Specify a dust threshold for coin selection

- Prevents spending any dust ouputs using a conservative threshold. This is necessary because we don't have a way to track unconfirmed space outputs yet.

- Simplify coin selection further removing the needing for adding foreign utxos as we have switched to just two set of descriptors.

Author: buffrr

node/src/wallets.rs

@@ -154,7 +154,8 @@ impl RpcWallet {
             balance,
             dust: unspent
                 .into_iter()
-                .filter(|output| output.is_spaceout)
+                .filter(|output| output.is_spaceout ||
+                    output.output.txout.value <= SpacesAwareCoinSelection::DUST_THRESHOLD)
                 .map(|output| output.output.txout.value)
                 .sum(),
         };
@@ -361,13 +362,17 @@ impl RpcWallet {
         wallet: &mut SpacesWallet,
         state: &mut LiveSnapshot,
     ) -> anyhow::Result<SpacesAwareCoinSelection> {
-        // exclude all space outs
+        // Filters out all "space outs" from the selection.
+        // Note: This exclusion only applies to confirmed space outs; unconfirmed ones are not excluded.
+        // In practice, this should be fine since Spaces coin selection skips dust by default,
+        // so explicitly excluding space outs may be redundant.
         let excluded = Self::list_unspent(wallet, state)?
             .into_iter()
             .filter(|out| out.is_spaceout)
             .map(|out| out.output.outpoint)
             .collect::<Vec<_>>();
-        Ok(SpacesAwareCoinSelection::new(Vec::new(), excluded))
+
+        Ok(SpacesAwareCoinSelection::new(excluded))
     }
 
     fn list_unspent(
@@ -437,6 +442,15 @@ impl RpcWallet {
         store: &mut LiveSnapshot,
         tx: RpcWalletTxBuilder,
     ) -> anyhow::Result<WalletResponse> {
+        if let Some(dust) = tx.dust {
+            if dust > SpacesAwareCoinSelection::DUST_THRESHOLD {
+                // Allowing higher dust may space outs to be accidentally
+                // spent during coin selection
+                return Err(anyhow!("dust cannot be higher than {}",
+                    SpacesAwareCoinSelection::DUST_THRESHOLD));
+            }
+        }
+
         let fee_rate = match tx.fee_rate.as_ref() {
             None => match Self::estimate_fee_rate(source) {
                 None => return Err(anyhow!("could not estimate fee rate")),

wallet/src/builder.rs

@@ -1,5 +1,4 @@
 use std::{
-    cell::RefCell,
     cmp::min,
     collections::BTreeMap,
     default::Default,
@@ -235,7 +234,7 @@ impl<'a, Cs: CoinSelectionAlgorithm> TxBuilderSpacesUtils<'a, Cs> for TxBuilder<
             placeholder.auction.outpoint.vout as u8,
             &offer,
         )?)
-        .expect("compressed psbt script bytes");
+            .expect("compressed psbt script bytes");
 
         let carrier = ScriptBuf::new_op_return(&compressed_psbt);
 
@@ -895,23 +894,25 @@ impl Builder {
     }
 }
 
-/// A coin selection algorithm that guarantees required utxos are ordered first
-/// appends any funding/change outputs to the end of the selected utxos
-/// also enables adding additional optional foreign utxos for funding
+/// A coin selection algorithm that :
+/// 1. Guarantees required utxos are ordered first appending
+/// any funding/change outputs to the end of the selected utxos.
+/// 2. Excludes all dust outputs to avoid accidentally spending space utxos
+/// 3. Enables adding additional output exclusions
 #[derive(Debug, Clone)]
 pub struct SpacesAwareCoinSelection {
     pub default_algorithm: DefaultCoinSelectionAlgorithm,
-    // Additional UTXOs to fund the transaction
-    pub other_optional_utxos: RefCell<Vec<WeightedUtxo>>,
     // Exclude outputs
     pub exclude_outputs: Vec<OutPoint>,
 }
 
 impl SpacesAwareCoinSelection {
-    pub fn new(optional_utxos: Vec<WeightedUtxo>, excluded: Vec<OutPoint>) -> Self {
+    // Will skip any outputs with value less than the dust threshold
+    // to avoid accidentally spending space outputs
+    pub const DUST_THRESHOLD: Amount = Amount::from_sat(1200);
+    pub fn new(excluded: Vec<OutPoint>) -> Self {
         Self {
             default_algorithm: DefaultCoinSelectionAlgorithm::default(),
-            other_optional_utxos: RefCell::new(optional_utxos),
             exclude_outputs: excluded,
         }
     }
@@ -931,12 +932,10 @@ impl CoinSelectionAlgorithm for SpacesAwareCoinSelection {
             .map(|w| w.utxo.clone())
             .collect::<Vec<_>>();
 
-        // Extend optional outputs
-        optional_utxos.extend(self.other_optional_utxos.borrow().iter().cloned());
-
-        // Filter out excluded outputs
+        // Filter out UTXOs that are either explicitly excluded or below the dust threshold
         optional_utxos.retain(|weighted_utxo| {
-            !self
+            weighted_utxo.utxo.txout().value > SpacesAwareCoinSelection::DUST_THRESHOLD
+                && !self
                 .exclude_outputs
                 .contains(&weighted_utxo.utxo.outpoint())
         });
@@ -952,9 +951,6 @@ impl CoinSelectionAlgorithm for SpacesAwareCoinSelection {
         let mut optional = Vec::with_capacity(result.selected.len() - required.len());
         for utxo in result.selected.drain(..) {
             if !required.iter().any(|u| u == &utxo) {
-                self.other_optional_utxos
-                    .borrow_mut()
-                    .retain(|x| x.utxo != utxo);
                 optional.push(utxo);
             }
         }