test(key-wallet): cover the change-address guard

key-wallet/src/wallet/managed_wallet_info/transaction_builder.rs

@@ -157,6 +157,11 @@ impl TransactionBuilder {
     /// must therefore not be held across an `await` between `add_funding` and
     /// `build_signed` or `assemble_unsigned`, since suspending there reopens the
     /// read-then-reserve window for a concurrent build.
+    ///
+    /// A change address already set by [`set_change_address`](Self::set_change_address)
+    /// wins: the funding account is only asked for one when the builder has none.
+    /// Deriving unconditionally would both discard the caller's choice and burn a
+    /// pool address, since `next_change_address` advances the pool state.
     pub fn add_funding(self, funds_acc: &mut ManagedCoreFundsAccount, acc: &Account) -> Self {
         self.fund_from(funds_acc, acc, true)
     }
@@ -2023,12 +2028,29 @@ mod tests {
 
         funds.reservations().reserve(&[reserved.outpoint], 200, ReservationToken::next());
 
-        let builder =
-            TransactionBuilder::new().set_current_height(200).add_funding(&mut funds, &account);
+        // Set change address to test the branch that avoids it being replaced
+        // by add_funding()
+        let explicit = Address::dummy(Network::Testnet, 1);
+        let builder = TransactionBuilder::new()
+            .set_current_height(200)
+            .set_change_address(explicit.clone())
+            .add_funding(&mut funds, &account);
 
         let candidates: Vec<OutPoint> = builder.inputs.iter().map(|utxo| utxo.outpoint).collect();
         assert!(!candidates.contains(&reserved.outpoint));
         assert!(candidates.contains(&free.outpoint));
+
+        assert_eq!(
+            builder.change_addr.as_ref(),
+            Some(&explicit),
+            "funding must not override a change address the caller chose"
+        );
+        // Nor may it burn a pool address to derive one it then discards.
+        let mut control = ManagedCoreFundsAccount::dummy_bip44();
+        assert_eq!(
+            funds.next_change_address(Some(&account.account_xpub), true).expect("change address"),
+            control.next_change_address(Some(&account.account_xpub), true).expect("change address"),
+        );
     }
 
     #[test]