From 9f0b1c5cbece117d0a5e424b444a66ba870eacf2 Mon Sep 17 00:00:00 2001
From: Thomas Eizinger <thomas@eizinger.io>
Date: Mon, 1 Mar 2021 15:35:45 +1100
Subject: [PATCH] Calculate max_giveable based on spending script size

---
 swap/src/bin/swap_cli.rs   |  4 ++--
 swap/src/bitcoin/lock.rs   | 12 ++++++++++++
 swap/src/bitcoin/wallet.rs |  8 ++------
 3 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/swap/src/bin/swap_cli.rs b/swap/src/bin/swap_cli.rs
index 5b238b64..c2a9ed4c 100644
--- a/swap/src/bin/swap_cli.rs
+++ b/swap/src/bin/swap_cli.rs
@@ -19,7 +19,7 @@ use std::{path::Path, sync::Arc, time::Duration};
 use structopt::StructOpt;
 use swap::{
     bitcoin,
-    bitcoin::Amount,
+    bitcoin::{Amount, TxLock},
     cli::{
         command::{Arguments, Command},
         config::{read_config, Config},
@@ -112,7 +112,7 @@ async fn main() -> Result<()> {
                 debug!("Received {}", bitcoin_wallet.balance().await?);
             }
 
-            let send_bitcoin = bitcoin_wallet.max_giveable().await?;
+            let send_bitcoin = bitcoin_wallet.max_giveable(TxLock::script_size()).await?;
 
             info!("Swapping {} ...", send_bitcoin);
 
diff --git a/swap/src/bitcoin/lock.rs b/swap/src/bitcoin/lock.rs
index 46666fe3..486c23ca 100644
--- a/swap/src/bitcoin/lock.rs
+++ b/swap/src/bitcoin/lock.rs
@@ -3,7 +3,9 @@ use crate::bitcoin::{
 };
 use ::bitcoin::{util::psbt::PartiallySignedTransaction, OutPoint, TxIn, TxOut, Txid};
 use anyhow::Result;
+use ecdsa_fun::fun::Point;
 use miniscript::{Descriptor, DescriptorTrait};
+use rand::thread_rng;
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
@@ -42,6 +44,16 @@ impl TxLock {
         OutPoint::new(self.txid(), self.lock_output_vout() as u32)
     }
 
+    /// Calculate the size of the script used by this transaction.
+    pub fn script_size() -> usize {
+        build_shared_output_descriptor(
+            Point::random(&mut thread_rng()),
+            Point::random(&mut thread_rng()),
+        )
+        .script_pubkey()
+        .len()
+    }
+
     /// Retreive the index of the locked output in the transaction outputs
     /// vector
     fn lock_output_vout(&self) -> usize {
diff --git a/swap/src/bitcoin/wallet.rs b/swap/src/bitcoin/wallet.rs
index 466a5f4c..1e018b49 100644
--- a/swap/src/bitcoin/wallet.rs
+++ b/swap/src/bitcoin/wallet.rs
@@ -132,17 +132,13 @@ impl Wallet {
     /// We define this as the maximum amount we can pay to a single output,
     /// already accounting for the fees we need to spend to get the
     /// transaction confirmed.
-    pub async fn max_giveable(&self) -> Result<Amount> {
+    pub async fn max_giveable(&self, locking_script_size: usize) -> Result<Amount> {
         let wallet = self.inner.lock().await;
 
         let mut tx_builder = wallet.build_tx();
 
-        // create a dummy script to make the txbuilder pass
-        // we don't intend to send this transaction, we just want to know the max amount
-        // we can spend
-        let dummy_script = Script::default();
+        let dummy_script = Script::from(vec![0u8; locking_script_size]);
         tx_builder.set_single_recipient(dummy_script);
-
         tx_builder.drain_wallet();
         tx_builder.fee_rate(self.select_feerate());
         let (_, details) = tx_builder.finish()?;