sync/mpmc/zero.rs b/library/std/src/sync/mpmc/zero.rs
index 446881291e6..577997c07a6 100644
--- a/library/std/src/sync/mpmc/zero.rs
+++ b/library/std/src/sync/mpmc/zero.rs
@@ -190,7 +190,8 @@ impl<T> Channel<T> {
             drop(inner);
 
             // Block the current thread.
-            let sel = cx.wait_until(deadline);
+            // SAFETY: the context belongs to the current thread.
+            let sel = unsafe { cx.wait_until(deadline) };
 
             match sel {
                 Selected::Waiting => unreachable!(),
@@ -257,7 +258,8 @@ impl<T> Channel<T> {
             drop(inner);
 
             // Block the current thread.
-            let sel = cx.wait_until(deadline);
+            // SAFETY: the context belongs to the current thread.
+            let sel = unsafe { cx.wait_until(deadline) };
 
             match sel {
                 Selected::Waiting => unreachable!(),