rbf: modify is_cancel candidates logic

src/commands/mod.rs

@@ -1036,17 +1036,28 @@ impl DaemonControl {
                 }
             };
         }
-        // We take all previous inputs as mandatory candidates and, if not self-send, include
-        // confirmed coins as optional.
-        // TODO: For `is_cancel`, shall we include just one previous input?
-        let candidate_coins: Vec<CandidateCoin> = prev_coins
+        // If `!is_cancel`, we take the previous coins as mandatory candidates and add confirmed coins as optional.
+        // Otherwise, we take the previous coins as optional candidates and let coin selection find the
+        // best solution that includes at least one of these. If there are insufficient funds to create the replacement
+        // transaction in this way, then we set candidates in the same way as for the `!is_cancel` case.
+        let mut candidate_coins: Vec<CandidateCoin> = prev_coins
             .values()
-            .chain(db_conn.coins(&[CoinStatus::Confirmed], &[]).values())
             .map(|c| CandidateCoin {
                 coin: *c,
-                must_select: prev_coins.contains_key(&c.outpoint),
+                must_select: !is_cancel,
             })
             .collect();
+        if !is_cancel {
+            candidate_coins.extend(
+                db_conn
+                    .coins(&[CoinStatus::Confirmed], &[])
+                    .into_values()
+                    .map(|c| CandidateCoin {
+                        coin: c,
+                        must_select: false,
+                    }),
+            );
+        }
         // Try with increasing feerate until fee paid by replacement transaction is high enough.
         // Replacement fee must be at least:
         // sum of fees paid by original transactions + incremental feerate * replacement size.
@@ -1057,13 +1068,34 @@ impl DaemonControl {
             // will ensure that the PSBT meets the required replacement fee and the loop will exit.
             let min_fee = mempool_entry.fees.descendant.to_sat() + rbf_vsize;
             println!("trying min_fee of {}", min_fee);
-            let rbf_psbt = self.create_rbf_spend(
+            let rbf_psbt = match self.create_rbf_spend(
                 &destinations,
                 &candidate_coins,
                 feerate_vb,
                 min_fee,
                 change_index,
-            )?;
+            ) {
+                Ok(psbt) => psbt,
+                // If we get a coin selection error due to insufficient funds and we want to cancel the
+                // transaction, then set all previous coins as mandatory and add confirmed coins as
+                // optional, unless we have already done this.
+                Err(CommandError::CoinSelectionError(_))
+                    if is_cancel && candidate_coins.iter().all(|c| !c.must_select) =>
+                {
+                    candidate_coins = prev_coins
+                        .values()
+                        .chain(db_conn.coins(&[CoinStatus::Confirmed], &[]).values())
+                        .map(|c| CandidateCoin {
+                            coin: *c,
+                            must_select: prev_coins.contains_key(&c.outpoint),
+                        })
+                        .collect();
+                    continue;
+                }
+                Err(e) => {
+                    return Err(e);
+                }
+            };
             rbf_vsize = {
                 // TODO: Make this a function (can be used also in `sanity_check`).
                 let witness_factor: u64 =