import contextlib
import datetime
import logging
from pathlib import Path

logger = logging.getLogger(__name__)


class LockAcquireException(Exception):
    """Raised when a lock cannot be acquired (held and not yet expired)."""
    pass


@contextlib.contextmanager
def try_lock(lock_path: Path, ttl: datetime.timedelta = None):
    logger.info(f'Trying to acquire lock at {lock_path}')
    now = datetime.datetime.now()
    if lock_path.exists():
        created_at = datetime.datetime.fromtimestamp(lock_path.stat().st_mtime)
        logger.info(
            f'Lockfile {lock_path} already exists. '
            f'Created at {created_at}, current time {now}, ttl {ttl}.')
        if not ttl or (now - created_at < ttl):
            logger.info(
                'Lockfile is not expired. raising LockAcquireException')
            raise LockAcquireException()
        logger.info('Lockfile is expired... Continue with lock')

    logger.info(f'Acquired lock at {lock_path}')
    lock_path.write_text(f'Locked at {now}')

    try:
        yield lock_path
    finally:
        lock_path.unlink(missing_ok=True)
        logger.info(f'Released lock at {lock_path}')