Driver based schedulers performing explicit acknowledgements cannot acknowledge updates from HTTP based executors.

It seems that the agent code sets StatusUpdate>slave_id but does not set the TaskStatus>slave_id if it's not already set. On the driver, when we receive such a status update and if it has explicit ACK enabled, it would pass the TaskStatus to the scheduler. But, the scheduler has no way of acking this update due to slave_id not being present. Note that, implicit acknowledgements still work since they use the slave_id from StatusUpdate. Hence, we never noticed this in our tests as all of them use implicit acknowledgements on the driver.