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" The original motivation for the "utils: introduce a loading_shared_values" series was a hinted handoff work where I needed an on-demand asynchronously loading key-value container (a replica address to a commitlog instance map). It turned out that we already have the classes that do almost what I needed: - utils::loading_cache - sstables::shared_index_lists Therefore it made sense to find a common ground, unify this functionality and reuse the code both in the classes above and in the new hinted handoff code. This series introduces the utils::loading_shared_values that generalizes the sstables::shared_index_lists API on top of bi::unordered_set with the rehashing logic from the utils::loading_cache triggered by an addition of an entry to the set (PATCH1). Then it reworks the sstables::shared_index_lists and utils::loading_cache on top of the new class (PATCH2 and PATCH3). PATCH4 optimizes the loading_cache for the long timer period use case. But then we have discovered that we have another "customer" for the loading_cache. Apparently our prepared statements cache had a birth flaw - it was unlimited in size - unless the corresponding keyspace and/or table are modified/dropped the entries are never evicted. We clearly need to limit its size and it would also make sense to evict the cache entries that haven't been used long enough. This seems like a perfect match for a utils::loading_cache except for prepared statements don't need to be reloaded after they are created. Patches starting from PATCH5 are dealing with adding the utils::loading_cache the missing functionality (like making the "reloading" conditional and adding the synchronous methods like find(key)) and then transitioning the CQL and Thrift prepared statements caches to utils::loading_cache. This also fixes #2474." * 'evict_unused_prepared-v5' of https://github.com/vladzcloudius/scylla: tests: loading_cache_test: initial commit cql3::query_processor: implement CQL and Thrift prepared statements caches using cql3::prepared_statements_cache cql3: prepared statements cache on top of loading_cache utils::loading_cache: make the size limitation more strict utils::loading_cache: added static_asserts for checking the callbacks signatures utils::loading_cache: add a bunch of standard synchronous methods utils::loading_cache: add the ability to create a cache that would not reload the values utils::loading_cache: add the ability to work with not-copy-constructable values utils::loading_cache: add EntrySize template parameter utils::loading_cache: rework on top of utils::loading_shared_values sstables::shared_index_list: use utils::loading_shared_values utils: introduce loading_shared_values