horizon-threads: the stack dump test samples only once the busy loop runs

The worker sent its handle and went straight into the loop, so the signal could land
while it was still waking the receiver (futex_wake in the mpsc send), and the busy
frame was not yet on its stack. It failed that way on CI (run 43194576407).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-threads/src/stack_dump.rs

@@ -183,8 +183,9 @@ mod tests {
     /// (it resumes from the return address), while every caller's frame stays.
     #[cfg(any(target_vendor = "apple", target_os = "linux", target_os = "android"))]
     #[inline(never)]
-    fn busy_in_a_named_frame(stop: &AtomicBool) {
+    fn busy_in_a_named_frame(started: &AtomicBool, stop: &AtomicBool) {
         let mut n = 0u64;
+        started.store(true, Ordering::Release);
         while !stop.load(Ordering::Relaxed) {
             n = std::hint::black_box(spin)(n);
         }
@@ -193,16 +194,21 @@ mod tests {
     #[cfg(any(target_vendor = "apple", target_os = "linux", target_os = "android"))]
     #[test]
     fn reads_the_stack_of_a_busy_thread_from_another() {
+        let started = Arc::new(AtomicBool::new(false));
         let stop = Arc::new(AtomicBool::new(false));
         let (tx, rx) = mpsc::channel();
         let worker = {
-            let stop = stop.clone();
+            let (started, stop) = (started.clone(), stop.clone());
             std::thread::spawn(move || {
                 tx.send(Watched::current()).unwrap();
-                busy_in_a_named_frame(&stop);
+                busy_in_a_named_frame(&started, &stop);
             })
         };
         let watched = rx.recv().unwrap();
+        // The send can still be waking the receiver when it returns: sample only once the loop runs.
+        while !started.load(Ordering::Acquire) {
+            std::hint::spin_loop();
+        }
         let stack = watched.stack(Duration::from_secs(10));
         stop.store(true, Ordering::Relaxed);
         worker.join().unwrap();