htlcswitch/link: add pending commit ticker for stall detection

This commit adds a PendingCommitTicker to the link config, which allows
us to control how quickly we fail the link if the commitment dance
stalls. Now that the mailbox has the ability to cancel packets, when the
link fails it will reset the mailbox packets on exit, forcing a
reevaluation of the HTLCs against their mailbox expiries.

htlcswitch/link.go

@@ -223,6 +223,11 @@ type ChannelLinkConfig struct {
 	// syncing.
 	FwdPkgGCTicker ticker.Ticker
 
+	// PendingCommitTicker is a ticker that allows the link to determine if
+	// a locally initiated commitment dance gets stuck waiting for the
+	// remote party to revoke.
+	PendingCommitTicker ticker.Ticker
+
 	// BatchSize is the max size of a batch of updates done to the link
 	// before we do a state update.
 	BatchSize uint32
@@ -1098,6 +1103,11 @@ func (l *channelLink) htlcManager() {
 				return
 			}
 
+		case <-l.cfg.PendingCommitTicker.Ticks():
+			l.fail(LinkFailureError{code: ErrRemoteUnresponsive},
+				"unable to complete dance")
+			return
+
 		// A message from the switch was just received. This indicates
 		// that the link is an intermediate hop in a multi-hop HTLC
 		// circuit.
@@ -1934,6 +1944,8 @@ func (l *channelLink) updateCommitTx() error {
 
 	theirCommitSig, htlcSigs, pendingHTLCs, err := l.channel.SignNextCommitment()
 	if err == lnwallet.ErrNoWindow {
+		l.cfg.PendingCommitTicker.Resume()
+
 		l.log.Tracef("revocation window exhausted, unable to send: "+
 			"%v, pend_updates=%v, dangling_closes%v",
 			l.channel.PendingLocalUpdateCount(),
@@ -1953,6 +1965,8 @@ func (l *channelLink) updateCommitTx() error {
 		return err
 	}
 
+	l.cfg.PendingCommitTicker.Pause()
+
 	// The remote party now has a new pending commitment, so we'll update
 	// the contract court to be aware of this new set (the prior old remote
 	// pending).