uint64_t getCounter() REQUIRES(mMutex) {
    static uint64_t counter = 0;
    return counter++;
}

getCounter() 维护 counter，它表示缓存的使用次序，用于 LRU 清除缓存。

void evictLeastRecentlyUsedBuffer() REQUIRES(mMutex) {
    auto itr = mBuffers.begin();
    uint64_t minCounter = itr->second;
    auto minBuffer = itr;
    itr++;

    while (itr != mBuffers.end()) {
        uint64_t counter = itr->second;
        if (counter < minCounter) {
            minCounter = counter;
            minBuffer = itr;
        }
        itr++;
    }
    uncacheLocked(minBuffer->first);
}