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mfc: avoid off-by-one rounding errors when finding change fee

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Since we round down in `amount_tx_fee`, find the change fee as the
difference between what we've already paid and what the combined/total
fee would be if the change weight were also added.
This commit is contained in:
niftynei
2020-11-24 18:38:48 -06:00
committed by Christian Decker
parent 3793687550
commit b040a893ed
1 changed files with 13 additions and 2 deletions
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15
plugins/spender/multifundchannel.c
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@@ -868,7 +868,7 @@ static struct command_result *
handle_mfc_change(struct multifundchannel_command *mfc)
{
size_t change_weight;
struct amount_sat change_fee;
struct amount_sat change_fee, fee_paid, total_fee;
struct amount_sat change_min_limit;
/* Determine if adding a change output is worth it.
@@ -877,7 +877,18 @@ handle_mfc_change(struct multifundchannel_command *mfc)
*/
change_weight = bitcoin_tx_output_weight(
BITCOIN_SCRIPTPUBKEY_P2WPKH_LEN);
change_fee = amount_tx_fee(mfc->feerate_per_kw, change_weight);
/* To avoid 'off-by-one' errors due to rounding down
* (which we do in `amount_tx_fee`), we find the total calculated
* fees (estimated_weight + change weight @ feerate) and subtract
* the originally calculated fees (estimated_weight @ feerate) */
fee_paid = amount_tx_fee(mfc->feerate_per_kw,
mfc->estimated_final_weight);
total_fee = amount_tx_fee(mfc->feerate_per_kw,
mfc->estimated_final_weight + change_weight);
if (!amount_sat_sub(&change_fee, total_fee, fee_paid))
abort();
/* The limit is equal to the change_fee plus the dust limit. */
if (!amount_sat_add(&change_min_limit,
change_fee, chainparams->dust_limit))