[1.11.x] backport of a series of blocking query readability and functionality improvements #12454
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To remove the TODO, and make it more readable. In general this reduces the scope of variables, making them easier to reason about. It also introduces more early returns so that we can see the flow from the structure of the function.
This safeguard should be safe to apply in general. We are already applying it to non-blocking queries that call blockingQuery, so it should be fine to apply it to others.
This helps keep the logic in blockingQuery more focused. In the future we may have a separate struct for RPC queries which may allow us to move this off of Server.
Remove some unnecessary comments around query_blocking metric. The only line that needs any comments in the atomic decrement. Cleanup the block and return comments and logic. The old comment about AbandonCh may have been relevant before, but it is expected behaviour now. The logic was simplified by inverting the err condition.
Follow the Go convention of accepting a small interface that documents the methods used by the function. Clarify the rules for implementing a query function passed to blockingQuery.
This test shows how blocking queries are not efficient when the query returns no results. The test fails with 100+ calls instead of the expected 2. This test is still a bit flaky because it depends on the timing of the writes. It can sometimes return 3 calls. A future commit should fix this and make blocking queries even more optimal for not-found results.
By using the query results as state. Blocking queries are efficient when the query matches some results, because the ModifyIndex of those results, returned as queryMeta.Mindex, will never change unless the items themselves change. Blocking queries for non-existent items are not efficient because the queryMeta.Index can (and often does) change when other entities are written. This commit reduces the churn of these queries by using a different comparison for "has changed". Instead of using the modified index, we use the existence of the results. If the previous result was "not found" and the new result is still "not found", we know we can ignore the modified index and continue to block. This is done by setting the minQueryIndex to the returned queryMeta.Index, which prevents the query from returning before a state change is observed.
Any query that returns a list of items is not part of this commit.
Otherwise when the query times out we might incorrectly send a value for the reply, when we should send an empty reply. Also document errNotFound and how to handle the result in that case.
rboyer
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kisunji
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Feb 25, 2022
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🍒 If backport labels were added before merging, cherry-picking will start automatically. To retroactively trigger a backport after merging, add backport labels and re-run https://circleci.com/gh/hashicorp/consul/597253. |
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This is an omnibus PR including all blocking query fixes found in: