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## Issue Addressed #4281 ## Proposed Changes - Change `ShufflingCache` implementation from using `LruCache` to a custom cache that removes entry with lowest epoch instead of oldest insertion time. - Protect the "enshrined" head shufflings when inserting new committee cache entries. The shuffling ids matching the head's previous, current, and future epochs will never be ejected from the cache during `Self::insert_cache_item`. ## Additional Info There is a bonus point on shuffling preferences in the issue description that hasn't been implemented yet, as I haven't figured out a good way to do this: > However I'm not convinced since there are some complexities around tie-breaking when two entries have the same epoch. Perhaps preferring entries in the canonical chain is best? We should be able to check if a block is on the canonical chain by: ```rust canonical_head .fork_choice_read_lock() .contains_block(root) ``` However we need to interleave the shuffling and fork choice locks, which may cause deadlocks if we're not careful (mentioned by @paulhauner). Alternatively, we could use the `state.block_roots` field of the `chain.canonical_head.snapshot.beacon_state`, which avoids deadlock but requires more work. I'd like to get some feedback on review & testing before I dig deeper into the preferences stuff, as having the canonical head preference may already be quite useful in preventing the issue raised. Co-authored-by: Jimmy Chen <jimmy@sigmaprime.io>