htlcswitch: ability to start link in shutdown mode

In this commit, we add the ability to start a link in shutdown mode.
This means that we immediately disable any new HTLC adds in the outgoing
direction and that we queue up a Shutdown message after the next
CommitSig message is sent (or immediately if no CommitSig message is
owed).

Author: ellemouton

htlcswitch/link.go

@@ -19,6 +19,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/contractcourt"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/htlcswitch/hodl"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
 	"github.com/lightningnetwork/lnd/invoices"
@@ -271,6 +272,14 @@ type ChannelLinkConfig struct {
 	// GetAliases is used by the link and switch to fetch the set of
 	// aliases for a given link.
 	GetAliases func(base lnwire.ShortChannelID) []lnwire.ShortChannelID
+
+	// PreviouslySentShutdown is an optional value that is set if, at the
+	// time of the link being started, persisted shutdown info was found for
+	// the channel. This value being set means that we previously sent a
+	// Shutdown message to our peer, and so we should do so again on
+	// re-establish and should not allow anymore HTLC adds on the outgoing
+	// direction of the link.
+	PreviouslySentShutdown fn.Option[lnwire.Shutdown]
 }
 
 // channelLink is the service which drives a channel's commitment update
@@ -1190,6 +1199,25 @@ func (l *channelLink) htlcManager() {
 		}
 	}
 
+	// If a shutdown message has previously been sent on this link, then we
+	// need to make sure that we have disabled any HTLC adds on the outgoing
+	// direction of the link and that we re-resend the same shutdown message
+	// that we previously sent.
+	l.cfg.PreviouslySentShutdown.WhenSome(func(shutdown lnwire.Shutdown) {
+		// Immediately disallow any new outgoing HTLCs.
+		if !l.DisableAdds(Outgoing) {
+			l.log.Warnf("Outgoing link adds already disabled")
+		}
+
+		// Re-send the shutdown message the peer. Since syncChanStates
+		// would have sent any outstanding CommitSig, it is fine for us
+		// to immediately queue the shutdown message now.
+		err := l.cfg.Peer.SendMessage(false, &shutdown)
+		if err != nil {
+			l.log.Warnf("Error sending shutdown message: %v", err)
+		}
+	})
+
 	// We've successfully reestablished the channel, mark it as such to
 	// allow the switch to forward HTLCs in the outbound direction.
 	l.markReestablished()