[Bugfix] Make suppress_stdout redirect fd 1 instead of sys.stdout.fileno() (#59336)

Signed-off-by: usmanmateen <usmanmateen255@gmail.com>
Co-authored-by: Misha Goin <mgoin64@gmail.com>

tests/utils_/test_system_utils.py

@@ -1,11 +1,18 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import contextlib
+import io
 import os
+import sys
 import tempfile
 from pathlib import Path
 
-from vllm.utils.system_utils import _maybe_force_spawn, unique_filepath
+from vllm.utils.system_utils import (
+    _maybe_force_spawn,
+    suppress_stdout,
+    unique_filepath,
+)
 
 
 def test_unique_filepath():
@@ -25,3 +32,19 @@ def test_numa_bind_forces_spawn(monkeypatch):
     monkeypatch.setattr("sys.argv", ["vllm", "serve", "--numa-bind"])
     _maybe_force_spawn()
     assert os.environ["VLLM_WORKER_MULTIPROC_METHOD"] == "spawn"
+
+
+def test_suppress_stdout_when_sys_stdout_has_no_fd(capfd):
+    with contextlib.redirect_stdout(io.StringIO()), suppress_stdout():
+        os.write(1, b"c library output\n")
+    assert "c library output" not in capfd.readouterr().out
+
+
+def test_suppress_stdout_keeps_stderr_when_sys_stdout_is_stderr(capfd, monkeypatch):
+    monkeypatch.setattr(sys, "stdout", sys.stderr)
+    with suppress_stdout():
+        os.write(1, b"c library output\n")
+        os.write(sys.stderr.fileno(), b"error message\n")
+    out, err = capfd.readouterr()
+    assert "c library output" not in out
+    assert "error message" in err

vllm/utils/system_utils.py

@@ -72,7 +72,8 @@ def suppress_stdout():
         yield
         return
 
-    stdout_fd = sys.stdout.fileno()
+    # C libraries write to fd 1, whatever sys.stdout is bound to.
+    stdout_fd = 1
     stdout_dup = os.dup(stdout_fd)
     devnull_fd = os.open(os.devnull, os.O_WRONLY)