htlcswitch: do not consider unknown failures an error

htlcswitch/failure.go

@@ -31,10 +31,10 @@ type ForwardingError struct {
 // returned.
 func (f *ForwardingError) Error() string {
 	if f.ExtraMsg == "" {
-		return f.FailureMessage.Error()
+		return fmt.Sprintf("%v", f.FailureMessage)
 	}
 
-	return fmt.Sprintf("%v: %v", f.FailureMessage.Error(), f.ExtraMsg)
+	return fmt.Sprintf("%v: %v", f.FailureMessage, f.ExtraMsg)
 }
 
 // ErrorDecrypter is an interface that is used to decrypt the onion encrypted
@@ -274,10 +274,14 @@ func (s *SphinxErrorDecrypter) DecryptError(reason lnwire.OpaqueReason) (
 		return nil, err
 	}
 
+	// Decode the failure. If an error occurs, we leave the failure message
+	// field nil.
 	r := bytes.NewReader(failure.Message)
 	failureMsg, err := lnwire.DecodeFailure(r, 0)
 	if err != nil {
-		return nil, err
+		return &ForwardingError{
+			FailureSourceIdx: failure.SenderIdx,
+		}, nil
 	}
 
 	return &ForwardingError{

htlcswitch/switch_test.go

@@ -2344,4 +2344,37 @@ func TestInvalidFailure(t *testing.T) {
 	case <-time.After(time.Second):
 		t.Fatal("err wasn't received")
 	}
+
+	// Modify the decryption to simulate that decryption went alright, but
+	// the failure cannot be decoded.
+	deobfuscator = SphinxErrorDecrypter{
+		OnionErrorDecrypter: &mockOnionErrorDecryptor{
+			sourceIdx: 2,
+			message:   []byte{200},
+		},
+	}
+
+	resultChan, err = s.GetPaymentResult(
+		paymentID, rhash, &deobfuscator,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	select {
+	case result := <-resultChan:
+		fErr, ok := result.Error.(*ForwardingError)
+		if !ok {
+			t.Fatal("expected ForwardingError")
+		}
+		if fErr.FailureSourceIdx != 2 {
+			t.Fatal("unexpected error source index")
+		}
+		if fErr.FailureMessage != nil {
+			t.Fatal("expected empty failure message")
+		}
+
+	case <-time.After(time.Second):
+		t.Fatal("err wasn't received")
+	}
 }