use spawn multiprocessing method for sidecar env server (#917)

* Use spawn multiprocessing method for sidecar env server

Switch from the default fork method to spawn when creating the env
server subprocess. Fork inherits all file descriptors from the parent,
which means multiple env server subprocesses can share sockets and
other fds, leading to hangs. Spawn starts a fresh interpreter and
avoids this class of issues.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* use mp

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: mikasenghaas

verifiers/envs/environment.py

@@ -14,7 +14,8 @@
 from collections.abc import Mapping
 from concurrent.futures import ThreadPoolExecutor
 from copy import deepcopy
-from multiprocessing import Process
+import multiprocessing as mp
+from multiprocessing.process import BaseProcess
 from pathlib import Path
 from typing import (
     TYPE_CHECKING,
@@ -146,7 +147,7 @@ def __init__(
         self.set_score_rollouts(score_rollouts)
 
         self.env_client: EnvClient | None = None
-        self.env_server_process: Process | None = None
+        self.env_server_process: BaseProcess | None = None
 
         # Dataset sources (builders) and built datasets
         # Use get_dataset()/get_eval_dataset() for access; build_dataset() to trigger build
@@ -1268,7 +1269,10 @@ async def start_server(
             depending on it directly.
         """
         address = address or f"tcp://127.0.0.1:{get_free_port()}"
-        self.env_server_process = Process(
+        # Use spawn to avoid inheriting file descriptors (e.g. sockets) from
+        # the parent process, which has caused hangs when multiple env server
+        # subprocesses share the same fds.
+        self.env_server_process = mp.get_context("spawn").Process(
             target=ZMQEnvServer.run_server,
             args=(
                 self.env_id,