A generator that yields exponential timeout values.

    Args:
        initial (float): The initial timeout.
        maximum (float): The maximum timeout.
        multiplier (float): The multiplier applied to the timeout.
        deadline (float): The overall deadline across all invocations.

    Yields:
        float: A timeout value.

def _exponential_timeout_generator(initial, maximum, multiplier, deadline):
    """A generator that yields exponential timeout values.

    Args:
        initial (float): The initial timeout.
        maximum (float): The maximum timeout.
        multiplier (float): The multiplier applied to the timeout.
        deadline (float): The overall deadline across all invocations.

    Yields:
        float: A timeout value.
    """
    if deadline is not None:
        deadline_datetime = datetime_helpers.utcnow() + datetime.timedelta(
            seconds=deadline
        )
    else:
        deadline_datetime = datetime.datetime.max

    timeout = initial
    while True:
        now = datetime_helpers.utcnow()
        yield min(
            # The calculated timeout based on invocations.
            timeout,
            # The set maximum timeout.
            maximum,
            # The remaining time before the deadline is reached.
            float((deadline_datetime - now).seconds),
        )
        timeout = timeout * multiplier