Replaces fcntl with filelock to avoid windows import issues

[AntoineRichard]

Description

The isaaclab.sim.spawners.from_files module imports fcntl unconditionally at module load. fcntl is Unix-only, so on Windows the import fails with:

ModuleNotFoundError: No module named 'fcntl'

This breaks any Windows usage of the spawner — including single-GPU runs that never take the lock path. Reproducer reported by a user (IsaacLab develop @ b258e87, IsaacSim 6.0.0rc41):

python scripts/reinforcement_learning/rl_games/train.py --task=Isaac-Cartpole-v0 --headless --video --video_length 100 --video_interval 500 --max_iterations 5

fcntl was introduced in #5032 to serialize USD download/stage composition across distributed ranks (preventing segfaults in Sdf_CrateFile::_MmapStream::Read on shared cached USD files). The intent is correct — only the implementation is Unix-only.

Change

• Replace fcntl.flock with filelock.FileLock, which uses fcntl on POSIX and msvcrt on Windows.
• Use contextlib.nullcontext for the single-rank path so the lock file is only created when actually needed (i.e., LOCAL_WORLD_SIZE > 1).
• Drop the manual try/finally and the # noqa: SIM115 on the bare open(...) — the with FileLock(...) form is exception-safe.
• Declare filelock in source/isaaclab/setup.py::INSTALL_REQUIRES. It was previously only transitively available via transformers → huggingface_hub; making it a direct dep so we don't depend on a transitive chain that could change.

The lock semantics are unchanged: an exclusive advisory lock on <tempdir>/isaaclab_usd_spawn.lock, held only while LOCAL_WORLD_SIZE > 1.

Original lock implementation: @ooctipus (#5032) — tagging for review since this changes the locking primitive.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Screenshots

N/A — import-time failure on Windows; no UI surface.

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation (N/A — internal change)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works — the failure is import fcntl at module load on Windows; a regression test would require a Windows CI runner, which IsaacLab does not currently exercise. Locally verified the module imports and FileLock resolves to filelock._unix.UnixFileLock on Linux (and would resolve to WindowsFileLock on Windows).
• I have added a changelog fragment under source/<pkg>/changelog.d/ for every touched package (do not edit CHANGELOG.rst or bump extension.toml — CI handles that)
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Greptile Summary

This PR fixes a Windows import failure in isaaclab.sim.spawners.from_files by swapping the Unix-only fcntl module for the cross-platform filelock library, and explicitly adding filelock to INSTALL_REQUIRES.

• _spawn_from_usd_file now uses FileLock (blocking, exclusive) when LOCAL_WORLD_SIZE > 1 and nullcontext() otherwise, preserving the original serialization semantics while avoiding lock-file creation on single-rank runs.
• try/finally manual unlock is removed in favour of the context manager's __exit__, which is exception-safe and cleaner.

Confidence Score: 5/5

The change is a minimal, well-scoped replacement of a Unix-only primitive with an equivalent cross-platform one; no behavioral differences are introduced on existing POSIX systems.

Both the locking path (FileLock — blocking, exclusive by default, released via __exit__) and the no-op path (nullcontext) correctly mirror the original code's intent. Exception safety is actually improved over the manual try/finally. The only loose end is the absence of a version floor on filelock, but the library has a stable API and this matches the style of other entries in INSTALL_REQUIRES.

No files require special attention; all three changed files are straightforward.

Important Files Changed

Filename	Overview
source/isaaclab/isaaclab/sim/spawners/from_files/from_files.py	Replaces Unix-only fcntl.flock with cross-platform filelock.FileLock; uses nullcontext for single-rank path — logic and exception safety are preserved.
source/isaaclab/setup.py	Adds filelock as an explicit install requirement; no version constraint specified (matches the style of most other entries in the list).
source/isaaclab/changelog.d/antoiner-fix-from-files-windows-filelock.rst	New changelog fragment documenting the Windows import fix and the added filelock dependency.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[_spawn_from_usd_file called] --> B{LOCAL_WORLD_SIZE > 1?}
    B -- Yes --> C[lock = FileLock\nisaaclab_usd_spawn.lock]
    B -- No --> D[lock = nullcontext]
    C --> E[with lock: acquire exclusive file lock]
    D --> E
    E --> F{file_status == 2?}
    F -- Yes --> G[retrieve_file_path\nforce_download=False]
    F -- No --> H[get_current_stage]
    G --> H
    H --> I{prim exists at prim_path?}
    I -- No --> J[create_prim]
    I -- Yes --> K[log warning]
    J --> L[release lock / exit context]
    K --> L
    L --> M[modify variants, rigid body,\ncollision, mass props...]

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_{Reviews (1): Last reviewed commit: "Fix from_files spawner import on Windows" | Re-trigger Greptile}