multi: fix inconsistent state in gossip syncer

[yyforyongyu]

Fixed a long standing issue, which is also sometimes found in our itest,

     --- FAIL: TestLightningNetworkDaemon/tranche09/156-of-269/bitcoind/receiver_blinded_error (57.25s)
        harness_node.go:403: Starting node (name=Alice) with PID=14745
        harness_node.go:403: Starting node (name=Bob) with PID=14777
        harness_node.go:403: Starting node (name=Carol) with PID=14805
        harness_node.go:403: Starting node (name=Dave) with PID=14878
        harness_assertion.go:1879: 
            	Error Trace:	/home/runner/work/lnd/lnd/lntest/harness_assertion.go:1879
            	            				/home/runner/work/lnd/lnd/lntest/harness_assertion.go:1937
            	            				/home/runner/work/lnd/lnd/lntest/harness.go:2353
            	            				/home/runner/work/lnd/lnd/lntest/harness.go:2219
            	            				/home/runner/work/lnd/lnd/itest/lnd_route_blinding_test.go:373
            	            				/home/runner/work/lnd/lnd/itest/lnd_route_blinding_test.go:631
            	            				/home/runner/work/lnd/lnd/lntest/harness.go:297
            	            				/home/runner/work/lnd/lnd/itest/lnd_test.go:130
            	Error:      	Received unexpected error:
            	            	channel 90033bd9f64e9aeff15f8730927a56b7192c3d835ddfa6b0ad9fbd7fff2ec131:0 not found in graph: rpc error: code = Unknown desc = edge not found: op=90033bd9f64e9aeff15f8730927a56b7192c3d835ddfa6b0ad9fbd7fff2ec131:0
            	Test:       	TestLightningNetworkDaemon/tranche09/156-of-269/bitcoind/receiver_blinded_error
            	Messages:   	Dave: timeout finding channel in graph

The Issue

When we receive a query response from the remote, we require the syncer to be in a waiting for reply state,

lnd/discovery/syncer.go

Lines 1515 to 1524 in e0a920a

	// Reply messages should only be expected in states where we're waiting
	// for a reply.
	case *lnwire.ReplyChannelRange, *lnwire.ReplyShortChanIDsEnd:
	syncState := g.syncState()
	if syncState != waitingQueryRangeReply &&
	syncState != waitingQueryChanReply {
	return fmt.Errorf("received unexpected query reply "+
	"message %T", msg)
	}

And this state is only set when we finishes sending the msg to the peer,

lnd/discovery/syncer.go

Lines 509 to 518 in e0a920a

	err = g.cfg.sendToPeer(queryRangeMsg)
	if err != nil {
	log.Errorf("Unable to send chan range "+
	"query: %v", err)
	return
	}
	// With the message sent successfully, we'll transition
	// into the next state where we wait for their reply.
	g.setSyncState(waitingQueryRangeReply)

However, we may get a reply from the remote peer before we finish setting the state above. The consequence is we never get out of the initial historical sync stage, causing the node to refuse to propagate channel announcements to its peers.

We barely notice this as the error is ignored here (and the error returned from ProcessRemoteAnnouncement is not processed anyway...more future PRs),

lnd/discovery/gossiper.go

Lines 831 to 836 in e0a920a

	// If we've found the message target, then we'll dispatch the
	// message directly to it.
	syncer.ProcessQueryMsg(m, peer.QuitSignal())
	errChan <- nil
	return errChan

The fix

Quite some debug logs have been added to help catch the bug, and the fix is to lock the state changes when sending msgs to the remote peer, so send msg -> update state is now atomic.

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[ziggie1984]

LGTM, nice catch

[ziggie1984]

could you explain why the chainSyncer remaining in the waitingQueryChanReply will cause the chan_announcments received from the peer directly to not be relayed ?

[yyforyongyu]

yeah it's very intertwined...so we skip broadcasting channel anns from our peers here,

lnd/discovery/gossiper.go

Lines 1574 to 1580 in e0a920a

	if newAnns != nil && shouldBroadcast {
	// TODO(roasbeef): exclude peer that sent.
	deDuped.AddMsgs(newAnns...)
	} else if newAnns != nil {
	log.Trace("Skipping broadcast of announcements received " +
	"during initial graph sync")
	}

and the shouldBroadcast is determined here,

lnd/discovery/gossiper.go

Lines 1533 to 1535 in e0a920a

	// We should only broadcast this message forward if it originated from
	// us or it wasn't received as part of our initial historical sync.
	shouldBroadcast := !nMsg.isRemote || d.syncMgr.IsGraphSynced()

which relies on this method,

lnd/discovery/sync_manager.go

Lines 806 to 811 in e0a920a

	// IsGraphSynced determines whether we've completed our initial historical sync.
	// The initial historical sync is done to ensure we've ingested as much of the
	// public graph as possible.
	func (m *SyncManager) IsGraphSynced() bool {
	return atomic.LoadInt32(&m.initialHistoricalSyncCompleted) == 1
	}

and the var initialHistoricalSyncCompleted is marked via,

lnd/discovery/sync_manager.go

Lines 800 to 804 in e0a920a

	// markGraphSynced allows us to report that the initial historical sync has
	// completed.
	func (m *SyncManager) markGraphSynced() {
	atomic.StoreInt32(&m.initialHistoricalSyncCompleted, 1)
	}

and the above method is called inside processChanRangeReply here,

lnd/discovery/syncer.go

Lines 951 to 954 in e0a920a

	// Ensure that the sync manager becomes aware that the
	// historical sync completed so synced_to_graph is updated over
	// rpc.
	g.cfg.markGraphSynced()

and the processChanRangeReply is called here,

lnd/discovery/syncer.go

Lines 528 to 537 in e0a920a

	select {
	case msg := <-g.gossipMsgs:
	// The remote peer is sending a response to our
	// initial query, we'll collate this response,
	// and see if it's the final one in the series.
	// If so, we can then transition to querying
	// for the new channels.
	queryReply, ok := msg.(*lnwire.ReplyChannelRange)
	if ok {
	err := g.processChanRangeReply(queryReply)

Note that it be blocked on case msg := <-g.gossipMsgs:, as the msg is never sent to the channel here,

lnd/discovery/syncer.go

Lines 1515 to 1532 in e0a920a

	// Reply messages should only be expected in states where we're waiting
	// for a reply.
	case *lnwire.ReplyChannelRange, *lnwire.ReplyShortChanIDsEnd:
	syncState := g.syncState()
	if syncState != waitingQueryRangeReply &&
	syncState != waitingQueryChanReply {
	return fmt.Errorf("received unexpected query reply "+
	"message %T", msg)
	}
	msgChan = g.gossipMsgs
	default:
	msgChan = g.gossipMsgs
	}
	select {
	case msgChan <- msg:

As the above case *lnwire.ReplyChannelRange, *lnwire.ReplyShortChanIDsEnd: will error out due to the state of the syncer not being updated to waitingQueryRangeReply yet.

[ziggie1984]

Thanks for the detailed explanation, agree really nested.

[ellemouton]

very nice investigation! 🔥 thanks for the detailed description

[ellemouton]

I'll try get around to updating the gossiper to use structured logging. That way we only need to add all this to the context once and then can just log.DebugS(ctx, "Processed NodeAnnouncement")