Adds support for fine-grained watches in blocking queries. #2671
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We can't actually return a fine-grained index from these tables unless support is added for tombstones. Otherwise, the index could slip backwards as things are deleted.
We always did an update before which caused excessive watch churn, even with our new fine-grained queries. This does a diff any only updates the node and service records if something actually changed.
Previously the blocking functions all closed over the state store from their first query, with would not have worked properly when a restore occurred. This makes sure they get a frest state store pointer each time, and that pointer is synchronized with the abandon watch.
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Amazing, good job @slackpad ! |
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This uses the newly-added watch support in go-memdb (via go-immutable-radix) to add fine-grained watches in blocking queries. This let us remove a bunch of hand-rolled code in the state store, and it means that when service X changes, it doesn't wake up callers blocking on service Y (fixes #1244). It also applies to all other areas of the state store.
In order to make this work, we also avoid no-op updates to node and service structures when only checks change. This helps prevent index churn on those upper structures and avoids extra unblocks.
The change in indexing did result in a slight change for KV watches, where there might be more spurious wakeups while waiting for a key to be created. This is the cheapest thing we can serve, so for now we will leave the code simple (it's at least a prefix watch), but we may want to consider a special case to make that more efficient if folks notice any performance impact from that.