From 5a856b82f3a1bf335e0e62d92c800d8436977af1 Mon Sep 17 00:00:00 2001
From: joboet <jonasboettiger@icloud.com>
Date: Thu, 7 Nov 2024 17:13:33 +0100
Subject: 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.
---
 library/std/src/sync/mpmc/context.rs | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

(limited to 'library/std/src/sync/mpmc/context.rs')

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() };
             }
         }
     }
-- 
cgit 1.4.1-3-g733a5