routing: add strict pruning as new flag, default for neutrino

In this commit we add a new flag that controls if lnd will do strict
zombie pruning or not. If true, then this will cause lnd to maintain a
tighter graph as it wants both edges to always be live. We enable this by
default for neutrino as without this, it's possible that a node never
sees both edges begin disabled, so those edges are never actually pruned
from the graph.

lncfg/routing.go

@@ -2,5 +2,7 @@ package lncfg
 
 // Routing holds the configuration options for routing.
 type Routing struct {
-	AssumeChannelValid bool `long:"assumechanvalid" description:"DEPRECATED: This is now turned on by default for Neutrino (use neutrino.validatechannels=true to turn off) and shouldn't be used for any other backend! (default: false)"`
+	AssumeChannelValid bool `long:"assumechanvalid" description:"Skip checking channel spentness during graph validation. This speedup comes at the risk of using an unvalidated view of the network for routing. (default: false)"`
+
+	StrictZombiePruning bool `long:"strictgraphpruning" description:"If true, then the graph will be pruned more aggressively for zombies. In practice this means that edges with a single stale edge will be considered a zombie."`
 }

sample-lnd.conf

@@ -489,6 +489,12 @@ bitcoin.node=btcd
 ; other backend!
 ; --routing.assumechanvalid=true
 
+; If set to true, then we'll prune a channel if only a single edge is seen as
+; being stale. This results in a more compact channel graph, and also is helpful
+; for neutrino nodes as it means they'll only maintain edges where both nodes are
+; seen as being live from it's PoV.
+; --routing.strictgraphpruning=true
+
 [Btcd]
 
 ; The base directory that contains the node's data, logs, configuration file,

server.go

@@ -768,6 +768,8 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s.controlTower = routing.NewControlTower(paymentControl)
 
+	strictPruning := (cfg.Bitcoin.Node == "neutrino" ||
+		cfg.Routing.StrictZombiePruning)
 	s.chanRouter, err = routing.New(routing.Config{
 		Graph:               chanGraph,
 		Chain:               cc.ChainIO,
@@ -784,6 +786,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		NextPaymentID:       sequencer.NextID,
 		PathFindingConfig:   pathFindingConfig,
 		Clock:               clock.NewDefaultClock(),
+		StrictZombiePruning: strictPruning,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("can't create router: %v", err)