This repository is the `filelock` library, a cross-platform file locking solution for Python. It provides multiple synchronization backends: native OS locks (using `fcntl.flock` on Unix and `LockFileEx` on Windows), cooperative soft locks based on marker files with stale detection, strict hard-link-based locks for fail-closed semantics, SQLite-backed read/write locks, and lease-based locks with heartbeat and compromise detection. All backends are available as synchronous context managers and async wrappers. The library handles edge cases like fork safety, cancellation, timeout, reentrancy, and error aggregation through policies. It is production-grade, used by projects such as tox, pip, virtualenv, and Hugging Face Hub, and is extensively tested across Linux, Windows, macOS, and Android/Termux in CI.