sweep: make sure nil tx is handled

After previous commit, it should be clear that the tx may be failed to
created in a `TxFailed` event. We now make sure to catch it to avoid
panic.

sweep/fee_bumper.go

@@ -75,7 +75,16 @@ const (
 	// TxPublished is sent when the broadcast attempt is finished.
 	TxPublished BumpEvent = iota
 
-	// TxFailed is sent when the broadcast attempt fails.
+	// TxFailed is sent when the tx has encountered a fee-related error
+	// during its creation or broadcast, or an internal error from the fee
+	// bumper. In either case the inputs in this tx should be retried with
+	// either a different grouping strategy or an increased budget.
+	//
+	// NOTE: We also send this event when there's a third party spend
+	// event, and the sweeper will handle cleaning this up.
+	//
+	// TODO(yy): Remove the above usage once we remove sweeping non-CPFP
+	// anchors.
 	TxFailed
 
 	// TxReplaced is sent when the original tx is replaced by a new one.
@@ -270,8 +279,11 @@ func (b *BumpResult) String() string {
 // Validate validates the BumpResult so it's safe to use.
 func (b *BumpResult) Validate() error {
 	// Every result must have a tx except the error or fail case.
-	if b.Tx == nil && b.Event != TxFatal {
-		return fmt.Errorf("%w: nil tx", ErrInvalidBumpResult)
+	if b.Tx == nil {
+		isFail := b.Event == TxFailed || b.Event == TxFatal
+		if !isFail {
+			return fmt.Errorf("%w: nil tx", ErrInvalidBumpResult)
+		}
 	}
 
 	// Every result must have a known event.

sweep/fee_bumper_test.go

@@ -91,13 +91,6 @@ func TestBumpResultValidate(t *testing.T) {
 	}
 	require.ErrorIs(t, b.Validate(), ErrInvalidBumpResult)
 
-	// A failed event without a tx will give an error.
-	b = BumpResult{
-		Event: TxFailed,
-		Err:   errDummy,
-	}
-	require.ErrorIs(t, b.Validate(), ErrInvalidBumpResult)
-
 	// A fatal event without a failure reason will give an error.
 	b = BumpResult{
 		Event: TxFailed,
@@ -118,6 +111,13 @@ func TestBumpResultValidate(t *testing.T) {
 	}
 	require.NoError(t, b.Validate())
 
+	// Tx is allowed to be nil in a TxFailed event.
+	b = BumpResult{
+		Event: TxFailed,
+		Err:   errDummy,
+	}
+	require.NoError(t, b.Validate())
+
 	// Tx is allowed to be nil in a TxFatal event.
 	b = BumpResult{
 		Event: TxFatal,

sweep/sweeper.go

@@ -1665,6 +1665,14 @@ func (s *UtxoSweeper) monitorFeeBumpResult(set InputSet,
 			// in sweeper and rely solely on this event to mark
 			// inputs as Swept?
 			if r.Event == TxConfirmed || r.Event == TxFailed {
+				// Exit if the tx is failed to be created.
+				if r.Tx == nil {
+					log.Debugf("Received %v for nil tx, "+
+						"exit monitor", r.Event)
+
+					return
+				}
+
 				log.Debugf("Received %v for sweep tx %v, exit "+
 					"fee bump monitor", r.Event,
 					r.Tx.TxHash())
@@ -1690,7 +1698,10 @@ func (s *UtxoSweeper) handleBumpEventTxFailed(resp *bumpResp) {
 	r := resp.result
 	tx, err := r.Tx, r.Err
 
-	log.Warnf("Fee bump attempt failed for tx=%v: %v", tx.TxHash(), err)
+	if tx != nil {
+		log.Warnf("Fee bump attempt failed for tx=%v: %v", tx.TxHash(),
+			err)
+	}
 
 	// NOTE: When marking the inputs as failed, we are using the input set
 	// instead of the inputs found in the tx. This is fine for current