import queue

from functools import wraps

from concurrent.futures import Executor, Future, CancelledError


class AsyncExecutorWrapper(object):
  """ This class will wrap a syncronous library transparently in a way which
      will move all calls off to an asynchronous Executor, and will change all
      returned values to be Future objects.
  """
  SYNC_FLAG_FIELD = '__AsyncExecutorWrapper__sync__'

  def __init__(self, delegate, executor):
    """ Wrap the specified synchronous delegate instance, and submit() all
        method calls to the specified Executor instance.
    """
    self._delegate = delegate
    self._executor = executor

  def __getattr__(self, attr_name):
    maybe_callable = getattr(self._delegate, attr_name) # Will raise proper attribute error
    if callable(maybe_callable):
      # Build a callable which when executed places the request
      # onto a queue
      @wraps(maybe_callable)
      def wrapped_method(*args, **kwargs):
        if getattr(maybe_callable, self.SYNC_FLAG_FIELD, False):
          sync_result = Future()
          try:
            sync_result.set_result(maybe_callable(*args, **kwargs))
          except Exception as ex:
            sync_result.set_exception(ex)
          return sync_result

        try:
          return self._executor.submit(maybe_callable, *args, **kwargs)
        except queue.Full as ex:
          queue_full = Future()
          queue_full.set_exception(ex)
          return queue_full

      return wrapped_method
    else:
      return maybe_callable

  @classmethod
  def sync(cls, f):
    """ Annotate the given method to flag it as synchronous so that AsyncExecutorWrapper
        will return the result immediately without submitting it to the executor.
    """
    setattr(f, cls.SYNC_FLAG_FIELD, True)
    return f


class NullExecutor(Executor):
  """ Executor instance which always returns a Future completed with a
      CancelledError exception. """
  def submit(self, _, *args, **kwargs):
    always_fail = Future()
    always_fail.set_exception(CancelledError('Null executor always fails.'))
    return always_fail