spend: add warning about fee for ancestor

src/spend.rs

@@ -543,6 +543,7 @@ pub enum SpendTxFees {
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub enum CreateSpendWarning {
     ChangeAddedToFee(u64),
+    AddtionalFeeForAncestors(u64),
 }
 
 impl fmt::Display for CreateSpendWarning {
@@ -554,6 +555,12 @@ impl fmt::Display for CreateSpendWarning {
                 amt,
                 if *amt > 1 {"s"} else {""},
             ),
+            CreateSpendWarning::AddtionalFeeForAncestors(amt) => write!(
+                f,
+                "An additional fee of {} sat{} has been added to pay for ancestors at the target feerate.",
+                amt,
+                if *amt > 1 {"s"} else {""},
+            ),
         }
     }
 }
@@ -673,7 +680,7 @@ pub fn create_spend(
         selected,
         change_amount,
         max_change_amount,
-        ..
+        fee_for_ancestors,
     } = {
         // At this point the transaction still has no input and no change output, as expected
         // by the coins selection helper function.
@@ -735,6 +742,12 @@ pub fn create_spend(
         ));
     }
 
+    if fee_for_ancestors.to_sat() > 0 {
+        warnings.push(CreateSpendWarning::AddtionalFeeForAncestors(
+            fee_for_ancestors.to_sat(),
+        ));
+    }
+
     // Iterate through selected coins and add necessary information to the PSBT inputs.
     let mut psbt_ins = Vec::with_capacity(selected.len());
     for cand in &selected {

tests/test_spend.py

@@ -171,7 +171,16 @@ def sign_and_broadcast(psbt):
     psbt = PSBT.from_base64(res["psbt"])
     sign_and_broadcast(psbt)
     assert len(psbt.o) == 4
-    assert len(res["warnings"]) == 0
+    assert bitcoind.rpc.getmempoolentry(deposit_d)["ancestorsize"] == 165
+    assert bitcoind.rpc.getmempoolentry(deposit_d)["fees"]["ancestor"] * COIN == 165
+    # ancestor vsize at feerate 2 sat/vb = ancestor_fee / 2 = 165 / 2 = 82
+    # extra_weight <= (extra vsize * witness factor) = (165 - 82) * 4 = 332
+    # additional fee at 2 sat/vb (0.5 sat/wu) = 332 * 0.5 = 166
+    assert len(res["warnings"]) == 1
+    assert (
+        res["warnings"][0]
+        == "An additional fee of 166 sats has been added to pay for ancestors at the target feerate."
+    )
 
     # All the spent coins must have been detected as such
     all_deposits = (deposit_a, deposit_b, deposit_c, deposit_d)