[GitHub] [incubator-pinot] sunithabeeram opened a new pull request #3942: Set processingException when all queried segments cannot be acquired

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sunithabeeram opened a new pull request #3942: Set processingException when all queried segments cannot be acquired
URL: https://github.com/apache/incubator-pinot/pull/3942
 
 
   **Motivation**: We have seen cases where due to bugs or delays wrt EV processing on the broker, broker sends a list of segments to the servers that are not hosted by it. This is technically a partial-response, but we currently don't indicate it as such. We would like to track this to the most extent possible.
   
   **Details**: There are cases where segments can be legitimately missing on the server; the known reason for this is when segments are deleted (either through retention or manually) and the broker's EV isn't updated yet. To not count partial responses in this scenario, we maintain an in-memory cache of deleted segment names. If a recently deleted segment is queried, we check our cache and not count it as an error if the segment-name exists in the cache. 
   NOTE: This can still miss cases where the server is restarted in the window between deletion and broker EV update. However, the chances of this are deemed low (and cost of catching this error deemed high).
   
   **Testing done**: Added unit tests to cover the new paths.
   
   **Impact**: Once this change is rolled out, num-segments-missing server metric should report lower numbers; we should see partial-responses broker metric go up if we hit missing-segments issue on the server.

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