htlcswitch: create error obfuscator with wrapped type for blinded

Create our error encrypter with a wrapped type if we have a blinding
point present. Doing this in the iterator allows us to track this
information when we have both pieces of information available to us,
compared to trying to handle this later down the line:
- Downstream link on failure: we know that we've set a blinding point
  for out outgoing HTLC, but not whether we're introduction or not
- Upstream link on failure: once the failure packet has been sent
  through the switch, we no longer know whether we were the introduction
  point (without looking it up / examining our payload again /
  propagating this information through the switch).

htlcswitch/hop/iterator.go

@@ -103,8 +103,8 @@ type Iterator interface {
 
 	// ExtractErrorEncrypter returns the ErrorEncrypter needed for this hop,
 	// along with a failure code to signal if the decoding was successful.
-	ExtractErrorEncrypter(ErrorEncrypterExtracter) (ErrorEncrypter,
-		lnwire.FailCode)
+	ExtractErrorEncrypter(extractor ErrorEncrypterExtracter,
+		introductionNode bool) (ErrorEncrypter, lnwire.FailCode)
 }
 
 // sphinxHopIterator is the Sphinx implementation of hop iterator which uses
@@ -235,9 +235,31 @@ func (r *sphinxHopIterator) HopPayload() (*Payload, RouteRole, error) {
 //
 // NOTE: Part of the HopIterator interface.
 func (r *sphinxHopIterator) ExtractErrorEncrypter(
-	extracter ErrorEncrypterExtracter) (ErrorEncrypter, lnwire.FailCode) {
+	extracter ErrorEncrypterExtracter, introductionNode bool) (
+	ErrorEncrypter, lnwire.FailCode) {
 
-	return extracter(r.ogPacket.EphemeralKey)
+	encrypter, errCode := extracter(r.ogPacket.EphemeralKey)
+	if errCode != lnwire.CodeNone {
+		return nil, errCode
+	}
+
+	// If we're in a blinded path, wrap the error encrypter that we just
+	// derived in a "marker" type which we'll use to know what type of
+	// error we're handling.
+	switch {
+	case introductionNode:
+		return &IntroductionErrorEncrypter{
+			ErrorEncrypter: encrypter,
+		}, errCode
+
+	case r.blindingKit.UpdateAddBlinding.IsSome():
+		return &RelayingErrorEncrypter{
+			ErrorEncrypter: encrypter,
+		}, errCode
+
+	default:
+		return encrypter, errCode
+	}
 }
 
 // BlindingProcessor is an interface that provides the cryptographic operations