multi: surface may add outgoing hltc

htlcswitch/interfaces.go

@@ -145,6 +145,10 @@ type ChannelLink interface {
 	// will use this function in forwarding decisions accordingly.
 	EligibleToForward() bool
 
+	// MayAddOutgoingHtlc returns an error if we may not add an outgoing
+	// htlc to the channel.
+	MayAddOutgoingHtlc() error
+
 	// AttachMailBox delivers an active MailBox to the link. The MailBox may
 	// have buffered messages.
 	AttachMailBox(MailBox)

htlcswitch/link.go

@@ -2156,6 +2156,12 @@ func (l *channelLink) Bandwidth() lnwire.MilliSatoshi {
 	return l.channel.AvailableBalance()
 }
 
+// MayAddOutgoingHtlc indicates whether we may add any more outgoing htlcs to
+// this channel.
+func (l *channelLink) MayAddOutgoingHtlc() error {
+	return l.channel.MayAddOutgoingHtlc()
+}
+
 // AttachMailBox updates the current mailbox used by this link, and hooks up
 // the mailbox's message and packet outboxes to the link's upstream and
 // downstream chans, respectively.

htlcswitch/mock.go

@@ -756,6 +756,7 @@ func (f *mockChannelLink) Peer() lnpeer.Peer                            { return
 func (f *mockChannelLink) ChannelPoint() *wire.OutPoint                 { return &wire.OutPoint{} }
 func (f *mockChannelLink) Stop()                                        {}
 func (f *mockChannelLink) EligibleToForward() bool                      { return f.eligible }
+func (f *mockChannelLink) MayAddOutgoingHtlc() error                    { return nil }
 func (f *mockChannelLink) setLiveShortChanID(sid lnwire.ShortChannelID) { f.shortChanID = sid }
 func (f *mockChannelLink) UpdateShortChanID() (lnwire.ShortChannelID, error) {
 	f.eligible = true

lnwallet/channel.go

@@ -4934,6 +4934,33 @@ func (lc *LightningChannel) AddHTLC(htlc *lnwire.UpdateAddHTLC,
 	return pd.HtlcIndex, nil
 }
 
+// MayAddOutgoingHtlc validates whether we can add an outgoing htlc to this
+// channel. We don't have a value or circuit for this htlc, because we just
+// want to test that we have slots for a potential htlc so we use a "mock"
+// htlc to validate a potential commitment state with one more outgoing htlc.
+func (lc *LightningChannel) MayAddOutgoingHtlc() error {
+	lc.Lock()
+	defer lc.Unlock()
+
+	// Create a "mock" outgoing htlc, using the smallest amount we can add
+	// to the commitment so that we validate commitment slots rather than
+	// available balance, since our actual htlc amount is unknown at this
+	// stage.
+	pd := lc.htlcAddDescriptor(
+		&lnwire.UpdateAddHTLC{
+			Amount: lc.channelState.LocalChanCfg.MinHTLC,
+		},
+		&channeldb.CircuitKey{},
+	)
+
+	if err := lc.validateAddHtlc(pd); err != nil {
+		lc.log.Debugf("May add outgoing htlc rejected: %v", err)
+		return err
+	}
+
+	return nil
+}
+
 // htlcAddDescriptor returns a payment descriptor for the htlc and open key
 // provided to add to our local update log.
 func (lc *LightningChannel) htlcAddDescriptor(htlc *lnwire.UpdateAddHTLC,