htlcswitch: reject HTLCs that use use as introduction if disabled

Reject any HTLCs that use us as an introduction point in a blinded
route if we have disabled route blinding. We have to do this after
we've processed the payload, because we only know we're an introduction
point once we've processed the payload itself.

htlcswitch/link.go

@@ -3309,6 +3309,27 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 
 		fwdInfo := pld.ForwardingInfo()
 
+		// Check whether the payload we've just processed uses our
+		// node as the introduction point (gave us a blinding key in
+		// the payload itself) and fail it back if we don't support
+		// route blinding.
+		if fwdInfo.NextBlinding.IsSome() &&
+			l.cfg.DisallowRouteBlinding {
+
+			failure := lnwire.NewInvalidBlinding(
+				onionBlob[:],
+			)
+			l.sendHTLCError(
+				pd, NewLinkError(failure), obfuscator, false,
+			)
+
+			l.log.Error("rejected htlc that uses use as an " +
+				"introduction point when we do not support " +
+				"route blinding")
+
+			continue
+		}
+
 		switch fwdInfo.NextHop {
 		case hop.Exit:
 			err := l.processExitHop(