Fix SubprocVecEnv ConnectionResetError with forkserver/spawn

[Lidang-Jiang]

Summary

Fix ConnectionResetError: [Errno 104] Connection reset by peer when creating highway-env environments via SubprocVecEnv (stable-baselines3) or any multiprocessing wrapper that uses the forkserver/spawn start method.

Root cause: SubprocVecEnv defaults to forkserver when available. Child processes started via forkserver or spawn do not inherit the parent's import highway_env, so gymnasium.register() calls in highway_env/__init__.py never execute in the subprocess. When gym.make("highway-fast-v0") runs in the child, the environment is not in the registry → NameNotFound → subprocess crash → ConnectionResetError in the parent.

Fix:

• Make _register_highway_envs() idempotent (skip if already registered) so that re-importing highway_env via gymnasium's module:env_name resolution does not raise duplicate-registration errors
• Add a gymnasium entry-point in pyproject.toml for potential future auto-discovery
• Users should use gym.make("highway_env:highway-fast-v0") (with module prefix) when using SubprocVecEnv — gymnasium's _find_spec imports the module in the subprocess before looking up the environment

Fixes #648

Before

=== Using bare env ID with SubprocVecEnv ===

Process ForkServerProcess-1:
Traceback (most recent call last):
  File "multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "stable_baselines3/common/vec_env/subproc_vec_env.py", line 29, in _worker
    env = _patch_env(env_fn_wrapper.var())
  File "stable_baselines3/common/env_util.py", line 95, in _init
    env = gym.make(env_id, **kwargs)
  File "gymnasium/envs/registration.py", line 681, in make
    env_spec = _find_spec(id)
  File "gymnasium/envs/registration.py", line 526, in _find_spec
    _check_version_exists(ns, name, version)
  File "gymnasium/envs/registration.py", line 392, in _check_version_exists
    _check_name_exists(ns, name)
  File "gymnasium/envs/registration.py", line 369, in _check_name_exists
    raise error.NameNotFound(
gymnasium.error.NameNotFound: Environment `highway-fast` doesn't exist.
Process ForkServerProcess-2:
Traceback (most recent call last):
  ...
gymnasium.error.NameNotFound: Environment `highway-fast` doesn't exist.

ConnectionResetError: [Errno 104] Connection reset by peer

After

=== Using module:env_name syntax with SubprocVecEnv ===

gymnasium supports "module:env_name" syntax which imports the module
in the subprocess before looking up the environment.

SubprocVecEnv created: obs shape=(2, 5, 5)
5 steps completed successfully
Environment closed cleanly

SUCCESS: No ConnectionResetError!

============================= test session starts ==============================
platform linux -- Python 3.12.12, pytest-9.0.2, pluggy-1.6.0

tests/envs/test_multiprocessing.py::test_env_in_subprocess[forkserver-highway_env:highway-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[forkserver-highway_env:highway-fast-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[forkserver-highway_env:merge-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[forkserver-highway_env:roundabout-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[forkserver-highway_env:intersection-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[forkserver-highway_env:parking-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[spawn-highway_env:highway-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[spawn-highway_env:highway-fast-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[spawn-highway_env:merge-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[spawn-highway_env:roundabout-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[spawn-highway_env:intersection-v0] PASSED
tests/envs/test_multiprocessing.py::test_env_in_subprocess[spawn-highway_env:parking-v0] PASSED

============================= 12 passed in 15.11s ==============================

============================= full test suite ==============================
73 passed, 1 skipped, 0 failed in 37.31s

Test plan

• 12 new multiprocessing tests: forkserver and spawn × 6 env IDs
• Full test suite: 73 passed, 1 skipped, 0 failed (no regression)
• Manual verification: SubprocVecEnv with stable-baselines3 works end-to-end

[eleurent]

Neat, thanks!