fix: make experimental file locks importable on Windows

[JWilksBooth]

verifiers.envs.experimental.utils.file_locks and .git_checkout_cache import fcntl unconditionally, so importing verifiers (and therefore vf.load_environment and the entire vf-eval CLI) crashes on Windows with "ModuleNotFoundError: No module named 'fcntl'", surfaced misleadingly as a prompt to install verifiers[all].

Description

Guard the fcntl import and fall back to msvcrt byte-range locking on Windows:

• Shared locks degrade to exclusive locks (msvcrt has no shared locks) — strictly more conservative, preserves the worktree pruning contract.
• Contended nonblocking acquisitions raise BlockingIOError, matching flock(LOCK_EX | LOCK_NB) semantics, so _prune_stale_worktrees keeps skipping in-use checkouts instead of crashing (msvcrt raises bare OSError on contention, which the pruning code doesn't catch — the fallback translates it).
• No behavior change on POSIX; the fcntl paths are untouched.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

Verified on Windows 11 / Python 3.10.9 against this branch's modules loaded by path:

• exclusive_file_lock acquire / release / re-acquire; shared_file_lock; shared/exclusive_path_lock (nonblocking)
• Cross-process contention: second process holding the lock → nonblocking acquisition raises BlockingIOError as the pruning code expects
• git_checkout_cache._acquire_in_use_lock / _release_all_in_use_locks (process-lifetime lock, idempotent re-entry)
• Reproduced the original crash against a pristine 0.1.14 wheel on the same machine; with this patch the modules import and locks work
• py_compile on both files. Full uv run pytest not run locally (Windows box; POSIX paths untouched and byte-identical in behavior).

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (N/A — no user-facing docs cover these internals)
• My changes generate no new warnings
• Any dependent changes have been merged and published (N/A)

Additional Notes

• verifiers/v1/runtimes/limiters.py also has a module-level import fcntl, but only loads with the v1 prime/modal runtimes (off the default import path) — left untouched; happy to guard it the same way if wanted.
• Known related Windows issue, not addressed here: the ZMQ env server never becomes healthy on Windows, so vf-eval needs --disable-env-server. With that flag plus this patch, vf-eval runs end to end on Windows.

---

Note

Medium Risk
Changes cross-process locking semantics on Windows (shared locks become exclusive) in git checkout pruning and experimental env utilities; POSIX behavior is untouched but concurrency contracts differ on the new platform.

Overview
Fixes Windows import crashes (ModuleNotFoundError: fcntl) for verifiers.envs.experimental.utils.file_locks and git_checkout_cache, which blocked vf.load_environment and the eval CLI on that platform.

file_locks.py now probes fcntl at import time and, when missing, uses msvcrt byte-range locking via a new _msvcrt_file_lock helper wired into shared_file_lock and exclusive_file_lock. On Windows, shared locks become exclusive (no shared mode in msvcrt), which the PR treats as stricter but safe. Nonblocking contention is normalized to BlockingIOError, matching flock(LOCK_NB) so callers like _prune_stale_worktrees keep skipping in-use checkouts instead of failing on bare OSError.

git_checkout_cache.py applies the same _HAVE_FCNTL guard for process-lifetime in-use locks in _acquire_in_use_lock, using msvcrt.LK_LOCK when fcntl is unavailable. POSIX paths are unchanged when fcntl imports successfully.

^{Reviewed by Cursor Bugbot for commit 0c30716. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix file lock imports in experimental utils to work on Windows

• Replaces unconditional fcntl imports in file_locks.py and git_checkout_cache.py with try/except blocks that fall back to msvcrt on Windows, gated by a _HAVE_FCNTL flag.
• Adds a _msvcrt_file_lock context manager that implements byte-range locking via msvcrt.locking, with blocking and nonblocking modes and BlockingIOError on contention to mirror fcntl semantics.
• Behavioral Change: on Windows, both shared_file_lock and exclusive_file_lock use exclusive 1-byte locks since msvcrt has no shared lock primitive.

^{Macroscope summarized 0c30716.}