[Bugfix] Register reducer even if transformers_modules not available (#19510)

Signed-off-by: Seiji Eicher <seiji@anyscale.com>

tests/config/test_mp_reducer.py

@@ -0,0 +1,57 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import sys
+from unittest.mock import patch
+
+from vllm.config import VllmConfig
+from vllm.engine.arg_utils import AsyncEngineArgs
+from vllm.v1.engine.async_llm import AsyncLLM
+
+
+def test_mp_reducer(monkeypatch):
+    """
+    Test that _reduce_config reducer is registered when AsyncLLM is instantiated
+    without transformers_modules. This is a regression test for
+    https://github.com/vllm-project/vllm/pull/18640.
+    """
+
+    # Use V1 AsyncLLM which calls maybe_register_config_serialize_by_value
+    monkeypatch.setenv('VLLM_USE_V1', '1')
+
+    # Ensure transformers_modules is not in sys.modules
+    if 'transformers_modules' in sys.modules:
+        del sys.modules['transformers_modules']
+
+    with patch('multiprocessing.reducer.register') as mock_register:
+        engine_args = AsyncEngineArgs(
+            model="facebook/opt-125m",
+            max_model_len=32,
+            gpu_memory_utilization=0.1,
+            disable_log_stats=True,
+            disable_log_requests=True,
+        )
+
+        async_llm = AsyncLLM.from_engine_args(
+            engine_args,
+            start_engine_loop=False,
+        )
+
+        assert mock_register.called, (
+            "multiprocessing.reducer.register should have been called")
+
+        vllm_config_registered = False
+        for call_args in mock_register.call_args_list:
+            # Verify that a reducer for VllmConfig was registered
+            if len(call_args[0]) >= 2 and call_args[0][0] == VllmConfig:
+                vllm_config_registered = True
+
+                reducer_func = call_args[0][1]
+                assert callable(
+                    reducer_func), "Reducer function should be callable"
+                break
+
+        assert vllm_config_registered, (
+            "VllmConfig should have been registered to multiprocessing.reducer"
+        )
+
+        async_llm.shutdown()