Ignore NO_OFFSET condition while commiting

[soumyajit-sahu]

While committing, we should ignore the NO_OFFSET error code. There is a note in the comments in the rdkafka.h file as follows, which corroborates this change: If no partitions had valid offsets to commit this callback will be called with \p err == RD_KAFKA_RESP_ERR__NO_OFFSET which is not to be considered an error.

[edenhill]

The reason for this behaviour is state synchronization: it allows the application to know when an async commit() operation has finished, regardless if it actually had to commit offsets or the requested offsets were only commited (NO_OFFSET).

[soumyajit-sahu]

We are calling the Consumer.Commit() synchronously to guarantee at-least once in our logic. I believe that a NO_OFFSET exception is raised when we don't have any new data in any partition. The Consumer.Commit() call should pass in this case too.

@edenhill if we shouldn't ignore the error in the SafeKafkaHandle.Commit(), then would you suggest to:

1. ignore it in the Consumer.Commit() or,
2. have the caller ignore it. In this case we can put a comment for Consumer.Commit() to document it just like it is done in rdkafka.h.

I would prefer the later to keep it alike to the c lib.

[edenhill]

Yep, I would go with option 2.

[soumyajit-sahu]

Thanks @edenhill . I have added a comment, and reverted the previous change.