[BugFix] Fix case where collective_rpc returns None (#22006)

Signed-off-by: Nick Hill <nhill@redhat.com>

tests/v1/engine/test_engine_core_client.py

@@ -305,10 +305,10 @@ def echo_dc(
     return_list: bool = False,
 ) -> Union[MyDataclass, list[MyDataclass]]:
     print(f"echo dc util function called: {msg}")
+    val = None if msg is None else MyDataclass(msg)
     # Return dataclass to verify support for returning custom types
     # (for which there is special handling to make it work with msgspec).
-    return [MyDataclass(msg) for _ in range(3)] if return_list \
-        else MyDataclass(msg)
+    return [val for _ in range(3)] if return_list else val
 
 
 @pytest.mark.asyncio(loop_scope="function")
@@ -351,6 +351,15 @@ async def test_engine_core_client_util_method_custom_return(
             assert isinstance(result, list) and all(
                 isinstance(r, MyDataclass) and r.message == "testarg2"
                 for r in result)
+
+            # Test returning None and list of Nones
+            result = await core_client.call_utility_async(
+                "echo_dc", None, False)
+            assert result is None
+            result = await core_client.call_utility_async(
+                "echo_dc", None, True)
+            assert isinstance(result, list) and all(r is None for r in result)
+
         finally:
             client.shutdown()
 

vllm/v1/serial_utils.py

@@ -49,7 +49,10 @@ def _log_insecure_serialization_warning():
                         "VLLM_ALLOW_INSECURE_SERIALIZATION=1")
 
 
-def _typestr(t: type):
+def _typestr(val: Any) -> Optional[tuple[str, str]]:
+    if val is None:
+        return None
+    t = type(val)
     return t.__module__, t.__qualname__
 
 
@@ -131,14 +134,13 @@ class MsgpackEncoder:
 
         if isinstance(obj, UtilityResult):
             result = obj.result
-            if not envs.VLLM_ALLOW_INSECURE_SERIALIZATION or result is None:
+            if not envs.VLLM_ALLOW_INSECURE_SERIALIZATION:
                 return None, result
             # Since utility results are not strongly typed, we also encode
             # the type (or a list of types in the case it's a list) to
             # help with correct msgspec deserialization.
-            cls = result.__class__
-            return _typestr(cls) if cls is not list else [
-                _typestr(type(v)) for v in result
+            return _typestr(result) if type(result) is not list else [
+                _typestr(v) for v in result
             ], result
 
         if not envs.VLLM_ALLOW_INSECURE_SERIALIZATION:
@@ -277,7 +279,9 @@ class MsgpackDecoder:
                 ]
         return UtilityResult(result)
 
-    def _convert_result(self, result_type: Sequence[str], result: Any):
+    def _convert_result(self, result_type: Sequence[str], result: Any) -> Any:
+        if result_type is None:
+            return result
         mod_name, name = result_type
         mod = importlib.import_module(mod_name)
         result_type = getattr(mod, name)