## 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>
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