sweep: handle missing inputs during fee bumping

This commit handles the case when the input is missing during the RBF
process, which could happen when the bumped tx has inputs being spent by
a third party. Normally we should be able to catch the spend early via
the spending notification and never attempt to fee bump the record.
However, due to the possible race between block notification and spend
notification, this cannot be guaranteed. Thus, we need to handle the
case during the RBF when seeing a `ErrMissingInputs`, which can only
happen when the inputs are spent by others.

contractcourt/anchor_resolver.go

@@ -108,7 +108,7 @@ func (c *anchorResolver) Resolve() (ContractResolver, error) {
 
 		// Anchor was swept by someone else. This is possible after the
 		// 16 block csv lock.
-		case sweep.ErrRemoteSpend:
+		case sweep.ErrRemoteSpend, sweep.ErrInputMissing:
 			c.log.Warnf("our anchor spent by someone else")
 			outcome = channeldb.ResolverOutcomeUnclaimed
 

sweep/fee_bumper.go

@@ -43,6 +43,10 @@ var (
 	// ErrUnknownSpent is returned when an unknown tx has spent an input in
 	// the sweeping tx.
 	ErrUnknownSpent = errors.New("unknown spend of input")
+
+	// ErrInputMissing is returned when a given input no longer exists,
+	// e.g., spending from an orphan tx.
+	ErrInputMissing = errors.New("input no longer exists")
 )
 
 var (
@@ -653,10 +657,68 @@ func (t *TxPublisher) createAndCheckTx(r *monitorRecord) (*sweepTxCtx, error) {
 		return sweepCtx, nil
 	}
 
+	// If the inputs are spent by another tx, we will exit with the latest
+	// sweepCtx and an error.
+	if errors.Is(err, chain.ErrMissingInputs) {
+		log.Debugf("Tx %v missing inputs, it's likely the input has "+
+			"been spent by others", sweepCtx.tx.TxHash())
+
+		// Make sure to update the record with the latest attempt.
+		t.updateRecord(r, sweepCtx)
+
+		return sweepCtx, ErrInputMissing
+	}
+
 	return sweepCtx, fmt.Errorf("tx=%v failed mempool check: %w",
 		sweepCtx.tx.TxHash(), err)
 }
 
+// handleMissingInputs handles the case when the chain backend reports back a
+// missing inputs error, which could happen when one of the input has been spent
+// in another tx, or the input is referencing an orphan. When the input is
+// spent, it will be handled via the TxUnknownSpend flow by creating a
+// TxUnknownSpend bump result, otherwise, a TxFatal bump result is returned.
+func (t *TxPublisher) handleMissingInputs(r *monitorRecord) *BumpResult {
+	// Get the spending txns.
+	spends := t.getSpentInputs(r)
+
+	// Attach the spending txns.
+	r.spentInputs = spends
+
+	// If there are no spending txns found and the input is missing, the
+	// input is referencing an orphan tx that's no longer valid, e.g., the
+	// spending the anchor output from the remote commitment after the local
+	// commitment has confirmed. In this case we will mark it as fatal and
+	// exit.
+	if len(spends) == 0 {
+		log.Warnf("Failing record=%v: found orphan inputs: %v\n",
+			r.requestID, inputTypeSummary(r.req.Inputs))
+
+		// Create a result that will be sent to the resultChan which is
+		// listened by the caller.
+		result := &BumpResult{
+			Event:     TxFatal,
+			Tx:        r.tx,
+			requestID: r.requestID,
+			Err:       ErrInputMissing,
+		}
+
+		return result
+	}
+
+	// Check that the spending tx matches the sweeping tx - given that the
+	// current sweeping tx has been failed due to missing inputs, the
+	// spending tx must be a different tx, thus it should NOT be matched. We
+	// perform a sanity check here to catch the unexpected state.
+	if !t.isUnknownSpent(r, spends) {
+		log.Errorf("Sweeping tx %v has missing inputs, yet the "+
+			"spending tx is the sweeping tx itself: %v",
+			r.tx.TxHash(), r.spentInputs)
+	}
+
+	return t.createUnknownSpentBumpResult(r)
+}
+
 // broadcast takes a monitored tx and publishes it to the network. Prior to the
 // broadcast, it will subscribe the tx's confirmation notification and attach
 // the event channel to the record. Any broadcast-related errors will not be
@@ -1052,6 +1114,11 @@ func (t *TxPublisher) handleInitialTxError(r *monitorRecord, err error) {
 	case errors.Is(err, ErrZeroFeeRateDelta):
 		result.Event = TxFailed
 
+	// When there are missing inputs, we'll create a TxUnknownSpend bump
+	// result here so the rest of the inputs can be retried.
+	case errors.Is(err, ErrInputMissing):
+		result = t.handleMissingInputs(r)
+
 	// Otherwise this is not a fee-related error and the tx cannot be
 	// retried. In that case we will fail ALL the inputs in this tx, which
 	// means they will be removed from the sweeper and never be tried
@@ -1782,6 +1849,16 @@ func (t *TxPublisher) handleReplacementTxError(r *monitorRecord,
 		return fn.None[BumpResult]()
 	}
 
+	// At least one of the inputs is missing, which means it has already
+	// been spent by another tx and confirmed. In this case we will handle
+	// it by returning a TxUnknownSpend bump result.
+	if errors.Is(err, ErrInputMissing) {
+		log.Warnf("Fail to fee bump tx %v: %v", oldTx.TxHash(), err)
+		bumpResult := t.handleMissingInputs(r)
+
+		return fn.Some(*bumpResult)
+	}
+
 	// If the error is not fee related, we will return a `TxFailed` event
 	// so this input can be retried.
 	result := fn.Some(BumpResult{