htlcswitch: move packet failure to mailbox

This commit moves the current logic for sending failures out of the link
and into the mailbox in preparation for our failing delayed htlcs. We do
so because the mailbox may need to fail packets while the link is
offline, and needs to be able to complete the task without member
methods on the link.

htlcswitch/link.go

@@ -1272,72 +1272,6 @@ func (l *channelLink) handleDownstreamPkt(pkt *htlcPacket) {
 			l.log.Warnf("Unable to handle downstream add HTLC: %v",
 				err)
 
-			var (
-				localFailure = false
-				reason       lnwire.OpaqueReason
-			)
-
-			// Create a temporary channel failure which we will send
-			// back to our peer if this is a forward, or report to
-			// the user if the failed payment was locally initiated.
-			failure := l.createFailureWithUpdate(
-				func(upd *lnwire.ChannelUpdate) lnwire.FailureMessage {
-					return lnwire.NewTemporaryChannelFailure(
-						upd,
-					)
-				},
-			)
-
-			// If the payment was locally initiated (which is
-			// indicated by a nil obfuscator), we do not need to
-			// encrypt it back to the sender.
-			if pkt.obfuscator == nil {
-				var b bytes.Buffer
-				err := lnwire.EncodeFailure(&b, failure, 0)
-				if err != nil {
-					l.log.Errorf("unable to encode "+
-						"failure: %v", err)
-					l.mailBox.AckPacket(pkt.inKey())
-					return
-				}
-				reason = lnwire.OpaqueReason(b.Bytes())
-				localFailure = true
-			} else {
-				// If the packet is part of a forward,
-				// (identified by a non-nil obfuscator) we need
-				// to encrypt the error back to the source.
-				var err error
-				reason, err = pkt.obfuscator.EncryptFirstHop(failure)
-				if err != nil {
-					l.log.Errorf("unable to "+
-						"obfuscate error: %v", err)
-					l.mailBox.AckPacket(pkt.inKey())
-					return
-				}
-			}
-
-			// Create a link error containing the temporary channel
-			// failure and a detail which indicates the we failed to
-			// add the htlc.
-			linkError := NewDetailedLinkError(
-				failure, OutgoingFailureDownstreamHtlcAdd,
-			)
-
-			failPkt := &htlcPacket{
-				incomingChanID: pkt.incomingChanID,
-				incomingHTLCID: pkt.incomingHTLCID,
-				circuit:        pkt.circuit,
-				sourceRef:      pkt.sourceRef,
-				hasSource:      true,
-				localFailure:   localFailure,
-				linkFailure:    linkError,
-				htlc: &lnwire.UpdateFailHTLC{
-					Reason: reason,
-				},
-			}
-
-			go l.forwardBatch(failPkt)
-
 			// Remove this packet from the link's mailbox, this
 			// prevents it from being reprocessed if the link
 			// restarts and resets it mailbox. If this response
@@ -1346,7 +1280,7 @@ func (l *channelLink) handleDownstreamPkt(pkt *htlcPacket) {
 			// the switch, since the circuit was never fully opened,
 			// and the forwarding package shows it as
 			// unacknowledged.
-			l.mailBox.AckPacket(pkt.inKey())
+			l.mailBox.FailAdd(pkt)
 
 			return
 		}