feat(companion): Apply cwd and env in spawned companion child

Child runs _apply_environment before the target: os.chdir(cwd) then
os.environ.update(env).

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

docs/design/companion-process-manager.md

@@ -672,7 +672,7 @@ No per-companion logic in Arbiter.
 - [x] Add status description helpers.
 - [x] Add `CompanionManager` skeleton.
 - [x] Spawn one companion process from the manager.
-- [ ] Apply `cwd` and `env` before target.
+- [x] Apply `cwd` and `env` before target.
 - [ ] Redirect `stdout` and `stderr`.
 - [ ] Reap exited companion processes.
 - [ ] Implement `STARTING -> RUNNING` using `startsecs`.

gunicorn/companion/manager.py

@@ -3,12 +3,18 @@
 # See the NOTICE for more information.
 
 
+from __future__ import annotations
+
 import importlib
 import os
 import time
+from typing import TYPE_CHECKING, Callable, Iterable, Union
 
 from gunicorn.companion.process import CompanionProcess, State
 
+if TYPE_CHECKING:
+    from gunicorn.companion.process import CompanionConfig
+
 
 class CompanionManager:
     """Forks and supervises companion processes.
@@ -19,12 +25,12 @@ class CompanionManager:
     socket, and the run loop arrive in later tasks.
     """
 
-    def __init__(self, configs, log):
+    def __init__(self, configs: Iterable[CompanionConfig], log):
         self.log = log
         self.pid = os.getpid()
         self.processes = {c.name: CompanionProcess(c) for c in configs}
 
-    def spawn_process(self, proc):
+    def spawn_process(self, proc: CompanionProcess) -> int:
         """Fork one companion.
 
         Parent records the pid and moves the companion to STARTING. Child
@@ -41,6 +47,7 @@ class CompanionManager:
             return pid
 
         try:
+            self._apply_environment(proc.config)
             target = self._resolve_target(proc.config.target)
             target()
         except SystemExit:
@@ -51,7 +58,20 @@ class CompanionManager:
         os._exit(0)
 
     @staticmethod
-    def _resolve_target(target):
+    def _apply_environment(config: CompanionConfig) -> None:
+        """Apply ``cwd`` and ``env`` in the child before running the target.
+
+        cwd is changed first so a relative path in env (or the target itself)
+        resolves against it. env is merged onto the inherited environment, not
+        replaced, so the companion keeps the manager's variables.
+        """
+        if config.cwd:
+            os.chdir(config.cwd)
+        if config.env:
+            os.environ.update(config.env)
+
+    @staticmethod
+    def _resolve_target(target: Union[Callable, str]) -> Callable:
         """Return the zero-arg callable for a companion target.
 
         Accepts an already-callable target or a ``"module:attr"`` import

tests/test_companion_manager.py

@@ -36,6 +36,24 @@ def test_resolve_target_rejects_bad_string():
         CompanionManager._resolve_target("no_colon")
 
 
+def test_apply_environment_sets_cwd_and_env():
+    config = CompanionConfig(name="rq", target=lambda: None,
+                             cwd="/tmp", env={"COMPANION_X": "1"})
+    with mock.patch("os.chdir") as chdir, \
+            mock.patch.dict("os.environ", {}, clear=False):
+        CompanionManager._apply_environment(config)
+        chdir.assert_called_once_with("/tmp")
+        import os
+        assert os.environ["COMPANION_X"] == "1"
+
+
+def test_apply_environment_noop_without_cwd_env():
+    config = CompanionConfig(name="rq", target=lambda: None)
+    with mock.patch("os.chdir") as chdir:
+        CompanionManager._apply_environment(config)
+        chdir.assert_not_called()
+
+
 def test_spawn_parent_records_pid_and_starting():
     mgr = make_manager("rq")
     proc = mgr.processes["rq"]