[MISC][V1] Register process killing handler only in the main thread (#14380)

Signed-off-by: Cody Yu <hao.yu.cody@gmail.com>

vllm/v1/engine/core_client.py

@@ -4,6 +4,7 @@ import asyncio
 import os
 import queue
 import signal
+import threading
 import uuid
 import weakref
 from abc import ABC, abstractmethod
@@ -260,7 +261,14 @@ class MPClient(EngineCoreClient):
                          "down. See stack trace above for root cause issue.")
             kill_process_tree(os.getpid())
 
-        signal.signal(signal.SIGUSR1, sigusr1_handler)
+        if threading.current_thread() == threading.main_thread():
+            signal.signal(signal.SIGUSR1, sigusr1_handler)
+        else:
+            logger.warning("SIGUSR1 handler not installed because we are not "
+                           "running in the main thread. In this case the "
+                           "forked engine process may not be killed when "
+                           "an exception is raised, and you need to handle "
+                           "the engine process shutdown manually.")
 
         # Serialization setup.
         self.encoder = MsgpackEncoder()