From e0c9abc2b32a44592446c8f6aa1ab72e2dafd096 Mon Sep 17 00:00:00 2001
From: Rusty Russell <rusty@rustcorp.com.au>
Date: Mon, 30 Oct 2023 15:51:59 +1030
Subject: [PATCH] wallet: routine to gather UTXOs to meet a certain feerate.

We want to use this for boosting txs: either attaching fees to
zero-fee HTLCs, or making anchor transactions.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 wallet/wallet.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++
 wallet/wallet.h | 19 +++++++++++++++++
 2 files changed, 75 insertions(+)

diff --git a/wallet/wallet.c b/wallet/wallet.c
index 1502b3ab0..2487a98aa 100644
--- a/wallet/wallet.c
+++ b/wallet/wallet.c
@@ -450,6 +450,62 @@ struct utxo *wallet_utxo_get(const tal_t *ctx, struct wallet *w,
 	return utxo;
 }
 
+/* Gather enough utxos to meet feerate, otherwise all we can. */
+struct utxo **wallet_utxo_boost(const tal_t *ctx,
+				struct wallet *w,
+				u32 blockheight,
+				struct amount_sat fee_amount,
+				u32 feerate_target,
+				size_t *weight)
+{
+	struct utxo **all_utxos = wallet_get_unspent_utxos(tmpctx, w);
+	struct utxo **utxos = tal_arr(ctx, struct utxo *, 0);
+	u32 feerate;
+
+	/* Select in random order */
+	tal_arr_randomize(all_utxos, struct utxo *);
+
+	/* Can't overflow, it's from our tx! */
+	if (!amount_feerate(&feerate, fee_amount, *weight))
+		abort();
+
+	for (size_t i = 0; i < tal_count(all_utxos); i++) {
+		u32 new_feerate;
+		size_t new_weight;
+		struct amount_sat new_fee_amount;
+		/* Convenience var */
+		struct utxo *utxo = all_utxos[i];
+
+		/* Are we already happy? */
+		if (feerate >= feerate_target)
+			break;
+
+		/* Don't add reserved ones */
+		if (utxo_is_reserved(utxo, blockheight))
+			continue;
+
+		/* UTXOs must be sane amounts */
+		if (!amount_sat_add(&new_fee_amount,
+				    fee_amount, utxo->amount))
+			abort();
+
+		new_weight = *weight + utxo_spend_weight(utxo, 0);
+		if (!amount_feerate(&new_feerate, new_fee_amount, new_weight))
+			abort();
+
+		/* Don't add uneconomic ones! */
+		if (new_feerate < feerate)
+			continue;
+
+		feerate = new_feerate;
+		*weight = new_weight;
+		fee_amount = new_fee_amount;
+		tal_arr_expand(&utxos, tal_steal(utxos, utxo));
+	}
+
+	return utxos;
+}
+
 static void db_set_utxo(struct db *db, const struct utxo *utxo)
 {
 	struct db_stmt *stmt;
diff --git a/wallet/wallet.h b/wallet/wallet.h
index c47e1e656..f88f12796 100644
--- a/wallet/wallet.h
+++ b/wallet/wallet.h
@@ -537,6 +537,25 @@ void wallet_unreserve_utxo(struct wallet *w, struct utxo *utxo,
 struct utxo *wallet_utxo_get(const tal_t *ctx, struct wallet *w,
 			     const struct bitcoin_outpoint *outpoint);
 
+/**
+ * wallet_utxo_boost - get (unreserved) utxos to meet a given feerate.
+ * @ctx: context to tal return array from
+ * @w: the wallet
+ * @blockheight: current height (to determine reserved status)
+ * @fee_amount: amount already paying in fees
+ * @feerate_target: feerate we want, in perkw.
+ * @weight: (in)existing weight before any utxos added, (out)final weight with utxos added.
+ *
+ * May not meet the feerate, but will spend all available utxos to try.
+ * You may also need to create change, as it may exceed.
+ */
+struct utxo **wallet_utxo_boost(const tal_t *ctx,
+				struct wallet *w,
+				u32 blockheight,
+				struct amount_sat fee_amount,
+				u32 feerate_target,
+				size_t *weight);
+
 /**
  * wallet_can_spend - Do we have the private key matching this scriptpubkey?
  *