How does Bitcoin Core's Coin Selection prevent change to be a dust output?

Question

Bitcoin Core's Coin Selection optimizes for minimal change outputs. How does Bitcoin Core prevent Change outputs of sizes below dust threshold from occurring?

This question is wrong, as Bitcoin Core actually creates a minimum change of 0.01 BTC, if it can't produce a direct match. — Murch, Oct 19 '16 at 22:55

score 2 · Accepted Answer · answered Sep 01 '15 at 11:58

See these lines:

// We do not move dust-change to fees, because the sender would end up paying more than requested.
// This would be against the purpose of the all-inclusive feature.
// So instead we raise the change and deduct from the recipient.
if (nSubtractFeeFromAmount > 0 && newTxOut.IsDust(::minRelayTxFee))
{
    CAmount nDust = newTxOut.GetDustThreshold(::minRelayTxFee) - newTxOut.nValue;
    newTxOut.nValue += nDust; // raise change until no more dust
    for (unsigned int i = 0; i < vecSend.size(); i++) // subtract from first recipient
    {
        if (vecSend[i].fSubtractFeeFromAmount)
        {
            txNew.vout[i].nValue -= nDust;
            if (txNew.vout[i].IsDust(::minRelayTxFee))
            {
                strFailReason = _("The transaction amount is too small to send after the fee has been deducted");
                return false;
            }
            break;
        }
    }
}

So Bitcoin core will give slightly less to the recipient rather than spend more than the payer originally requested, in the case of a dust-change output.

Then there's the question of whether or not this is a good policy... I don't believe it is. I think that as infrequently as dust change happens, just let miners have it and get a slightly faster confirmation time. — morsecoder, Sep 01 '15 at 11:59
It might not be good network policy, but it doesn't seem like users have an incentive to destroy dust UTXO's sent to them. — Nick ODell, Sep 02 '15 at 04:00

score 0 · Answer 2 · answered Oct 19 '16 at 22:57

Bitcoin Core actually creates a minimum change of 0.01 BTC (if it has sufficient funds), if it can't produce a direct match.

In the rare case that it doesn't have sufficient funds to produce a non-dust change output, it will do the weird stuff Stephen mentions.

How does Bitcoin Core's Coin Selection prevent change to be a dust output?

2 Answers2