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chaintopology: set a fee floor to avoid creating unrelayable txs. #1251

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Mar 20, 2018
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chaintopology: set a fee floor to avoid creating unrelayable txs. #1251

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chaintopology: set a fee floor to avoid creating unrelayable txs.
rustyrussell Mar 20, 2018
1d3b056
chaintopology: add debug message when we're adjusting due to feerate …
rustyrussell Mar 20, 2018
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53 changes: 50 additions & 3 deletions lightningd/chaintopology.c
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Original file line number Diff line number Diff line change
Expand Up @@ -268,6 +268,48 @@ static const char *feerate_name(enum feerate feerate)
/* Mutual recursion via timer. */
static void next_updatefee_timer(struct chain_topology *topo);

/* bitcoind considers 250 satoshi per kw to be the minimum acceptable fee:
* less than this won't even relay.
*/
#define BITCOIND_MINRELAYTXFEE_PER_KW 250
/*
* But bitcoind uses vbytes (ie. (weight + 3) / 4) for this
* calculation, rather than weight, meaning we can disagree since we do
* it sanely (as specified in BOLT #3).
*/
#define FEERATE_BITCOIND_SEES(feerate, weight) \
(((feerate) * (weight)) / 1000 * 1000 / ((weight) + 3))
/* ie. fee = (feerate * weight) // 1000
* bitcoind needs (worst-case): fee * 1000 / (weight + 3) >= 4000
*
* (feerate * weight) // 1000 * 1000 // (weight + 3) >= 4000
*
* The feerate needs to be higher for lower weight, and our minimum tx weight
* is 464 (version (4) + count_tx_in (1) + tx_in (32 + 4 + 1 + 4) +
* count_tx_out (1) + amount (8) + P2WSH (1 + 1 + 32) + witness 1 + 1 + <sig>
* + 1 + <key>). Assume it's 400 to give a significant safety margin (it
* only makes 1 difference in the result anyway).
*/
#define MINIMUM_TX_WEIGHT 400
/*
* This formula is satisfied by a feerate of 4030 (hand-search).
*/
#define FEERATE_FLOOR 253
static u32 feerate_floor(void)
{
/* Assert that bitcoind will see this as above minRelayTxFee */
BUILD_ASSERT(FEERATE_BITCOIND_SEES(FEERATE_FLOOR, MINIMUM_TX_WEIGHT)
>= BITCOIND_MINRELAYTXFEE_PER_KW);
/* And a lesser value won't do */
BUILD_ASSERT(FEERATE_BITCOIND_SEES(FEERATE_FLOOR-1, MINIMUM_TX_WEIGHT)
< BITCOIND_MINRELAYTXFEE_PER_KW);
/* And I'm right about it being OK for larger txs, too */
BUILD_ASSERT(FEERATE_BITCOIND_SEES(FEERATE_FLOOR, (MINIMUM_TX_WEIGHT*2))
>= BITCOIND_MINRELAYTXFEE_PER_KW);

return FEERATE_FLOOR;
}

/* We sanitize feerates if necessary to put them in descending order. */
static void update_feerates(struct bitcoind *bitcoind,
const u32 *satoshi_per_kw,
Expand All @@ -277,12 +319,17 @@ static void update_feerates(struct bitcoind *bitcoind,
bool changed = false;

for (size_t i = 0; i < NUM_FEERATES; i++) {
if (satoshi_per_kw[i] != topo->feerate[i])
u32 feerate = satoshi_per_kw[i];

if (feerate < feerate_floor())
feerate = feerate_floor();
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@ZmnSCPxj ZmnSCPxj Mar 20, 2018 •
edited
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Suggest adding a log_debug here that we are bumping the feerate up because hsyterical raisins. Or alternatively expand the existing log_debug message that we bumped it up and the actual feerate received from bitcoind.

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Done.


if (feerate != topo->feerate[i])
log_debug(topo->log, "%s feerate %u (was %u)",
feerate_name(i),
satoshi_per_kw[i], topo->feerate[i]);
feerate, topo->feerate[i]);
old_feerates[i] = topo->feerate[i];
topo->feerate[i] = satoshi_per_kw[i];
topo->feerate[i] = feerate;
}

for (size_t i = 0; i < NUM_FEERATES; i++) {
Expand Down