Function spend_anchor

lightningd/anchorspend.c:333–557

If it's possible and worth it, return signed tx. Otherwise NULL. */

/* If it's possible and worth it, return signed tx.  Otherwise NULL. */
static struct bitcoin_tx *spend_anchor(const tal_t *ctx,
				       struct channel *channel,
				       struct one_anchor *anch)
{
	struct lightningd *ld = channel->peer->ld;
	size_t base_weight, psbt_weight;
	struct amount_sat psbt_fee, diff;
	struct bitcoin_tx *tx;
	struct utxo **psbt_utxos;
	const struct hsm_utxo **hsm_utxos;
	struct wally_psbt *psbt, *signed_psbt;
	struct amount_msat total_value;
	const struct deadline_value *unimportant_deadline;
	const u8 *msg;

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

	total_value = AMOUNT_MSAT(0);
	psbt = NULL;
	unimportant_deadline = NULL;

	/* We start with furthest feerate target, and keep going backwards
	 * until it's either not worth making an anchor at that price for all
	 * the HTLCs from that point on, or we have an anchor for the closest
	 * HTLC deadline */
	for (int i = tal_count(anch->adet->vals) - 1; i >= 0; --i) {
		const struct deadline_value *val = &anch->adet->vals[i];
		u32 feerate, feerate_target;
		size_t weight;
		struct amount_sat fee;
		struct wally_psbt *candidate_psbt;
		struct utxo **utxos;

		/* We only cover important deadlines here */
		if (!val->important) {
			unimportant_deadline = val;
			continue;
		}


		if (!amount_msat_accumulate(&total_value, val->msat))
			abort();

		feerate_target = feerate_for_target(ld->topology, val->block);

		/* If the feerate for the commitment tx is already
		 * sufficient, don't try for anchor. */
		if (amount_feerate(&feerate,
				   anch->info.commitment_fee,
				   anch->info.commitment_weight)
		    && feerate >= feerate_target) {
			continue;