From 395842ecf3467ecf22c232d88c40f3a10c18c42e Mon Sep 17 00:00:00 2001
From: David Lattimore <dvdlttmr@gmail.com>
Date: Sat, 29 Nov 2025 15:15:54 +1100
Subject: [PATCH] fix: Avoid deadlock on downsizing of threadpool

---
 src/thread_pool.rs | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/src/thread_pool.rs b/src/thread_pool.rs
index dc7304e..c7c0fba 100644
--- a/src/thread_pool.rs
+++ b/src/thread_pool.rs
@@ -440,17 +440,19 @@ impl ThreadPool {
             }
             // The size decreased
             cmp::Ordering::Less => {
+                let mut heartbeat_handle = None;
+
                 // Halt the heartbeat thread when scaling to zero.
-                if let Some(control) = state.managed_threads.heartbeat.take() {
+                if new_size == 0
+                    && let Some(control) = state.managed_threads.heartbeat.take()
+                {
                     control.halt.store(true, Ordering::Relaxed);
-                    let _ = control.handle.join();
+                    heartbeat_handle = Some(control.handle);
                 }
 
                 // Pull the workers we intend to halt out of the thread manager.
                 let terminating_workers = state.managed_threads.workers.split_off(new_size);
 
-                drop(state);
-
                 // Terminate the workers.
                 for worker in &terminating_workers {
                     // Tell the worker to halt.
@@ -458,7 +460,11 @@ impl ThreadPool {
                 }
 
                 // Wake any sleeping workers to ensure they will eventually see the termination notice.
-                // self.job_is_ready.notify_all();
+                for seat in &state.seats {
+                    seat.data.sleep_controller.wake();
+                }
+
+                drop(state);
 
                 let own_lease = Worker::map_current(|worker| worker.lease.index);
 
@@ -470,6 +476,10 @@ impl ThreadPool {
                         let _ = worker.control.handle.join();
                     }
                 }
+
+                if let Some(handle) = heartbeat_handle {
+                    let _ = handle.join();
+                }
             }
         }
 
@@ -906,7 +916,7 @@ impl Worker {
     /// Cooperatively yields execution to the threadpool, allowing it to execute
     /// some work.
     ///
-    /// Tis function may execute either local or shared work: work already
+    /// This function may execute either local or shared work: work already
     /// queued on the worker, or work off-loaded by a different worker. If there
     /// is no work on the pool, this will lock the thread-pool mutex, so it
     /// should not be called within a hot loop. Consider using
@@ -1134,7 +1144,7 @@ impl Worker {
                 // the queue must point to `stack_job`, implying that
                 // `stack_job` cannot have been executed yet.
                 let a = unsafe { stack_job.unwrap() };
-                // Execute the closure directly and return the results. This is
+                // Execute the closure directly and return the results. This
                 // allows the compiler to inline and optimize `a`.
                 let result_a = a(self);
                 return (result_a, result_b);
@@ -1553,7 +1563,7 @@ fn heartbeat_loop(thread_pool: &'static ThreadPool, halt: Arc<AtomicBool>) {
                     queued_to_heartbeat = (seat_index + 1) % num_seats;
                 }
 
-                // Count every occupied slot, even if we didn't sent them a heartbeat.
+                // Count every occupied slot, even if we didn't send them a heartbeat.
                 num_occupied += 1;
             }
         }