// Gemma 4 31B QAT

▮ PILLAR BREAKDOWN

▮ CRITICAL BUGS

• Capacity breach: eviction does self.freq_map[self.min_freq].pop_tail() with NO guard that the bucket exists or is non-empty, and never prunes emptied freq buckets. After manual deletes empty the min-tier, pop_tail returns None silently -> eviction fails -> cache grows PAST capacity. This is the exact stale-min_freq capacity-breach the rubric flags.
• Non-conformant transaction API: Transaction has NO commit() or rollback() method (both required by spec). Commit happens via a separate cache.apply_transaction_changes(tx._state) — wrong surface; the test only passes because it uses this internal path.
• Isolation leak: Transaction.get falls back to the public cache.get, which bumps global frequency before commit — uncommitted tx reads alter global eviction order.
• Duplicated eviction logic in apply_transaction_changes re-introduces the stale-min_freq bug at line 211.
• Uses time.time() (system clock) via _get_now() instead of time.monotonic() — NTP jumps corrupt TTL (though _get_now is a clean single fix point).
• No __slots__ on Node/DoublyLinkedList/LFUCache/TransactionState/Transaction.
• Background sweep does list(self.cache.keys()) = O(N) per interval.
• Tests pass but use the non-spec commit path and don't probe capacity breach or isolation leak.

▮ RECOMMENDED USE

▮ REFACTORED PATCH

# FIX 1 (capacity breach): guard + prune empty buckets on eviction:
while self.min_freq in self.freq_map and self.freq_map[self.min_freq].size == 0:
    del self.freq_map[self.min_freq]
    self.min_freq += 1
    if not self.freq_map:
        break
if self.min_freq not in self.freq_map:
    return  # nothing to evict
evicted = self.freq_map[self.min_freq].pop_tail()
if evicted and self.freq_map[self.min_freq].size == 0:
    del self.freq_map[self.min_freq]

# FIX 2 (conformant API): add commit/rollback to Transaction:
async def commit(self):
    await self._cache.apply_transaction_changes(self._state)
    self._committed = True
def rollback(self):
    self._state.writes.clear()
    self._committed = True

# FIX 3 (isolation leak): tx.get should use a read-only global lookup
# (no _update_freq), not the public cache.get.
# FIX 4: _get_now returns time.monotonic().
# FIX 5: add __slots__ to all node/list classes.