common/close_tx: add create_simple_close_tx() for option_simple_close

Adds the BOLT3 simple closing transaction builder:
- sequence 0xFFFFFFFD (RBF-signalling)
- locktime from closing_complete
- closer pays fee (their output is reduced)
- dust outputs are omitted and a zero-value OP_RETURN is used when both outputs would be dust.

Author: nGoline

common/close_tx.c

@@ -4,6 +4,8 @@
 #include <common/close_tx.h>
 #include <common/permute_tx.h>
 #include <common/psbt_keypath.h>
+#include <common/utils.h>
+#include <wally_script.h>
 
 struct bitcoin_tx *create_close_tx(const tal_t *ctx,
 				   const struct chainparams *chainparams,
@@ -91,3 +93,98 @@ struct bitcoin_tx *create_close_tx(const tal_t *ctx,
 	assert(bitcoin_tx_check(tx));
 	return tx;
 }
+
+/* BOLT #3:
+ *
+ * ## Closing Transaction
+ *
+ * This variant is used for `closing_complete` and `closing_sig` messages
+ * (i.e. where `option_simple_close` is negotiated).
+ * ...
+ * The side with lesser funds can opt to omit their own output.
+ * * version: 2
+ * * locktime: `locktime` from the `closing_complete` message
+ * * txin count: 1
+ *   * `txin[0]` outpoint: `txid` and `output_index` of the channel output
+ *   * `txin[0]` sequence: 0xFFFFFFFD
+ *   * `txin[0]` script bytes: 0
+ *   * `txin[0]` witness: `0 <signature_for_pubkey1> <signature_for_pubkey2>`
+ */
+struct bitcoin_tx *create_simple_close_tx(const tal_t *ctx,
+					  u32 *local_wallet_index,
+					  const struct ext_key *local_wallet_ext_key,
+					  const u8 *closer_script,
+					  const u8 *closee_script,
+					  const u8 *funding_wscript,
+					  const struct bitcoin_outpoint *funding,
+					  struct amount_sat funding_sats,
+					  struct amount_sat closer_amount,
+					  struct amount_sat closee_amount,
+					  u32 locktime)
+{
+	struct bitcoin_tx *tx;
+	size_t num_outputs = 0;
+	u8 *script;
+
+	/* Sequence 0xFFFFFFFD signals RBF and satisfies the nSequence
+	 * requirement for nLockTime. */
+	tx = bitcoin_tx(ctx, chainparams, 1, 2, locktime);
+
+	bitcoin_tx_add_input(tx, funding,
+		/* RBF-enabled, not final */
+		0xFFFFFFFD,
+		NULL, funding_sats, NULL, funding_wscript);
+
+	/* BOLT #3:
+	 * * The closer output: * `txout` amount:
+	 *   * 0 if the `scriptpubkey` starts with `OP_RETURN`
+	 *   * otherwise the final balance for the closer, minus `closing_complete.fee_satoshis`, rounded down to whole satoshis
+	 * * `txout` script: as specified in `closer_scriptpubkey` from the `closing_complete` message
+	 */
+	if (closer_script) {
+		struct amount_sat amt = closer_amount;
+		/* OP_RETURN output must have zero value */
+		if (tal_count(closer_script) > 0
+		    && closer_script[0] == OP_RETURN)
+			amt = AMOUNT_SAT(0);
+
+		script = tal_dup_talarr(tx, u8, closer_script);
+		bitcoin_tx_add_output(tx, script, NULL, amt);
+		assert((local_wallet_index == NULL) == (local_wallet_ext_key == NULL));
+		if (local_wallet_index) {
+			size_t script_len = tal_bytelen(script);
+			if (!psbt_add_keypath_to_last_output(
+				    tx, *local_wallet_index, local_wallet_ext_key,
+				    is_p2tr(script, script_len, NULL)))
+				return tal_free(tx);
+		}
+		num_outputs++;
+	}
+
+	/* BOLT #3:
+	 * * The closee output:
+	 *   * `txout` amount:
+	 *     * 0 if the `scriptpubkey` starts with `OP_RETURN`
+	 *     * otherwise the final balance for the closee, rounded down to whole satoshis
+	 *   * `txout` script: as specified in `closee_scriptpubkey` from the `closing_complete` message
+	 */
+	if (closee_script) {
+		struct amount_sat amt = closee_amount;
+		if (tal_count(closee_script) > 0
+		    && closee_script[0] == OP_RETURN)
+			amt = AMOUNT_SAT(0);
+
+		script = tal_dup_talarr(tx, u8, closee_script);
+		bitcoin_tx_add_output(tx, script, NULL, amt);
+		num_outputs++;
+	}
+
+	if (num_outputs == 0)
+		return tal_free(tx);
+
+	permute_outputs(tx, NULL, NULL);
+
+	bitcoin_tx_finalize(tx);
+	assert(bitcoin_tx_check(tx));
+	return tx;
+}

common/close_tx.h

@@ -19,4 +19,19 @@ struct bitcoin_tx *create_close_tx(const tal_t *ctx,
 				   struct amount_sat to_us,
 				   struct amount_sat to_them,
 				   struct amount_sat dust_limit);
+/* Create simple close tx (option_simple_close) to spend the anchor tx output.
+ * The closer pays the fee; closee output is omitted if closee_script is NULL.
+ * Closer output is omitted if closer_script is NULL (or OP_RETURN with amount 0).
+ * Uses sequence 0xFFFFFFFD (RBF) and the specified locktime. */
+struct bitcoin_tx *create_simple_close_tx(const tal_t *ctx,
+					  u32 *local_wallet_index,
+					  const struct ext_key *local_wallet_ext_key,
+					  const u8 *closer_script,
+					  const u8 *closee_script,
+					  const u8 *funding_wscript,
+					  const struct bitcoin_outpoint *funding,
+					  struct amount_sat funding_sats,
+					  struct amount_sat closer_amount,
+					  struct amount_sat closee_amount,
+					  u32 locktime);
 #endif /* LIGHTNING_COMMON_CLOSE_TX_H */