Skip signal handler registration when not on main thread (#18752)

python/sglang/srt/entrypoints/engine.py

@@ -882,6 +882,8 @@ def _set_envs_and_config(server_args: ServerArgs):
                 "Please reinstall the latest version with `pip install sgl-kernel --force-reinstall`",
             )
 
+    # Signal handlers can only be registered from the main thread.
+    if threading.current_thread() is threading.main_thread():
         if server_args.custom_sigquit_handler is None:
             # Register the signal handler.
             # The child processes will send SIGQUIT to this process when any error happens
@@ -901,6 +903,12 @@ def _set_envs_and_config(server_args: ServerArgs):
                 f"Using custom SIGQUIT handler: {server_args.custom_sigquit_handler}"
             )
             signal.signal(signal.SIGQUIT, server_args.custom_sigquit_handler)
+    else:
+        logger.warning(
+            "Signal handler is not added because the engine is not in the "
+            "main thread. This disables the SIGQUIT handler for cleaning up "
+            "the process tree when a child process fails."
+        )
 
     # Set mp start method
     mp.set_start_method("spawn", force=True)