htlcswitch+peer: set and read aux custom records

This is the final step, we actually call the interface and either
provide or retrieve the custom features over the message. We also notify
the aux components when channel reestablish is received.

peer/brontide.go

@@ -456,6 +456,11 @@ type Config struct {
 	// used to modify the way the co-op close transaction is constructed.
 	AuxChanCloser fn.Option[chancloser.AuxChanCloser]
 
+	// AuxChannelNegotiator is an optional interface that allows aux channel
+	// implementations to inject and process custom records over channel
+	// related wire messages.
+	AuxChannelNegotiator fn.Option[lnwallet.AuxChannelNegotiator]
+
 	// ShouldFwdExpEndorsement is a closure that indicates whether
 	// experimental endorsement signals should be set.
 	ShouldFwdExpEndorsement func() bool
@@ -1454,8 +1459,9 @@ func (p *Brontide) addLink(chanPoint *wire.OutPoint,
 		ShouldFwdExpEndorsement: p.cfg.ShouldFwdExpEndorsement,
 		DisallowQuiescence: p.cfg.DisallowQuiescence ||
 			!p.remoteFeatures.HasFeature(lnwire.QuiescenceOptional),
-		AuxTrafficShaper:  p.cfg.AuxTrafficShaper,
-		QuiescenceTimeout: p.cfg.QuiescenceTimeout,
+		AuxTrafficShaper:     p.cfg.AuxTrafficShaper,
+		AuxChannelNegotiator: p.cfg.AuxChannelNegotiator,
+		QuiescenceTimeout:    p.cfg.QuiescenceTimeout,
 	}
 
 	// Before adding our new link, purge the switch of any pending or live
@@ -4537,6 +4543,19 @@ func (p *Brontide) handleInitMsg(msg *lnwire.Init) error {
 		return fmt.Errorf("data loss protection required")
 	}
 
+	// If we have an AuxChannelNegotiator and the peer sent aux features,
+	// process them.
+	p.cfg.AuxChannelNegotiator.WhenSome(
+		func(acn lnwallet.AuxChannelNegotiator) {
+			err = acn.ProcessInitRecords(
+				p.cfg.PubKeyBytes, msg.CustomRecords.Copy(),
+			)
+		},
+	)
+	if err != nil {
+		return fmt.Errorf("could not process init records: %w", err)
+	}
+
 	return nil
 }
 
@@ -4597,6 +4616,30 @@ func (p *Brontide) sendInitMsg(legacyChan bool) error {
 		features.RawFeatureVector,
 	)
 
+	var err error
+
+	// If we have an AuxChannelNegotiator, get custom feature bits to
+	// include in the init message.
+	p.cfg.AuxChannelNegotiator.WhenSome(
+		func(negotiator lnwallet.AuxChannelNegotiator) {
+			var auxRecords lnwire.CustomRecords
+			auxRecords, err = negotiator.GetInitRecords(
+				p.cfg.PubKeyBytes,
+			)
+			if err != nil {
+				p.log.Warnf("Failed to get aux init features: "+
+					"%v", err)
+				return
+			}
+
+			mergedRecs := msg.CustomRecords.MergedCopy(auxRecords)
+			msg.CustomRecords = mergedRecs
+		},
+	)
+	if err != nil {
+		return err
+	}
+
 	return p.writeMessage(msg)
 }