Fix SubprocVecEnv ConnectionResetError with forkserver/spawn

highway_env/__init__.py

@@ -1,7 +1,7 @@
 import os
 import sys
 
-from gymnasium.envs.registration import register
+from gymnasium.envs.registration import register, registry
 
 
 __version__ = "1.10.2"
@@ -20,7 +20,15 @@
 
 
 def _register_highway_envs():
-    """Import the envs module so that envs register themselves."""
+    """Import the envs module so that envs register themselves.
+
+    This function is idempotent: calling it multiple times (e.g. when
+    gymnasium resolves a ``"highway_env:env-id"`` spec in a subprocess)
+    will not raise duplicate-registration errors.
+    """
+    # Skip if environments are already registered (idempotent)
+    if "highway-v0" in registry:
+        return
 
     from highway_env.envs.common.abstract import MultiAgentWrapper
 

pyproject.toml

@@ -44,6 +44,9 @@ Repository = "https://github.com/eleurent/highway-env"
 Documentation = "https://highway-env.farama.org/"
 "Bug Report" = "https://github.com/eleurent/highway-env/issues"
 
+[project.entry-points."gymnasium.envs.__root__"]
+highway-env = "highway_env:_register_highway_envs"
+
 [tool.setuptools]
 include-package-data = true
 

tests/envs/test_multiprocessing.py

@@ -0,0 +1,59 @@
+"""Test that highway-env environments work with multiprocessing (forkserver/spawn).
+
+When using SubprocVecEnv from stable-baselines3 or similar vectorized
+environment wrappers, child processes are started via ``forkserver`` or
+``spawn`` and do **not** inherit the parent's ``import highway_env``.
+
+Gymnasium's ``module:env_name`` syntax (e.g. ``"highway_env:highway-v0"``)
+triggers an import of the module in the subprocess, which registers the
+environments on demand.
+
+See: https://github.com/Farama-Foundation/HighwayEnv/issues/648
+"""
+
+import multiprocessing as mp
+
+import gymnasium as gym
+import pytest
+
+
+def _make_env_in_subprocess(env_id: str, result_queue: mp.Queue) -> None:
+    """Create and step an environment inside a subprocess (no prior import of highway_env)."""
+    try:
+        env = gym.make(env_id)
+        obs, _info = env.reset()
+        _obs, _reward, _terminated, _truncated, _info = env.step(
+            env.action_space.sample()
+        )
+        env.close()
+        result_queue.put(("ok", str(type(obs))))
+    except Exception as exc:
+        result_queue.put(("error", f"{type(exc).__name__}: {exc}"))
+
+
+@pytest.mark.parametrize(
+    "env_id",
+    [
+        "highway_env:highway-v0",
+        "highway_env:highway-fast-v0",
+        "highway_env:merge-v0",
+        "highway_env:roundabout-v0",
+        "highway_env:intersection-v0",
+        "highway_env:parking-v0",
+    ],
+)
+@pytest.mark.parametrize("start_method", ["forkserver", "spawn"])
+def test_env_in_subprocess(env_id: str, start_method: str) -> None:
+    """Environments should be creatable in forkserver/spawn subprocesses via module:name syntax."""
+    if start_method not in mp.get_all_start_methods():
+        pytest.skip(f"{start_method} not available on this platform")
+
+    ctx = mp.get_context(start_method)
+    q: mp.Queue = ctx.Queue()
+    p = ctx.Process(target=_make_env_in_subprocess, args=(env_id, q))
+    p.start()
+    p.join(timeout=30)
+
+    assert not q.empty(), "Subprocess produced no result (likely crashed)"
+    status, detail = q.get()
+    assert status == "ok", f"Subprocess failed: {detail}"