Repository: bitcoin/bitcoin
File: src/coins.cpp (C++)

Code before:
                if (erase) {
                    // The `move` call here is purely an optimization; we rely on the
                    // `mapCoins.erase` call in the `for` expression to actually remove
                    // the entry from the child map.
                    entry.coin = std::move(it->second.coin);
                } else {
                    entry.coin = it->second.coin;
                }
                cachedCoinsUsage += entry.coin.DynamicMemoryUsage();
                entry.flags = CCoinsCacheEntry::DIRTY;
                // We can mark it FRESH in the parent if it was FRESH in the child
                // Otherwise it might have just been flushed from the parent's cache
                // and already exist in the grandparent
                if (it->second.flags & CCoinsCacheEntry::FRESH) {
                    entry.flags |= CCoinsCacheEntry::FRESH;
                }
            }
        } else {
            // Found the entry in the parent cache
            if ((it->second.flags & CCoinsCacheEntry::FRESH) && !itUs->second.coin.IsSpent()) {
                // The coin was marked FRESH in the child cache, but the coin
                // exists in the parent cache. If this ever happens, it means
                // the FRESH flag was misapplied and there is a logic error in
                // the calling code.
                throw std::logic_error("FRESH flag misapplied to coin that exists in parent cache");
            }

            if ((itUs->second.flags & CCoinsCacheEntry::FRESH) && it->second.coin.IsSpent()) {
                // The grandparent cache does not have an entry, and the coin
                // has been spent. We can just delete it from the parent cache.
                cachedCoinsUsage -= itUs->second.coin.DynamicMemoryUsage();
                cacheCoins.erase(itUs);
            } else {
                // A normal modification.
                cachedCoinsUsage -= itUs->second.coin.DynamicMemoryUsage();
                if (erase) {
                    // The `move` call here is purely an optimization; we rely on the
                    // `mapCoins.erase` call in the `for` expression to actually remove
                    // the entry from the child map.
                    itUs->second.coin = std::move(it->second.coin);
                } else {
                    itUs->second.coin = it->second.coin;
                }
                cachedCoinsUsage += itUs->second.coin.DynamicMemoryUsage();
                itUs->second.flags |= CCoinsCacheEntry::DIRTY;
                // NOTE: It isn't safe to mark the coin as FRESH in the parent
                // cache. If it already existed and was spent in the parent
                // cache then marking it FRESH would prevent that spentness
                // from being flushed to the grandparent.
            }
        }

Review comment (performance):
Thinking about this more, is it not more than a pure optimization? If we copy instead of move, then we are increasing the memory footprint of our dbcache. For a large batch write won't all dirty entries exist in memory twice, potentially increasing the memory requirements of the node temporarily until the parent cache writes to disk?

Code after:

void CCoinsViewCache::SetBestBlock(const uint256 &hashBlockIn) {
    hashBlock = hashBlockIn;
}

bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn, bool erase) {
    for (CCoinsMap::iterator it = mapCoins.begin();
            it != mapCoins.end();
            it = erase ? mapCoins.erase(it) : std::next(it)) {
        // Ignore non-dirty entries (optimization).
        if (!(it->second.flags & CCoinsCacheEntry::DIRTY)) {
            continue;
        }
        CCoinsMap::iterator itUs = cacheCoins.find(it->first);
        if (itUs == cacheCoins.end()) {
            // The parent cache does not have an entry, while the child cache does.
            // We can ignore it if it's both spent and FRESH in the child
            if (!(it->second.flags & CCoinsCacheEntry::FRESH && it->second.coin.IsSpent())) {
                // Create the coin in the parent cache, move the data up
                // and mark it as dirty.
                CCoinsCacheEntry& entry = cacheCoins[it->first];
                if (erase) {
                    // The `move` call here is purely an optimization; we rely on the
                    // `mapCoins.erase` call in the `for` expression to actually remove
                    // the entry from the child map.
                    entry.coin = std::move(it->second.coin);
                } else {
                    entry.coin = it->second.coin;
                }
                cachedCoinsUsage += entry.coin.DynamicMemoryUsage();
                entry.flags = CCoinsCacheEntry::DIRTY;
                // We can mark it FRESH in the parent if it was FRESH in the child
                // Otherwise it might have just been flushed from the parent's cache
                // and already exist in the grandparent
                if (it->second.flags & CCoinsCacheEntry::FRESH) {
                    entry.flags |= CCoinsCacheEntry::FRESH;
                }
            }
        } else {
            // Found the entry in the parent cache
            if ((it->second.flags & CCoinsCacheEntry::FRESH) && !itUs->second.coin.IsSpent()) {
                // The coin was marked FRESH in the child cache, but the coin
                // exists in the parent cache. If this ever happens, it means
                // the FRESH flag was misapplied and there is a logic error in
                // the calling code.
                throw std::logic_error("FRESH flag misapplied to coin that exists in parent cache");
            }

            if ((itUs->second.flags & CCoinsCacheEntry::FRESH) && it->second.coin.IsSpent()) {
                // The grandparent cache does not have an entry, and the coin
                // has been spent. We can just delete it from the parent cache.