server+htlcswitch: prevent privacy leaks, allow alias routing

This intent of this change is to prevent privacy leaks when routing
with aliases and also to allow routing when using an alias. The
aliases are our aliases.

Introduces are two maps:

* aliasToReal:
  This is an N->1 mapping for a channel. The keys are the set of
  aliases and the value is the confirmed, on-chain SCID.

* baseIndex:
  This is also an N->1 mapping for a channel. The keys are the set
  of aliases and the value is the "base" SCID (whatever is in the
  OpenChannel.ShortChannelID field). There is also a base->base
  mapping, so not all keys are aliases.

The above maps are populated when a link is added to the switch and
when the channel has confirmed on-chain. The maps are not removed
from if the link is removed, but this is fine since forwarding won't
occur.

* getLinkByMapping
  This function is introduced to adhere to the spec requirements that
  using the confirmed SCID of a private, scid-alias-feature-bit
  channel does not work. Lnd implements a stricter version of the spec
  and disallows this behavior if the feature-bit was negotiated, rather
  than just the channel type. The old, privacy-leak behavior is
  preserved.

The spec also requires that if we must fail back an HTLC, the
ChannelUpdate must use the SCID of whatever was in the onion, to avoid
a privacy leak. This is also done by passing in the relevant SCID to
the mailbox and link. Lnd will also cancel back on the "incoming" side
if the InterceptableSwitch was used or if the link failed to decrypt
the onion. In this case, we are cautious and replace the SCID if an
alias exists.

htlcswitch/mailbox.go

@@ -91,10 +91,6 @@ type mailBoxConfig struct {
 	// belongs to.
 	shortChanID lnwire.ShortChannelID
 
-	// fetchUpdate retrieves the most recent channel update for the channel
-	// this mailbox belongs to.
-	fetchUpdate func(lnwire.ShortChannelID) (*lnwire.ChannelUpdate, error)
-
 	// forwardPackets send a varidic number of htlcPackets to the switch to
 	// be routed. A quit channel should be provided so that the call can
 	// properly exit during shutdown.
@@ -107,6 +103,11 @@ type mailBoxConfig struct {
 	// have not been yet been delivered. The computed deadline will expiry
 	// this long after the Adds are added via AddPacket.
 	expiry time.Duration
+
+	// failMailboxUpdate is used to fail an expired HTLC and use the
+	// correct SCID if the underlying channel uses aliases.
+	failMailboxUpdate func(outScid,
+		mailboxScid lnwire.ShortChannelID) lnwire.FailureMessage
 }
 
 // memoryMailBox is an implementation of the MailBox struct backed by purely
@@ -710,13 +711,9 @@ func (m *memoryMailBox) FailAdd(pkt *htlcPacket) {
 	// 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.
-	var failure lnwire.FailureMessage
-	update, err := m.cfg.fetchUpdate(m.cfg.shortChanID)
-	if err != nil {
-		failure = &lnwire.FailTemporaryNodeFailure{}
-	} else {
-		failure = lnwire.NewTemporaryChannelFailure(update)
-	}
+	failure := m.cfg.failMailboxUpdate(
+		pkt.originalOutgoingChanID, m.cfg.shortChanID,
+	)
 
 	// If the payment was locally initiated (which is indicated by a nil
 	// obfuscator), we do not need to encrypt it back to the sender.
@@ -817,10 +814,6 @@ type mailOrchConfig struct {
 	// properly exit during shutdown.
 	forwardPackets func(chan struct{}, ...*htlcPacket) error
 
-	// fetchUpdate retrieves the most recent channel update for the channel
-	// this mailbox belongs to.
-	fetchUpdate func(lnwire.ShortChannelID) (*lnwire.ChannelUpdate, error)
-
 	// clock is a time source for the generated mailboxes.
 	clock clock.Clock
 
@@ -828,6 +821,11 @@ type mailOrchConfig struct {
 	// have not been yet been delivered. The computed deadline will expiry
 	// this long after the Adds are added to a mailbox via AddPacket.
 	expiry time.Duration
+
+	// failMailboxUpdate is used to fail an expired HTLC and use the
+	// correct SCID if the underlying channel uses aliases.
+	failMailboxUpdate func(outScid,
+		mailboxScid lnwire.ShortChannelID) lnwire.FailureMessage
 }
 
 // newMailOrchestrator initializes a fresh mailOrchestrator.
@@ -881,11 +879,11 @@ func (mo *mailOrchestrator) exclusiveGetOrCreateMailBox(
 	mailbox, ok := mo.mailboxes[chanID]
 	if !ok {
 		mailbox = newMemoryMailBox(&mailBoxConfig{
-			shortChanID:    shortChanID,
-			fetchUpdate:    mo.cfg.fetchUpdate,
-			forwardPackets: mo.cfg.forwardPackets,
-			clock:          mo.cfg.clock,
-			expiry:         mo.cfg.expiry,
+			shortChanID:       shortChanID,
+			forwardPackets:    mo.cfg.forwardPackets,
+			clock:             mo.cfg.clock,
+			expiry:            mo.cfg.expiry,
+			failMailboxUpdate: mo.cfg.failMailboxUpdate,
 		})
 		mailbox.Start()
 		mo.mailboxes[chanID] = mailbox