"""
Task lock
=========

This is an implementation of cache locking for tasks that need to be
run in blocking fashion. Ex: As a cron that should not start again
while still running.
"""


from dogpile.cache.api import NO_VALUE

from hashlib import md5

from labelaudit import config
from labelaudit.connectors import cache


class TaskLock:

    def __init__(self, task_name):
        task_hexdigest = md5(task_name.encode('utf-8')).hexdigest()
        # The cache key consists of the fully qualified task name and the
        # MD5 digest of the task name.
        self.lock_id = '{}-{}-lock-{}'.format(
            config.ENVIRONMENT, task_name, task_hexdigest)

    def acquire_task(self):
        """See if task can be acquired, or if it's already locked.

        If task is available, lock it, and return True.

        Return:
            boolean (True if task is available and False if it is not.)
        """

        if cache.region.get(self.lock_id) == NO_VALUE:
            cache.region.set(self.lock_id, 'locked')
            return True

        return False

    def delete(self):
        """Delete the cache lock.

        This operation is idempotent (can be called multiple times, or on a
        non-existent key, safely).
        """

        return cache.region.delete(self.lock_id)