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

Code before:
            } 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.
            }
        }
    }
    hashBlock = hashBlockIn;
    return true;
}

bool CCoinsViewCache::Flush() {
    bool fOk = base->BatchWrite(cacheCoins, hashBlock, /*erase*/ true);
    cacheCoins.clear();
    cachedCoinsUsage = 0;
    return fOk;
}

bool CCoinsViewCache::Sync()
{
    bool fOk = base->BatchWrite(cacheCoins, hashBlock, /*erase*/ false);
    // Instead of clearing `cacheCoins` as we would in Flush(), just clear the
    // FRESH/DIRTY flags of any coin that isn't spent.
    for (auto it = cacheCoins.begin(); it != cacheCoins.end(); ) {
        if (it->second.coin.IsSpent()) {
            cachedCoinsUsage -= it->second.coin.DynamicMemoryUsage();
            it = cacheCoins.erase(it);
        } else {
            it->second.flags = 0;
            ++it;
        }
    }
    return fOk;
}

void CCoinsViewCache::Uncache(const COutPoint& hash)
{
    CCoinsMap::iterator it = cacheCoins.find(hash);

Review comment (performance):
The following `TODO` comment in `src/validation.cpp` was removed in #23738. Making a note here so the `TODO` can be either be done in this PR or in a follow-up.
```
    coins_cache.Flush(); // TODO: if #17487 is merged, add erase=false here for better performance.
```


Code after:
        } 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.
            }
        }
    }
    hashBlock = hashBlockIn;
    return true;
}

bool CCoinsViewCache::Flush() {
    bool fOk = base->BatchWrite(cacheCoins, hashBlock, /*erase=*/ true);
    cacheCoins.clear();
    cachedCoinsUsage = 0;
    return fOk;
}

bool CCoinsViewCache::Sync()
{
    bool fOk = base->BatchWrite(cacheCoins, hashBlock, /*erase=*/ false);
    // Instead of clearing `cacheCoins` as we would in Flush(), just clear the
    // FRESH/DIRTY flags of any coin that isn't spent.