Add Fair Lock Recipe?

[grantjenks]

I'm not convinced of its usefulness but here's a prototype. I'm not sure how to handle two scenarios:

1. The cache key expires and the get() returns None. Maybe we should try to add() and see if it's our turn now.
2. We get skipped! Depends how expiry is handled. Maybe this means getting a new ticket.

class FairLock(object):
    """Recipe for cross-process and cross-thread fair lock.

    # TODO: I don't think this is correct yet. How to handle expiry?

    Based on the "Ticket Lock" algorithm:
    https://en.wikipedia.org/wiki/Ticket_lock

    """
    def __init__(self):
        self._cache.add('tickets', 0)
        self._cache.add('serving', 1)

    def acquire(self):
        while True:
            ticket = self._cache.incr(
                self._tickets, expire=self._expire, tag=self._tag, retry=True,
            )
            while True:
                serving = self._cache.get(self._serving)
                if serving is None:
                    # Expiration occurred!
                    pass  # TODO
                elif serving < ticket:
                    time.sleep(0.001)
                elif serving == ticket:
                    return
                else:
                    assert serving > ticket
                    # We got skipped!
                    pass  # TODO

    def release(self):
        self._cache.incr(
            self._serving, expire=self._expire, tag=self._tag, retry=True,
        )

[grantjenks]

See #115