from __future__ import annotations

import fcntl
import os
import time
from collections.abc import Iterator
from contextlib import contextmanager
from pathlib import Path


class LockUnavailable(RuntimeError):
    """Raised when another process already owns the sync lock."""


@contextmanager
def exclusive_file_lock(path: Path) -> Iterator[None]:
    path.parent.mkdir(parents=True, exist_ok=True)
    with path.open("a+", encoding="utf-8") as lock_file:
        try:
            fcntl.flock(lock_file.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
        except BlockingIOError as exc:
            raise LockUnavailable(f"Lock is already held: {path}") from exc

        try:
            lock_file.seek(0)
            lock_file.truncate()
            lock_file.write(f"pid={os.getpid()} time={int(time.time())}\n")
            lock_file.flush()
            yield
        finally:
            fcntl.flock(lock_file.fileno(), fcntl.LOCK_UN)