[Bugfix] Fix spawn_new_process_for_each_test silently swallowing test failures

tests/utils.py

@@ -18,7 +18,7 @@
 import time
 import warnings
 from collections.abc import Callable, Iterable, Sequence
-from contextlib import ExitStack, contextmanager, suppress
+from contextlib import ExitStack, contextmanager
 from multiprocessing import Process
 from pathlib import Path
 from typing import Any, Literal
@@ -1414,52 +1414,65 @@ def wrapper(*args: _P.args, **kwargs: _P.kwargs) -> None:
 
 
 def spawn_new_process_for_each_test(f: Callable[_P, None]) -> Callable[_P, None]:
-    """Decorator to spawn a new process for each test function."""
+    """Decorator to spawn a new process for each test function.
+
+    Uses subprocess with cloudpickle to serialize the test function and
+    propagates exceptions back to the parent, so test failures are never
+    silently swallowed (fixes https://github.com/vllm-project/vllm/issues/41415).
+    """
 
     @functools.wraps(f)
     def wrapper(*args: _P.args, **kwargs: _P.kwargs) -> None:
-        # Check if we're already in a subprocess
-        if os.environ.get("RUNNING_IN_SUBPROCESS") == "1":
-            # If we are, just run the function directly
-            return f(*args, **kwargs)
-
-        import torch.multiprocessing as mp
-
-        with suppress(RuntimeError):
-            mp.set_start_method("spawn")
-
-        # Get the module
-        module_name = f.__module__
-
-        # Create a process with environment variable set
-        env = os.environ.copy()
-        env["RUNNING_IN_SUBPROCESS"] = "1"
-
-        with tempfile.TemporaryDirectory() as tempdir:
-            output_filepath = os.path.join(tempdir, "new_process.tmp")
+        with tempfile.NamedTemporaryFile(delete=False, suffix=".tb", mode="wb") as tmp:
+            tb_file = tmp.name
 
-            # `cloudpickle` allows pickling complex functions directly
-            input_bytes = cloudpickle.dumps((f, output_filepath))
+        try:
+            # Serialize the function + args with cloudpickle so closures work
+            payload = cloudpickle.dumps((f, args, kwargs, tb_file))
+
+            child_script = (
+                "import sys, cloudpickle, traceback\n"
+                "try:\n"
+                "    from _pytest.outcomes import Skipped\n"
+                "except ImportError:\n"
+                "    class Skipped(BaseException): pass\n"
+                "f, args, kwargs, tb_file = "
+                "cloudpickle.loads(sys.stdin.buffer.read())\n"
+                "try:\n"
+                "    f(*args, **kwargs)\n"
+                "except Skipped:\n"
+                "    sys.exit(0)\n"
+                "except BaseException:\n"
+                "    open(tb_file, 'w').write(traceback.format_exc())\n"
+                "    sys.exit(1)\n"
+            )
 
             repo_root = str(VLLM_PATH.resolve())
-
-            env = dict(env or os.environ)
+            env = os.environ.copy()
             env["PYTHONPATH"] = repo_root + os.pathsep + env.get("PYTHONPATH", "")
 
-            cmd = [sys.executable, "-m", f"{module_name}"]
-
-            returned = subprocess.run(
-                cmd, input=input_bytes, capture_output=True, env=env
+            result = subprocess.run(
+                [sys.executable, "-c", child_script],
+                input=payload,
+                capture_output=True,
+                env=env,
             )
 
-            # check if the subprocess is successful
-            try:
-                returned.check_returncode()
-            except Exception as e:
-                # wrap raised exception to provide more information
+            if result.returncode != 0:
+                # Read traceback written by child, fall back to stderr
+                tb = ""
+                if os.path.exists(tb_file) and os.path.getsize(tb_file) > 0:
+                    with open(tb_file) as fp:
+                        tb = fp.read()
+                else:
+                    tb = result.stderr.decode()
                 raise RuntimeError(
-                    f"Error raised in subprocess:\n{returned.stderr.decode()}"
-                ) from e
+                    f"Test subprocess '{f.__name__}' failed "
+                    f"(exit code {result.returncode}):\n{tb}"
+                )
+        finally:
+            with contextlib.suppress(OSError):
+                os.remove(tb_file)
 
     return wrapper
 

tests/utils_/test_spawn_decorator.py

@@ -0,0 +1,33 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Tests for spawn_new_process_for_each_test decorator."""
+
+import pytest
+
+from tests.utils import spawn_new_process_for_each_test
+
+
+@spawn_new_process_for_each_test
+def test_spawn_decorator_passing():
+    """Passing function should complete normally."""
+    assert 1 + 1 == 2
+
+
+@pytest.mark.xfail(raises=RuntimeError, strict=True)
+@spawn_new_process_for_each_test
+def test_spawn_decorator_failure_is_caught():
+    """Failing function should raise RuntimeError, never silently pass."""
+    raise ValueError("intentional failure")
+
+
+@spawn_new_process_for_each_test
+def test_spawn_decorator_skip():
+    """pytest.skip inside subprocess should propagate correctly."""
+    pytest.skip("intentional skip")
+
+
+@spawn_new_process_for_each_test
+@pytest.mark.parametrize("x,y,expected", [(1, 2, 3), (0, 0, 0)])
+def test_spawn_decorator_parametrized(x, y, expected):
+    """Args and kwargs must be forwarded correctly to subprocess."""
+    assert x + y == expected