Releasing the messages immediately if prefetch is disabled and if there is no (active) downstream

[anuchandy]

Description

• Based on the implementation thoughts from Connie on this pr Releasing expired messages instead of serving them downstream. #26894
• Releasing the messages immediately if there is no active downstream, which reduces the chances of serving expired messages, see Do not serve expired messages to the user #26632

What is not in the scope of this pr

• There might be corner cases where we benefit from doing the release (such as when the client closes/terminates), but those are not in the scope of this pr. This pr explicitly targets the case when there is no active downstream.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Title of the pull request is clear and informative.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

[conniey]

Thanks for fixing my PR @anuchandy ! One test case I had troubles testing was whether it released messages if they expired while the same current work was polling and that the next work item would get new messages...

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[anuchandy]

Hi @conniey, I've added a UT. Regarding the expired messages, I later learned from Ramya that we could not release if it's already expired, so release should happen asap if there are no active receivers.

[anuchandy]

The below diagram shows what is with this new release feature, adding it for future reference -

quite_period: a period in the sliding window with no active receivers, any messages during this duration are released.

As illustrated, we continue to use the sliding window, i.e., the prefetch queue is shared, and there is no correlation such as this set of messages belonging to this specific receive call. By design, a receive call can receive messages that arrived as a result of an earlier credit call (e.g., receive_3); those messages are unlikely to be expired as they just arrived