Major Description
The JavaDoc for commitSync() states the following:
Note that asynchronous offset commits sent previously with the
Unknown macro: {@link #commitAsync(OffsetCommitCallback)}(or similar) are guaranteed to have their callbacks invoked prior to completion of this method.
But should we happen to call the method with an empty offset map
(i.e. commitSync(Collections.emptyMap())) the callbacks for any incomplete async commits will not be invoked because of an early return in ConsumerCoordinator.commitOffsetsSync() when the input map is empty.
If users are doing manual offset commits and relying on commitSync as a barrier for in-flight async commits prior to a rebalance, this could be an important (though somewhat implementation-dependent) detail.