7bd1bcd779
loading_shared_values/loading_cache'es iterators interface is dangerous/fragile because iterator doesn't "lock" the entry it points to and if there is a preemption point between aquiring non-end() iterator and its dereferencing the corresponding cache entry may had already got evicted (for whatever reason, e.g. cache size constraints or expiration) and then dereferencing may end up in a use-after-free and we don't have any protection against it in the value_extractor_fn today. And this is in addition to #8920. So, instead of trying to fix the iterator interface this patch kills two birds in a single shot: we are ditching the iterators interface completely and return value_ptr from find(...) instead - the same one we are returning from loading_cache::get_ptr(...) asyncronous APIs. A similar rework is done to a loading_shared_values loading_cache is based on: we drop iterators interface and return loading_shared_values::entry_ptr from find(...) instead. loading_cache::value_ptr already takes care of "lock"ing the returned value so that it would relain readable even if it's evicted from the cache by the time one tries to read it. And of course it also takes care of updating the last read time stamp and moving the corresponding item to the top of the MRU list. Fixes #8920 Signed-off-by: Vlad Zolotarov <vladz@scylladb.com> Message-Id: <20210817222404.3097708-1-vladz@scylladb.com>
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