htlcswitch+routing: implement expiry_too_far failure

In this commit we add a check to HtlcSatifiesPolicy to verify that the
time lock for the outgoing htlc that is requested in the onion packet
isn't too far in the future.

Without this check, anyone could force an unreasonably long time lock on
the forwarding node.

lnwire/onion_error.go

@@ -77,6 +77,7 @@ const (
 	CodeFinalExpiryTooSoon            FailCode = 17
 	CodeFinalIncorrectCltvExpiry      FailCode = 18
 	CodeFinalIncorrectHtlcAmount      FailCode = 19
+	CodeExpiryTooFar                  FailCode = 21
 )
 
 // String returns the string representation of the failure code.
@@ -145,6 +146,9 @@ func (c FailCode) String() string {
 	case CodeFinalIncorrectHtlcAmount:
 		return "FinalIncorrectHtlcAmount"
 
+	case CodeExpiryTooFar:
+		return "ExpiryTooFar"
+
 	default:
 		return "<unknown>"
 	}
@@ -1038,6 +1042,26 @@ func (f *FailFinalIncorrectHtlcAmount) Encode(w io.Writer, pver uint32) error {
 	return writeElement(w, f.IncomingHTLCAmount)
 }
 
+// FailExpiryTooFar is returned if the CLTV expiry in the HTLC is too far in the
+// future.
+//
+// NOTE: May be returned by any node in the payment route.
+type FailExpiryTooFar struct{}
+
+// Code returns the failure unique code.
+//
+// NOTE: Part of the FailureMessage interface.
+func (f FailExpiryTooFar) Code() FailCode {
+	return CodeExpiryTooFar
+}
+
+// Returns a human readable string describing the target FailureMessage.
+//
+// NOTE: Implements the error interface.
+func (f FailExpiryTooFar) Error() string {
+	return f.Code().String()
+}
+
 // DecodeFailure decodes, validates, and parses the lnwire onion failure, for
 // the provided protocol version.
 func DecodeFailure(r io.Reader, pver uint32) (FailureMessage, error) {
@@ -1199,6 +1223,10 @@ func makeEmptyOnionError(code FailCode) (FailureMessage, error) {
 
 	case CodeFinalIncorrectHtlcAmount:
 		return &FailFinalIncorrectHtlcAmount{}, nil
+
+	case CodeExpiryTooFar:
+		return &FailExpiryTooFar{}, nil
+
 	default:
 		return nil, errors.Errorf("unknown error code: %v", code)
 	}