peer: always send channel update on reconnect

[Roasbeef]

We have existing logic to attempt to reliably send a channel update to the remote peer. In the wild, we've seen this fail, as it's possible right when we send the update the peer disconnects.

In this commit, we implement a simple fix which is just to send the chan update each time we connect to the remote party.

Fixes #6870.

[coderabbitai]

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[yyforyongyu]

Following this analysis, even a disconnection happens, the msg won't be deleted and will be retried again during the next connection? Unless it's considered stale here,

lnd/discovery/reliable_sender.go

Lines 255 to 256 in b7c59b3

	if s.cfg.IsMsgStale(msg) {
	err := s.cfg.MessageStore.DeleteMessage(msg, peerPubKey)

which checks the channel_update via,

lnd/discovery/gossiper.go

Lines 2136 to 2168 in b7c59b3

	case *lnwire.ChannelUpdate:
	_, p1, p2, err := d.cfg.Graph.GetChannelByID(msg.ShortChannelID)
	// If the channel cannot be found, it is most likely a leftover
	// message for a channel that was closed, so we can consider it
	// stale.
	if errors.Is(err, channeldb.ErrEdgeNotFound) {
	return true
	}
	if err != nil {
	log.Debugf("Unable to retrieve channel=%v from graph: "+
	"%v", msg.ShortChannelID, err)
	return false
	}
	// Otherwise, we'll retrieve the correct policy that we
	// currently have stored within our graph to check if this
	// message is stale by comparing its timestamp.
	var p *models.ChannelEdgePolicy
	if msg.ChannelFlags&lnwire.ChanUpdateDirection == 0 {
	p = p1
	} else {
	p = p2
	}
	// If the policy is still unknown, then we can consider this
	// policy fresh.
	if p == nil {
	return false
	}
	timestamp := time.Unix(int64(msg.Timestamp), 0)
	return p.LastUpdate.After(timestamp)

which seems to be related to #7261.

[Roasbeef]

Following #6870 (comment), even a disconnection happens, the msg won't be deleted and will be retried again during the next connection? Unless it's considered stale here,

Hmm, I think you're right.

For channel updates, staleness comes down to if the on disk update is newer than the one we're trying to send:

lnd/discovery/gossiper.go

Lines 2172 to 2173 in c262b1b

	timestamp := time.Unix(int64(msg.Timestamp), 0)
	return p.LastUpdate.After(timestamp)

.

So if we haven't sent a newer channel update (compared to the default one we generated once everything confirmed), then this the update won't be seen as stale, and we'll continue to try and send it. However, we do have dynamic enable/disable logic, so if that triggers and disables/enables the channel, it'll generate a newer update, marking the one written to disk as stale.

With all that said, I think this PR does still cover an existing gap:

• Peer is offline for > 20 mins, causing us to generate a new channel update on disk (enable/disable).
• Peer comes back online, we try to send, but it actually doesn't get there.
• Newer update generated for the first step, so we delete it, but they might have not gotten any of the updates.

[Roasbeef]

Pushed revision 3 here: https://reviewable.io/reviews/lightningnetwork/lnd/8963

[yyforyongyu]

Cool pending linter and unit tests fix, otherwise it's good to go!

[Crypt-iQ]

looks good just need to add FetchLastChanUpdate to the test config

[Roasbeef]

Fixed the unit test issue, and also added a slim one to exercise this new feature (TestAlwaysSendChannelUpdate).

Linter also should be good now....

[yyforyongyu]

LGTM🦐 Need to fix the new race inTestAlwaysSendChannelUpdate before merging tho.

[yyforyongyu]

🙏