From 3d65a4aecdcfdd72ff1657317657a4cd665bba5f Mon Sep 17 00:00:00 2001
From: Ouri Half <112880714+ouriamzn@users.noreply.github.com>
Date: Thu, 4 Dec 2025 19:10:09 +0200
Subject: [PATCH] Fix deadlock in IO thread shutdown during panic (#2898)

## Problem
IO thread shutdown can deadlock during server panic when the main thread
calls `pthread_cancel()` while the IO thread holds its mutex, preventing
the thread from observing the cancellation.

## Solution
Release the IO thread mutex before cancelling to ensure clean thread
termination.

## Testing
Reproducer:
```
bash
./src/valkey-server --io-threads 2 --enable-debug-command yes
./src/valkey-cli debug panic
```

Before: Server hangs indefinitely
After: Server terminates cleanly

Signed-off-by: Ouri Half <ourih@amazon.com>
---
 src/io_threads.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/io_threads.c b/src/io_threads.c
index a9f59bf41..b5c9d1f03 100644
--- a/src/io_threads.c
+++ b/src/io_threads.c
@@ -285,6 +285,10 @@ static void shutdownIOThread(int id) {
     if (tid == pthread_self()) return;
     if (tid == 0) return;
 
+    /* Only unlock mutex for inactive threads. Active threads are already unlocked. */
+    if (id >= server.active_io_threads_num) {
+        pthread_mutex_unlock(&io_threads_mutex[id]);
+    }
     pthread_cancel(tid);
 
     if ((err = pthread_join(tid, NULL)) != 0) {