graph+server: add template for new graph Builder sub-system

This is preparation for an upcoming commit that will move over various
responsibilities from the ChannelRouter to the graph Builder. So that
that commit can be a pure code-move commit, the template for the new
sub-system is added up front here.

graph/builder.go

@@ -0,0 +1,57 @@
+package graph
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// Config holds the configuration required by the Builder.
+type Config struct{}
+
+// Builder builds and maintains a view of the Lightning Network graph.
+type Builder struct {
+	started atomic.Bool
+	stopped atomic.Bool
+
+	cfg *Config
+
+	quit chan struct{}
+	wg   sync.WaitGroup
+}
+
+// NewBuilder constructs a new Builder.
+func NewBuilder(cfg *Config) (*Builder, error) {
+	return &Builder{
+		cfg:  cfg,
+		quit: make(chan struct{}),
+	}, nil
+}
+
+// Start launches all the goroutines the Builder requires to carry out its
+// duties. If the builder has already been started, then this method is a noop.
+func (b *Builder) Start() error {
+	if !b.started.CompareAndSwap(false, true) {
+		return nil
+	}
+
+	log.Info("Builder starting")
+
+	return nil
+}
+
+// Stop signals to the Builder that it should halt all routines. This method
+// will *block* until all goroutines have excited. If the builder has already
+// stopped then this method will return immediately.
+func (b *Builder) Stop() error {
+	if !b.stopped.CompareAndSwap(false, true) {
+		return nil
+	}
+
+	log.Info("Builder shutting down...")
+	defer log.Debug("Builder shutdown complete")
+
+	close(b.quit)
+	b.wg.Wait()
+
+	return nil
+}

graph/log.go

@@ -0,0 +1,47 @@
+package graph
+
+import (
+	"github.com/btcsuite/btclog"
+	"github.com/lightningnetwork/lnd/build"
+)
+
+// log is a logger that is initialized with no output filters.  This means the
+// package will not perform any logging by default until the caller requests
+// it.
+var log btclog.Logger
+
+const Subsystem = "GRPH"
+
+// The default amount of logging is none.
+func init() {
+	UseLogger(build.NewSubLogger(Subsystem, nil))
+}
+
+// DisableLog disables all library log output.  Logging output is disabled by
+// by default until UseLogger is called.
+func DisableLog() {
+	UseLogger(btclog.Disabled)
+}
+
+// UseLogger uses a specified Logger to output package logging info.  This
+// should be used in preference to SetLogWriter if the caller is also using
+// btclog.
+func UseLogger(logger btclog.Logger) {
+	log = logger
+}
+
+// logClosure is used to provide a closure over expensive logging operations so
+// don't have to be performed when the logging level doesn't warrant it.
+type logClosure func() string
+
+// String invokes the underlying function and returns the result.
+func (c logClosure) String() string {
+	return c()
+}
+
+// newLogClosure returns a new closure over a function that returns a string
+// which itself provides a Stringer interface so that it can be used with the
+// logging system.
+func newLogClosure(c func() string) logClosure {
+	return logClosure(c)
+}

log.go

@@ -18,6 +18,7 @@ import (
 	"github.com/lightningnetwork/lnd/contractcourt"
 	"github.com/lightningnetwork/lnd/discovery"
 	"github.com/lightningnetwork/lnd/funding"
+	"github.com/lightningnetwork/lnd/graph"
 	"github.com/lightningnetwork/lnd/healthcheck"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/invoices"
@@ -179,6 +180,7 @@ func SetupLoggers(root *build.RotatingLogWriter, interceptor signal.Interceptor)
 	AddSubLogger(root, btcwallet.Subsystem, interceptor, btcwallet.UseLogger)
 	AddSubLogger(root, rpcwallet.Subsystem, interceptor, rpcwallet.UseLogger)
 	AddSubLogger(root, peersrpc.Subsystem, interceptor, peersrpc.UseLogger)
+	AddSubLogger(root, graph.Subsystem, interceptor, graph.UseLogger)
 }
 
 // AddSubLogger is a helper method to conveniently create and register the

server.go

@@ -41,6 +41,7 @@ import (
 	"github.com/lightningnetwork/lnd/feature"
 	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/funding"
+	"github.com/lightningnetwork/lnd/graph"
 	"github.com/lightningnetwork/lnd/healthcheck"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
@@ -271,6 +272,8 @@ type server struct {
 
 	missionControl *routing.MissionControl
 
+	graphBuilder *graph.Builder
+
 	chanRouter *routing.ChannelRouter
 
 	controlTower routing.ControlTower
@@ -973,6 +976,11 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 	strictPruning := cfg.Bitcoin.Node == "neutrino" ||
 		cfg.Routing.StrictZombiePruning
 
+	s.graphBuilder, err = graph.NewBuilder(&graph.Config{})
+	if err != nil {
+		return nil, fmt.Errorf("can't create graph builder: %w", err)
+	}
+
 	s.chanRouter, err = routing.New(routing.Config{
 		SelfNode:            selfNode.PubKeyBytes,
 		RoutingGraph:        graphsession.NewRoutingGraph(chanGraph),
@@ -2019,6 +2027,12 @@ func (s *server) Start() error {
 		}
 		cleanup = cleanup.add(s.authGossiper.Stop)
 
+		if err := s.graphBuilder.Start(); err != nil {
+			startErr = err
+			return
+		}
+		cleanup = cleanup.add(s.graphBuilder.Stop)
+
 		if err := s.chanRouter.Start(); err != nil {
 			startErr = err
 			return