From 21aba1793634938e9bca8cab46f32545b5e8889a Mon Sep 17 00:00:00 2001
From: Julien Portalier <julien@portalier.com>
Date: Tue, 11 Feb 2025 17:00:00 +0100
Subject: [PATCH] Fix: MT may fail to wake other schedulers after evloop run

---
 src/fiber/execution_context/multi_threaded/scheduler.cr | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/fiber/execution_context/multi_threaded/scheduler.cr b/src/fiber/execution_context/multi_threaded/scheduler.cr
index 866f0e9399c2..7bf532d3c571 100644
--- a/src/fiber/execution_context/multi_threaded/scheduler.cr
+++ b/src/fiber/execution_context/multi_threaded/scheduler.cr
@@ -142,9 +142,13 @@ module Fiber::ExecutionContext
           yield @global_queue.grab?(@runnables, divisor: @execution_context.size)
 
           if @execution_context.lock_evloop? { @event_loop.run(pointerof(list), blocking: false) }
-            if fiber = enqueue_many(pointerof(list))
+            unless list.empty?
+              # must stop spinning before calling enqueue_many that may call
+              # wake_scheduler which returns immediately if a thread is
+              # spinning... but we're spinning, so that would always fail to
+              # wake sleeping schedulers despite having runnable fibers
               spin_stop
-              yield fiber
+              yield enqueue_many(pointerof(list))
             end
           end