kibertoad / layered-loader / 28658774958

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Fix race conditions and correctness issues in caching layers (#417)

* Fix race conditions and correctness issues in caching layers

Synthesizes PR #414 (invalidation/in-flight race audit) and PR #415
(correctness audit), combining best-of-breed fixes from both.

Race conditions between invalidation and in-flight loads:
- getAsyncOnly (flat + group) fences its write-back: the resolved value is
  persisted only if the dedup entry still belongs to that load, so any
  invalidation or forced write that evicted the entry cancels the write-back
  while awaiting callers still receive the value.
- Invalidation paths (invalidateCache/invalidateCacheFor/invalidateCacheForMany/
  invalidateCacheForGroup) evict running loads before and after the awaited
  async delete and clear the in-memory entry last, so a get racing a slow async
  delete cannot repopulate memory after the invalidation resolves.
- deleteGroupRunningLoad only drops the group entry when it still points at the
  settling load's map, so a load settling against a stale map cannot evict a
  newer load's map and break deduplication.

Correctness fixes:
- InMemoryCache.getMany / InMemoryGroupCache.getManyFromGroup treated cached
  falsy values (0, '', false, null) as misses via a truthy check; use an
  undefined check instead, consistent with single get and RedisCache.
- InMemoryGroupCache reads (getFromGroup, getManyFromGroup,
  getExpirationTimeFromGroup, deleteFromGroup) no longer insert empty groups
  into the LRU on a miss, which could evict a populated group's still-valid data.
- Loader/GroupLoader preemptive-refresh expiration lookups now have a .catch, so
  an async-cache error no longer becomes an unhandled promise rejection.
- RedisGroupCache.deleteGroup expires the group index with groupTtlInMsecs
  instead of the entry ttlInMsecs, consistent with setForGroup/setManyForGroup.

Redis notification consumers:
- Register the message listener before subscribing, so no message is dropped in
  the ... (continued)

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