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lightningd: rewrite anchor spend to use multiple UTXOs if needed.
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Closes: #6747
Changelog-EXPERIMENTAL: Fixed anchor spending to be able to use more than one UTXO.
Signed-off-by: Rusty Russell <[email protected]>
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@rustyrussell
rustyrussell committed Oct 24, 2023
1 parent f67c948 commit ee41768
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Showing 5 changed files with 113 additions and 94 deletions.
10 changes: 6 additions & 4 deletions bitcoin/tx.c
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Original file line number Diff line number Diff line change
Expand Up @@ -966,17 +966,19 @@ size_t bitcoin_tx_2of2_input_witness_weight(void)
);
}

size_t change_weight(void)
{
return bitcoin_tx_output_weight(chainparams->is_elements ? BITCOIN_SCRIPTPUBKEY_P2WPKH_LEN : BITCOIN_SCRIPTPUBKEY_P2TR_LEN);
}

struct amount_sat change_fee(u32 feerate_perkw, size_t total_weight)
{
size_t outweight;
struct amount_sat fee;

/* Must be able to pay for its own additional weight */
outweight = bitcoin_tx_output_weight(chainparams->is_elements ? BITCOIN_SCRIPTPUBKEY_P2WPKH_LEN : BITCOIN_SCRIPTPUBKEY_P2TR_LEN);

/* Rounding can cause off by one errors, so we do this */
if (!amount_sat_sub(&fee,
amount_tx_fee(feerate_perkw, outweight + total_weight),
amount_tx_fee(feerate_perkw, change_weight() + total_weight),
amount_tx_fee(feerate_perkw, total_weight)))
abort();
return fee;
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5 changes: 5 additions & 0 deletions bitcoin/tx.h
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Original file line number Diff line number Diff line change
Expand Up @@ -319,6 +319,11 @@ size_t bitcoin_tx_simple_input_weight(bool p2sh);
/* The witness for our 2of2 input (closing or commitment tx). */
size_t bitcoin_tx_2of2_input_witness_weight(void);

/**
* change_weight - what's the weight of a change output?
*/
size_t change_weight(void);

/**
* change_fee - what's the cost to add a change output to this tx?
* @feerate_perkw: feerate.
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4 changes: 2 additions & 2 deletions hsmd/libhsmd.c
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Original file line number Diff line number Diff line change
Expand Up @@ -511,8 +511,8 @@ static void sign_our_inputs(struct utxo **utxos, struct wally_psbt *psbt)
psbt_set_version(psbt, 0);
hsmd_status_failed(STATUS_FAIL_INTERNAL_ERROR,
"Received wally_err attempting to "
"sign utxo with key %s. PSBT: %s",
type_to_string(tmpctx, struct pubkey,
"sign input %zu with key %s. PSBT: %s",
j, type_to_string(tmpctx, struct pubkey,
&pubkey),
type_to_string(tmpctx, struct wally_psbt,
psbt));
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186 changes: 100 additions & 86 deletions lightningd/anchorspend.c
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,7 @@
#include <lightningd/lightningd.h>
#include <lightningd/log.h>
#include <lightningd/peer_control.h>
#include <wally_psbt.h>

/* This is attached to each anchor tx retransmission */
struct one_anchor {
Expand Down Expand Up @@ -209,15 +210,51 @@ struct anchor_details *create_anchor_details(const tal_t *ctx,
return adet;
}

static u32 commit_feerate(const struct local_anchor_info *info)
/* total_weight includes the commitment tx we're trying to push! */
static struct wally_psbt *anchor_psbt(const tal_t *ctx,
struct channel *channel,
struct one_anchor *anch,
struct utxo **utxos,
u32 feerate_target,
size_t total_weight)
{
u32 feerate;
struct lightningd *ld = channel->peer->ld;
struct wally_psbt *psbt;
struct amount_sat change, fee;
struct pubkey final_key;

/* Fee should not overflow! */
if (!amount_feerate(&feerate, info->commitment_fee, info->commitment_weight))
abort();
/* PSBT knows how to spend utxos. */
psbt = psbt_using_utxos(ctx, ld->wallet, utxos,
default_locktime(ld->topology),
BITCOIN_TX_RBF_SEQUENCE, NULL);

return feerate;
/* BOLT #3:
* #### `to_local_anchor` and `to_remote_anchor` Output (option_anchors)
*...
* The amount of the output is fixed at 330 sats, the default
* dust limit for P2WSH.
*/
psbt_append_input(psbt, &anch->info.anchor_point, BITCOIN_TX_RBF_SEQUENCE,
NULL, anch->adet->anchor_wscript, NULL);
psbt_input_set_wit_utxo(psbt, psbt->num_inputs - 1,
scriptpubkey_p2wsh(tmpctx, anch->adet->anchor_wscript),
AMOUNT_SAT(330));
psbt_input_add_pubkey(psbt, psbt->num_inputs - 1, &channel->local_funding_pubkey, false);

