ZIP 401 uses serialized size to calculate cost but the zcashd implementation uses RecursiveDynamicUsage

[daira]

I just noticed that our implementation of ZIP 401 (mempool size limiting) uses the RecursiveDynamicUsage of each transaction to calculate its cost, not the serialized size:

WeightedTxInfo WeightedTxInfo::from(const CTransaction& tx, const CAmount& fee)
{
    size_t memUsage = RecursiveDynamicUsage(tx);
    int64_t cost = std::max((int64_t) memUsage, (int64_t) MIN_TX_COST);
    int64_t evictionWeight = cost;
    if (fee < DEFAULT_FEE) {
        evictionWeight += LOW_FEE_PENALTY;
    }
    return WeightedTxInfo(tx.GetHash(), TxWeight(cost, evictionWeight));
}

The ZIP specifies to use the serialized size.

It looks like the implementation has been like that since ZIP 401 was first implemented, and I even made a comment on the relevant line, so I'm surprised that I didn't notice the discrepancy then. The implementation using dynamic usage might be better than what is specified, but if so then we should make an explicit decision about that and update the ZIP accordingly.

[daira]

This interacts with #565, because when we chose 4000 (bytes) for the minimum transaction cost for mempool limiting, we were thinking of it as a minimum relative to the serialized size, not relative to the size in memory.

[daira]

I also notice that CTxMemPoolEntry uses RecursiveDynamicUsage(*tx) + memusage::DynamicUsage(tx). Should the cost calculation be doing the same? Does DEFAULT_MEMPOOL_TOTAL_COST_LIMIT need to be adjusted?