feat(companion): Redirect companion stdout and stderr

Child calls _redirect_output after env setup: each configured log path is
opened append-mode and dup2'd onto fd 1/2. None/inherit keeps the inherited
fd; stderr stdout shares stdout's fd. Rotation stays external.

Add tests for inherit, append flags, file dup2, and stderr-to-stdout.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/design/companion-process-manager.md

@@ -673,7 +673,7 @@ No per-companion logic in Arbiter.
 - [x] Add `CompanionManager` skeleton.
 - [x] Spawn one companion process from the manager.
 - [x] Apply `cwd` and `env` before target.
-- [ ] Redirect `stdout` and `stderr`.
+- [x] Redirect `stdout` and `stderr`.
 - [ ] Reap exited companion processes.
 - [ ] Implement `STARTING -> RUNNING` using `startsecs`.
 - [ ] Implement `BACKOFF` with fixed `companion_restart_delay`.

gunicorn/companion/manager.py

@@ -48,6 +48,7 @@ class CompanionManager:
 
         try:
             self._apply_environment(proc.config)
+            self._redirect_output(proc.config)
             target = self._resolve_target(proc.config.target)
             target()
         except SystemExit:
@@ -70,6 +71,33 @@ class CompanionManager:
         if config.env:
             os.environ.update(config.env)
 
+    @staticmethod
+    def _redirect_output(config: CompanionConfig) -> None:
+        """Send the companion's stdout and stderr to its configured log files.
+
+        By default a companion just inherits the manager's stdout/stderr, so
+        leaving these unset (or ``"inherit"``) keeps that. Give a file path and
+        we append the output there instead. For stderr you can also pass
+        ``"stdout"`` to fold the two streams into one file.
+        """
+        out = CompanionManager._open_output(config.stdout)
+        if out is not None:
+            os.dup2(out, 1)
+        if config.stderr == "stdout":
+            os.dup2(1, 2)
+        else:
+            err = CompanionManager._open_output(config.stderr)
+            if err is not None:
+                os.dup2(err, 2)
+
+    @staticmethod
+    def _open_output(value):
+        """Open one log file for writing, or return None to leave the stream
+        as-is when the companion should keep inheriting it."""
+        if value in (None, "inherit"):
+            return None
+        return os.open(value, os.O_WRONLY | os.O_CREAT | os.O_APPEND, 0o644)
+
     @staticmethod
     def _resolve_target(target: Union[Callable, str]) -> Callable:
         """Return the zero-arg callable for a companion target.

tests/test_companion_manager.py

@@ -2,6 +2,7 @@
 # This file is part of gunicorn released under the MIT license.
 # See the NOTICE for more information.
 
+import os
 from unittest import mock
 
 import pytest
@@ -54,6 +55,46 @@ def test_apply_environment_noop_without_cwd_env():
         chdir.assert_not_called()
 
 
+def test_open_output_inherit_returns_none():
+    assert CompanionManager._open_output(None) is None
+    assert CompanionManager._open_output("inherit") is None
+
+
+def test_open_output_path_opens_append():
+    with mock.patch("os.open", return_value=9) as op:
+        fd = CompanionManager._open_output("/var/log/rq.log")
+    assert fd == 9
+    flags = op.call_args.args[1]
+    assert flags & os.O_APPEND and flags & os.O_CREAT
+
+
+def test_redirect_output_files():
+    config = CompanionConfig(name="rq", target=lambda: None,
+                             stdout="/o.log", stderr="/e.log")
+    with mock.patch("os.open", side_effect=[10, 11]), \
+            mock.patch("os.dup2") as dup2:
+        CompanionManager._redirect_output(config)
+    dup2.assert_any_call(10, 1)
+    dup2.assert_any_call(11, 2)
+
+
+def test_redirect_output_stderr_to_stdout():
+    config = CompanionConfig(name="rq", target=lambda: None,
+                             stdout="/o.log", stderr="stdout")
+    with mock.patch("os.open", return_value=10), \
+            mock.patch("os.dup2") as dup2:
+        CompanionManager._redirect_output(config)
+    dup2.assert_any_call(10, 1)
+    dup2.assert_any_call(1, 2)
+
+
+def test_redirect_output_inherit_noop():
+    config = CompanionConfig(name="rq", target=lambda: None)
+    with mock.patch("os.dup2") as dup2:
+        CompanionManager._redirect_output(config)
+    dup2.assert_not_called()
+
+
 def test_spawn_parent_records_pid_and_starting():
     mgr = make_manager("rq")
     proc = mgr.processes["rq"]