/* A zero-output tx is invalid: we must have change, even if not really economic */
change = psbt_compute_fee(psbt);
/* Assume we add a change output, what would the total fee be? */
fee = amount_tx_fee(feerate_target, total_weight + change_weight());
if (!amount_sat_sub(&change, change, fee)
|| amount_sat_less(change, chainparams->dust_limit)) {
change = chainparams->dust_limit;
}

bip32_pubkey(ld, &final_key, channel->final_key_idx);
psbt_append_output(psbt,
scriptpubkey_p2wpkh(tmpctx, &final_key),
change);
return psbt;
}

/* If it's possible and worth it, return signed tx. Otherwise NULL. */
Expand All @@ -227,59 +264,93 @@ static struct bitcoin_tx *spend_anchor(const tal_t *ctx,
{
struct lightningd *ld = channel->peer->ld;
struct utxo **utxos;
size_t weight;
struct amount_sat fee, diff, change;
size_t base_weight, weight;
struct amount_sat fee, diff;
struct bitcoin_tx *tx;
bool worthwhile;
struct wally_psbt *psbt;
struct amount_msat total_value;
struct pubkey final_key;
const u8 *msg;

/* Estimate weight of spend tx plus commitment_tx */
weight = bitcoin_tx_core_weight(2, 1)
+ bitcoin_tx_input_weight(false, bitcoin_tx_simple_input_witness_weight())
/* Estimate weight of spend tx plus commitment_tx (not including any UTXO we add) */
base_weight = bitcoin_tx_core_weight(2, 1)
+ bitcoin_tx_input_weight(false,
bitcoin_tx_input_sig_weight()
+ 1 + tal_bytelen(anch->adet->anchor_wscript))
+ bitcoin_tx_output_weight(BITCOIN_SCRIPTPUBKEY_P2WPKH_LEN)
+ anch->info.commitment_weight;

worthwhile = false;
total_value = AMOUNT_MSAT(0);
for (int i = tal_count(anch->adet->vals) - 1; i >= 0 && !worthwhile; --i) {
psbt = NULL;
for (int i = tal_count(anch->adet->vals) - 1; i >= 0; --i) {
const struct deadline_value *val = &anch->adet->vals[i];
u32 feerate;
u32 feerate, feerate_target;
struct wally_psbt *candidate_psbt;

/* Calculate the total value for the current deadline
* and all the following */
if (!amount_msat_add(&total_value, total_value, val->msat))
return NULL;

feerate = feerate_for_target(ld->topology, val->block);
/* Would the commit tx make that feerate by itself? */
if (commit_feerate(&anch->info) >= feerate)
feerate_target = feerate_for_target(ld->topology, val->block);

/* Ask for some UTXOs which could meet this feerate */
weight = base_weight;
utxos = wallet_utxo_boost(tmpctx,
ld->wallet,
get_block_height(ld->topology),
anch->info.commitment_fee,
feerate_target,
&weight);

/* Create a new candidate PSBT */
candidate_psbt = anchor_psbt(tmpctx, channel, anch, utxos, feerate_target, weight);
if (!candidate_psbt)
continue;

/* Get the fee required to meet the current block */
fee = amount_tx_fee(feerate, weight);
fee = psbt_compute_fee(candidate_psbt);

/* Is it even worth spending this fee to meet the deadline? */
if (!amount_msat_greater_sat(total_value, fee)) {
log_debug(channel->log,
"Not worth fee %s for %s commit tx to get %s in %u blocks at feerate %uperkw",
fmt_amount_sat(tmpctx, fee),
anch->commit_side == LOCAL ? "local" : "remote",
fmt_amount_msat(tmpctx, val->msat),
val->block, feerate);
break;
}

