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There is a bug with the `StreamChangeCache` where it would incorrectly return that all entities had changed if asked for entities changed *since* the earliest stream position. Note that for streams we use the inequalities: `$min_stream_id < stream_id <= $max_stream_id`, i.e. when we ask the stream change cache for all things that have changed since `$stream_id` we don't care for events that happened *at* `$stream_id`. Specifically: `_earliest_known_stream_pos` is the position at which we know that we'll have entries for all changes since that point, we can use the cache for any stream IDs that equal `_earliest_known_stream_pos`. `_earliest_known_stream_pos` is set in three places: - On startup we set it either to: - the current maximum stream ID, with not prefilled values; or - the minimum of the latest N values we pulled from the DB - When we evict items from the bottom, we set it to the stream ID of the evicted items. This was changed in https://github.com/matrix-org/synapse/pull/14435, but I think we were overly conservative there. --------- Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
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Fix performance of streams that don't change often.
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