[BugFix] Fix frontend multiprocessing hang (#7217)

Signed-off-by: Max de Bayser <mbayser@br.ibm.com>
Co-authored-by: Robert Shaw <114415538+robertgshaw2-neuralmagic@users.noreply.github.com>

tests/entrypoints/openai/test_mp_crash.py

@@ -0,0 +1,35 @@
+from typing import Any
+
+import pytest
+
+from vllm.engine.async_llm_engine import AsyncLLMEngine
+from vllm.entrypoints.openai.api_server import build_async_engine_client
+from vllm.entrypoints.openai.cli_args import make_arg_parser
+from vllm.utils import FlexibleArgumentParser
+
+
+def crashing_from_engine_args(
+    cls,
+    engine_args: Any = None,
+    start_engine_loop: Any = None,
+    usage_context: Any = None,
+    stat_loggers: Any = None,
+) -> "AsyncLLMEngine":
+    raise Exception("foo")
+
+
+@pytest.mark.asyncio
+async def test_mp_crash_detection(monkeypatch):
+
+    with pytest.raises(RuntimeError) as excinfo, monkeypatch.context() as m:
+        m.setattr(AsyncLLMEngine, "from_engine_args",
+                  crashing_from_engine_args)
+        parser = FlexibleArgumentParser(
+            description="vLLM's remote OpenAI server.")
+        parser = make_arg_parser(parser)
+        args = parser.parse_args([])
+
+        async with build_async_engine_client(args):
+            pass
+    assert "The server process died before responding to the readiness probe"\
+          in str(excinfo.value)