/* We already have part of the fee in commitment_tx. */
if (amount_sat_sub(&fee, fee, anch->info.commitment_fee)
&& amount_msat_greater_sat(total_value, fee)) {
worthwhile = true;
if (!amount_feerate(&feerate, fee, weight))
abort();

if (feerate < feerate_target) {
/* We might have had lower feerates which worked: only complain if
* we have *nothing* */
if (tal_count(utxos) == 0 && !psbt) {
log_unusual(channel->log,
"No utxos to bump commit_tx to feerate %uperkw!",
feerate_target);
break;
}

log_unusual(channel->log,
"We want to bump commit_tx to feerate %uperkw, but can only bump to %uperkw!",
feerate_target, feerate);
psbt = candidate_psbt;
/* We don't expect to do any better at higher feerates */
break;
}

log_debug(channel->log, "%s fee %s for %s commit tx to get %s in %u blocks at feerate %uperkw",
worthwhile ? "Worth" : "Not worth",
log_debug(channel->log, "Worth fee %s for %s commit tx to get %s in %u blocks at feerate %uperkw",
fmt_amount_sat(tmpctx, fee),
anch->commit_side == LOCAL ? "local" : "remote",
fmt_amount_msat(tmpctx, val->msat),
val->block, feerate);
psbt = candidate_psbt;
}

/* Not worth it? */
if (!worthwhile)
/* No psbt was worth it? */
if (!psbt)
return NULL;

/* Higher enough than previous to be valid RBF?
Expand All @@ -299,63 +370,6 @@ static struct bitcoin_tx *spend_anchor(const tal_t *ctx,
(fee.satoshis + anch->info.commitment_fee.satoshis) /* Raw: debug log */
* 1000 / weight);

/* FIXME: Use more than one utxo! */
utxos = tal_arr(tmpctx, struct utxo *, 1);
utxos[0] = wallet_find_utxo(utxos, ld->wallet,
get_block_height(ld->topology),
NULL,
0, /* FIXME: unused! */
0, false, NULL);
if (!utxos[0]) {
log_unusual(channel->log,
"We want to bump commit_tx fee, but no funds!");
return NULL;
}

/* FIXME: we get a random UTXO. We should really allow
* multiple UTXOs here */
if (amount_sat_less(utxos[0]->amount, fee)) {
log_unusual(channel->log,
"We want to bump commit_tx with fee %s, but utxo %s is only %s!",
fmt_amount_sat(tmpctx, fee),
type_to_string(tmpctx, struct bitcoin_outpoint,
&utxos[0]->outpoint),
fmt_amount_sat(tmpctx, utxos[0]->amount));
return NULL;
}

/* PSBT knows how to spend utxos. */
psbt = psbt_using_utxos(tmpctx, ld->wallet, utxos,
default_locktime(ld->topology),
BITCOIN_TX_RBF_SEQUENCE, NULL);

/* BOLT #3:
* #### `to_local_anchor` and `to_remote_anchor` Output (option_anchors)
*...
* The amount of the output is fixed at 330 sats, the default
* dust limit for P2WSH.
*/
psbt_append_input(psbt, &anch->info.anchor_point, BITCOIN_TX_RBF_SEQUENCE,
NULL, anch->adet->anchor_wscript, NULL);
psbt_input_set_wit_utxo(psbt, 1,
scriptpubkey_p2wsh(tmpctx, anch->adet->anchor_wscript),
AMOUNT_SAT(330));
psbt_input_add_pubkey(psbt, 1, &channel->local_funding_pubkey, false);

if (!amount_sat_add(&change, utxos[0]->amount, AMOUNT_SAT(330))
|| !amount_sat_sub(&change, change, fee)) {
log_broken(channel->log,
"Error calculating anchorspend change: utxo %s fee %s",
fmt_amount_sat(tmpctx, utxos[0]->amount),
fmt_amount_sat(tmpctx, fee));
return NULL;
}

bip32_pubkey(ld, &final_key, channel->final_key_idx);
psbt_append_output(psbt,
scriptpubkey_p2wpkh(tmpctx, &final_key),
change);

/* OK, HSM, sign it! */
msg = towire_hsmd_sign_anchorspend(NULL,
&channel->peer->id,
Expand Down
2 changes: 0 additions & 2 deletions tests/test_closing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3703,7 +3703,6 @@ def ignore_sendrawtx(r):
wait_for(lambda: len(bitcoind.rpc.getrawmempool()) == 1)


@pytest.mark.xfail(strict=True)
@unittest.skipIf(TEST_NETWORK != 'regtest', 'elementsd anchors unsupported')
def test_closing_anchorspend_htlc_tx_rbf(node_factory, bitcoind):
# We want an outstanding HTLC for l1, so it uses anchor to push.
Expand Down Expand Up @@ -3898,7 +3897,6 @@ def test_closing_minfee(node_factory, bitcoind):
bitcoind.generate_block(1, wait_for_mempool=txid)


@pytest.mark.xfail(strict=True)
@unittest.skipIf(TEST_NETWORK != 'regtest', 'elementsd anchors not supportd')
def test_peer_anchor_push(node_factory, bitcoind, executor, chainparams):
"""Test that we use anchor on peer's commit to CPFP tx"""
Expand Down

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