core: Simplify use_current_thread clean-up.

Do it from Registry::terminate() instead of exposing a new API.

rayon-core/src/lib.rs

@@ -100,8 +100,8 @@ pub use self::scope::{in_place_scope, scope, Scope};
 pub use self::scope::{in_place_scope_fifo, scope_fifo, ScopeFifo};
 pub use self::spawn::{spawn, spawn_fifo};
 pub use self::thread_pool::{
-    clean_up_use_current_thread, current_thread_has_pending_tasks, current_thread_index,
-    yield_local, yield_now, ThreadPool, Yield,
+    current_thread_has_pending_tasks, current_thread_index, yield_local, yield_now, ThreadPool,
+    Yield,
 };
 
 use self::registry::{CustomSpawn, DefaultSpawn, ThreadSpawn};
@@ -548,11 +548,8 @@ impl<S> ThreadPoolBuilder<S> {
     /// # Cleaning up a local thread-pool
     ///
     /// In order to properly clean-up the worker thread state, for local thread-pools you should
-    /// call [`clean_up_use_current_thread()`] from the same thread that built the thread-pool.
-    /// See that function's documentation for more details.
-    ///
-    /// This call is not required, but without it the registry will leak even if the pool is
-    /// otherwise terminated.
+    /// drop the thread pool from the same thread it was created on. Otherwise the registry will
+    /// leak (even if the pool is otherwise terminated).
     pub fn use_current_thread(mut self) -> Self {
         self.use_current_thread = true;
         self

rayon-core/src/registry.rs

@@ -610,9 +610,21 @@ impl Registry {
     /// dropped. The worker threads will gradually terminate, once any
     /// extant work is completed.
     pub(super) fn terminate(&self) {
-        if self.terminate_count.fetch_sub(1, Ordering::AcqRel) == 1 {
-            for (i, thread_info) in self.thread_infos.iter().enumerate() {
-                unsafe { OnceLatch::set_and_tickle_one(&thread_info.terminate, self, i) };
+        if self.terminate_count.fetch_sub(1, Ordering::AcqRel) != 1 {
+            return;
+        }
+        for (i, thread_info) in self.thread_infos.iter().enumerate() {
+            unsafe { OnceLatch::set_and_tickle_one(&thread_info.terminate, self, i) };
+        }
+        if self.used_creator_thread {
+            if let Some(t) = self.current_thread() {
+                if t.index == 0 {
+                    unsafe {
+                        wait_until_out_of_work(t);
+                        let _ = Box::from_raw(WorkerThread::current() as *mut WorkerThread);
+                    }
+                    assert!(WorkerThread::current().is_null());
+                }
             }
         }
     }
@@ -621,10 +633,6 @@ impl Registry {
     pub(super) fn notify_worker_latch_is_set(&self, target_worker_index: usize) {
         self.sleep.notify_worker_latch_is_set(target_worker_index);
     }
-
-    pub(super) fn used_creator_thread(&self) -> bool {
-        self.used_creator_thread
-    }
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]

rayon-core/src/thread_pool/mod.rs

@@ -461,39 +461,6 @@ pub fn yield_local() -> Option<Yield> {
     }
 }
 
-/// Waits for termination of the thread-pool (if pending), and cleans up resources allocated by
-/// [`ThreadPoolBuilder::use_current_thread()`]. Should only be called from the thread that built
-/// the thread-pool, and only when [`ThreadPoolBuilder::use_current_thread()`] is used.
-///
-/// Calling this function from a thread pool job will block indefinitely.
-///
-/// Calling this function before before the thread-pool has been dropped will cause the thread to
-/// not return control flow to the caller until that happens (stealing work as necessary).
-///
-/// # Panics
-///
-/// If the calling thread is no the creator thread of a thread-pool, or not part of that
-/// thread-pool, via [`ThreadPoolBuilder::use_current_thread()`].
-pub fn clean_up_use_current_thread() {
-    unsafe {
-        let thread = WorkerThread::current()
-            .as_ref()
-            .expect("Should be called from a worker thread");
-        assert!(
-            thread.registry().used_creator_thread(),
-            "Should only be used to clean up the pool creator constructor thread"
-        );
-        assert_eq!(
-            thread.index(),
-            0,
-            "Should be called from the thread that created the pool"
-        );
-        crate::registry::wait_until_out_of_work(thread);
-        let _ = Box::from_raw(WorkerThread::current() as *mut WorkerThread);
-    }
-    assert!(WorkerThread::current().is_null());
-}
-
 /// Result of [`yield_now()`] or [`yield_local()`].
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Yield {

rayon-core/tests/use_current_thread.rs

@@ -38,7 +38,6 @@ fn use_current_thread_basic() {
         .wait_while(pair.0.lock().unwrap(), |ran| !*ran)
         .unwrap();
     std::mem::drop(pool); // Drop the pool.
-    rayon_core::clean_up_use_current_thread(); // Clean-up
     assert_eq!(rayon_core::current_thread_index(), None);
 
     // Wait until all threads have actually exited. This is not really needed, other than to
@@ -47,71 +46,3 @@ fn use_current_thread_basic() {
         let _ = handle.join();
     }
 }
-
-#[test]
-fn bogus_use_current_thread_cleanup_wrong_thread() {
-    assert_eq!(rayon_core::current_thread_index(), None);
-    let pair = Arc::new((Mutex::new(None), Condvar::new()));
-    let pair2 = Arc::clone(&pair);
-    let pool = ThreadPoolBuilder::new()
-        .num_threads(2)
-        .use_current_thread()
-        .panic_handler(move |e| {
-            let &(ref panic_message, ref condvar) = &*pair2;
-            *panic_message.lock().unwrap() = Some(e.downcast::<String>().unwrap());
-            condvar.notify_one();
-        })
-        .build()
-        .unwrap();
-    assert_eq!(rayon_core::current_thread_index(), Some(0));
-
-    pool.spawn(move || {
-        rayon_core::clean_up_use_current_thread();
-        // Should never get here.
-    });
-
-    std::mem::drop(pool); // Drop the pool.
-    let panic_message = pair
-        .1
-        .wait_while(pair.0.lock().unwrap(), |panic_message| {
-            panic_message.is_none()
-        })
-        .unwrap();
-    let panic_message = panic_message.as_ref().unwrap();
-    assert!(
-        panic_message.contains("Should be called from the thread that created the pool"),
-        "{panic_message}"
-    );
-}
-
-#[test]
-#[should_panic]
-fn bogus_use_current_thread_cleanup_no_thread() {
-    assert_eq!(rayon_core::current_thread_index(), None);
-    rayon_core::clean_up_use_current_thread();
-}
-
-#[test]
-#[ignore = "Hard to test things that should just infinitely block"]
-fn bogus_use_current_thread_cleanup_reentrant() {
-    let pool = Arc::new(
-        ThreadPoolBuilder::new()
-            .num_threads(1)
-            .use_current_thread()
-            .build()
-            .unwrap(),
-    );
-    assert_eq!(rayon_core::current_thread_index(), Some(0));
-    let pool_clone = Arc::clone(&pool);
-    let handle = std::thread::spawn(move || {
-        pool_clone.spawn(move || {
-            // This should not execute until we clean_up_use_current_thread, so it'll get called
-            // reentrantly. This shouldn't result in UB or a use-after-free (should just infinitely
-            // block).
-            rayon_core::clean_up_use_current_thread();
-        });
-    });
-    handle.join().unwrap();
-    std::mem::drop(pool);
-    rayon_core::clean_up_use_current_thread();
-}