KAFKA-12427: Don't update connection idle time for muted connections

[splett2]

Selector.poll() will always call pollSelectionKeys() for channels with buffered data. pollSelectionKeys() will always update connection last idle time, even if the channel is muted and we don't actually read from the channel.

There is an existing unit test idleExpiryWithBufferedReceives that fails to catch this behavior because the MockTime object used in the test is updated in a large enough increment to expire a connection between calls to poll(). After updating the test to advance time in smaller increments, the test fails without the Selector change.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[splett2]

cc @rajinisivaram for reviews/thoughts

This approach is fairly simple, and we still end up calling pollSelectionKeys() with muted channels. An alternative that I was thinking of would be to remove a kafka channel from keysWithBufferedReads when explicitly muting, and then checking whether the channel has buffered data when unmuting.

[rajinisivaram]

@splett2 Thanks for the PR. I think it would be better to store channels in keysWithBufferedReads only when the channel is not explicitly muted since we care about this state only when we are ready to read from the channel. So we could check if channel is explicitly muted when adding to keysWithBufferedReads and add the key to keysWithBufferedReads when unmuting. This would avoid processing the channel unnecessarily.

[rajinisivaram]

This is limiting the times when we perform the update. For example when sending response, the channel is active, but explicitly muted. Don't think we want to skip update in this case.

[splett2]

@rajinisivaram
thanks for the review, that makes sense to me.

[rajinisivaram]

@splett2 Thanks for the update, LGTM. Merging to trunk.