commands: add a change_index field to listspendtxs entries

doc/API.md

@@ -152,6 +152,7 @@ This command does not take any parameter for now.
 
 ##### Spend tx entry
 
-| Field         | Type              | Description                                                 |
-| ------------- | ----------------- | ----------------------------------------------------------- |
-| `psbt`        | string            | Base64-encoded PSBT of the Spend transaction.               |
+| Field          | Type              | Description                                                             |
+| -------------- | ----------------- | ----------------------------------------------------------------------- |
+| `psbt`         | string            | Base64-encoded PSBT of the Spend transaction.                           |
+| `change_index` | int or null       | Index of the change output in the transaction outputs, if there is one. |

src/commands/mod.rs

@@ -9,7 +9,7 @@ use crate::{
     database::{Coin, DatabaseInterface},
     descriptors, DaemonControl, VERSION,
 };
-use utils::{deser_amount_from_sats, deser_psbt_base64, ser_amount, ser_base64};
+use utils::{change_index, deser_amount_from_sats, deser_psbt_base64, ser_amount, ser_base64};
 
 use std::{
     collections::{BTreeMap, HashMap},
@@ -287,7 +287,7 @@ impl DaemonControl {
                 script_pubkey: address.script_pubkey(),
             });
             // TODO: if it's an address of ours, signal it as change to signing devices by adding
-            // the BIP32 derivation path to the PSBT input.
+            // the BIP32 derivation path to the PSBT output.
             psbt_outs.push(PsbtOut::default());
         }
 
@@ -424,7 +424,10 @@ impl DaemonControl {
         let spend_txs = db_conn
             .list_spend()
             .into_iter()
-            .map(|psbt| ListSpendEntry { psbt })
+            .map(|psbt| {
+                let change_index = change_index(&psbt).map(|i| i.try_into().expect("insane usize"));
+                ListSpendEntry { psbt, change_index }
+            })
             .collect();
         ListSpendResult { spend_txs }
     }
@@ -476,6 +479,7 @@ pub struct CreateSpendResult {
 pub struct ListSpendEntry {
     #[serde(serialize_with = "ser_base64", deserialize_with = "deser_psbt_base64")]
     pub psbt: Psbt,
+    pub change_index: Option<u32>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]

src/commands/utils.rs

@@ -32,3 +32,27 @@ where
     let psbt = consensus::deserialize(&s).map_err(de::Error::custom)?;
     Ok(psbt)
 }
+
+// Utility to gather the index of a change output in a Psbt, if there is one.
+// FIXME: this is temporary! This is based on create_spend's behaviour that reuses the
+// first coin address and doesn't shuffle the outputs!
+pub fn change_index(psbt: &Psbt) -> Option<usize> {
+    // We always set the witness UTxO in the PSBTs we create.
+    let first_coin_spk = match psbt.inputs[0]
+        .witness_utxo
+        .as_ref()
+        .map(|o| &o.script_pubkey)
+    {
+        Some(spk) => spk,
+        None => return None,
+    };
+
+    let tx = &psbt.global.unsigned_tx;
+    for i in (0..tx.output.len()).rev() {
+        if &tx.output[i].script_pubkey == first_coin_spk {
+            return Some(i);
+        }
+    }
+
+    None
+}

tests/test_rpc.py

@@ -96,23 +96,26 @@ def test_create_spend(minisafed, bitcoind):
 
 
 def test_list_spend(minisafed, bitcoind):
-    # Start by creating two conflicting Spend PSBTs
+    # Start by creating two conflicting Spend PSBTs. The first one will have a change
+    # output but not the second one.
     addr = minisafed.rpc.getnewaddress()["address"]
-    bitcoind.rpc.sendtoaddress(addr, 0.2567)
+    value_a = 0.2567
+    bitcoind.rpc.sendtoaddress(addr, value_a)
     wait_for(lambda: len(minisafed.rpc.listcoins()["coins"]) == 1)
     outpoints = [c["outpoint"] for c in minisafed.rpc.listcoins()["coins"]]
     destinations = {
-        bitcoind.rpc.getnewaddress(): 200_000,
+        bitcoind.rpc.getnewaddress(): int(value_a * COIN // 2),
     }
     res = minisafed.rpc.createspend(outpoints, destinations, 6)
     assert "psbt" in res
 
     addr = minisafed.rpc.getnewaddress()["address"]
-    bitcoind.rpc.sendtoaddress(addr, 0.0987)
+    value_b = 0.0987
+    bitcoind.rpc.sendtoaddress(addr, value_b)
     wait_for(lambda: len(minisafed.rpc.listcoins()["coins"]) == 2)
     outpoints = [c["outpoint"] for c in minisafed.rpc.listcoins()["coins"]]
     destinations = {
-        bitcoind.rpc.getnewaddress(): 400_000,
+        bitcoind.rpc.getnewaddress(): int((value_a + value_b) * COIN - 1_000),
     }
     res_b = minisafed.rpc.createspend(outpoints, destinations, 2)
     assert "psbt" in res_b
@@ -122,12 +125,14 @@ def test_list_spend(minisafed, bitcoind):
     minisafed.rpc.updatespend(res["psbt"])
     minisafed.rpc.updatespend(res_b["psbt"])
 
-    # Listing all Spend transactions will list them both.
+    # Listing all Spend transactions will list them both. It'll tell us which one has
+    # change and which one doesn't.
     list_res = minisafed.rpc.listspendtxs()["spend_txs"]
     assert len(list_res) == 2
-    all_psbts = [entry["psbt"] for entry in list_res]
-    assert res["psbt"] in all_psbts
-    assert res_b["psbt"] in all_psbts
+    first_psbt = next(entry for entry in list_res if entry["psbt"] == res["psbt"])
+    assert first_psbt["change_index"] == 1
+    second_psbt = next(entry for entry in list_res if entry["psbt"] == res_b["psbt"])
+    assert second_psbt["change_index"] is None
 
 
 def test_update_spend(minisafed, bitcoind):