multi: add base lookup option to AddLocalAlias

We add an extra option to the AddLocalAlias method which only controls
whether we store a reverse lookup from the alias back to the base scid
it corresponds to. The previous flag "gossip" is still maintained, and
in a way supercedes the new flag (it will also store the base scid
lookup even if the base lookup flag isn't set). The only call that sets
this option is the XAddLocalChanAlias RPC endpoint, where we want to
make sure that a reverse lookup is stored in the alias manager in order
to later expose it via the new RPC method.

aliasmgr/aliasmgr.go

@@ -239,14 +239,43 @@ func (m *Manager) populateMaps() error {
 	return nil
 }
 
+// addAliasCfg is a struct that hosts various options related to adding a local
+// alias to the alias manager.
+type addAliasCfg struct {
+	// baseLookup signals that the alias should also store a reverse look-up
+	// to the base scid.
+	baseLookup bool
+}
+
+// AddLocalAliasOption is a functional option that modifies the configuration
+// for adding a local alias.
+type AddLocalAliasOption func(cfg *addAliasCfg)
+
+// WithBaseLookup is a functional option that controls whether a reverse lookup
+// will be stored from the alias to the base scid.
+func WithBaseLookup() AddLocalAliasOption {
+	return func(cfg *addAliasCfg) {
+		cfg.baseLookup = true
+	}
+}
+
 // AddLocalAlias adds a database mapping from the passed alias to the passed
 // base SCID. The gossip boolean marks whether or not to create a mapping
 // that the gossiper will use. It is set to false for the upgrade path where
 // the feature-bit is toggled on and there are existing channels. The linkUpdate
-// flag is used to signal whether this function should also trigger an update
-// on the htlcswitch scid alias maps.
+// flag is used to signal whether this function should also trigger an update on
+// the htlcswitch scid alias maps.
+//
+// NOTE: The following aliases will not be persisted (will be lost on restart):
+//   - Aliases that were created without gossip flag.
+//   - Aliases that correspond to confirmed channels.
 func (m *Manager) AddLocalAlias(alias, baseScid lnwire.ShortChannelID,
-	gossip, linkUpdate bool) error {
+	gossip, linkUpdate bool, opts ...AddLocalAliasOption) error {
+
+	cfg := addAliasCfg{}
+	for _, opt := range opts {
+		opt(&cfg)
+	}
 
 	// We need to lock the manager for the whole duration of this method,
 	// except for the very last part where we call the link updater. In
@@ -302,8 +331,9 @@ func (m *Manager) AddLocalAlias(alias, baseScid lnwire.ShortChannelID,
 	// Update the aliasToBase and baseToSet maps.
 	m.baseToSet[baseScid] = append(m.baseToSet[baseScid], alias)
 
-	// Only store the gossiper map if gossip is true.
-	if gossip {
+	// Only store the gossiper map if gossip is true, or if the caller
+	// explicitly asked to store this reverse mapping.
+	if gossip || cfg.baseLookup {
 		m.aliasToBase[alias] = baseScid
 	}
 
@@ -342,7 +372,9 @@ func (m *Manager) GetAliases(
 }
 
 // FindBaseSCID finds the base SCID for a given alias. This is used in the
-// gossiper to find the correct SCID to lookup in the graph database.
+// gossiper to find the correct SCID to lookup in the graph database. It can
+// also be used to look up the base for manual aliases that were added over the
+// RPC.
 func (m *Manager) FindBaseSCID(
 	alias lnwire.ShortChannelID) (lnwire.ShortChannelID, error) {
 
@@ -446,7 +478,7 @@ func (m *Manager) DeleteLocalAlias(alias,
 	}
 
 	// Finally, we'll delete the aliasToBase mapping from the Manager's
-	// cache (but this is only set if we gossip the alias).
+	// cache.
 	delete(m.aliasToBase, alias)
 
 	// We definitely need to unlock the Manager before calling the link

aliasmgr/aliasmgr_test.go

@@ -179,10 +179,19 @@ func TestAliasLifecycle(t *testing.T) {
 	require.Equal(t, StartingAlias, firstRequested)
 
 	// We now manually add the next alias from the range as a custom alias.
+	// This time we also use the base lookup option, in order to be able to
+	// go from alias back to the base scid.
 	secondAlias := getNextScid(firstRequested)
-	err = aliasStore.AddLocalAlias(secondAlias, baseScid, false, true)
+	err = aliasStore.AddLocalAlias(
+		secondAlias, baseScid, false, true, WithBaseLookup(),
+	)
 	require.NoError(t, err)
 
+	baseLookup, err := aliasStore.FindBaseSCID(secondAlias)
+	require.NoError(t, err)
+
+	require.Equal(t, baseScid, baseLookup)
+
 	// When we now request another alias from the allocation list, we expect
 	// the third one (tx position 2) to be returned.
 	thirdRequested, err := aliasStore.RequestAlias()