ipn/ipnlocal: add basic support for netmap caching (#18530)

This commit is based on ff0978ab, and extends #18497 to connect network map
caching to the LocalBackend. As implemented, only "whole" netmap values are
stored, and we do not yet handle incremental updates. As-written, the feature must
be explicitly enabled via the TS_USE_CACHED_NETMAP envknob, and must be
considered experimental.

Updates #12639

Co-Authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I48a1e92facfbf7fb3a8e67cff7f2c9ab4ed62c83
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

ipn/ipnlocal/local.go

@@ -271,6 +271,7 @@ type LocalBackend struct {
 	// of [LocalBackend]'s own state that is not tied to the node context.
 	currentNodeAtomic atomic.Pointer[nodeBackend]
 
+	diskCache        diskCache
 	conf             *conffile.Config // latest parsed config, or nil if not in declarative mode
 	pm               *profileManager  // mu guards access
 	lastFilterInputs *filterInputs
@@ -1573,7 +1574,13 @@ func (b *LocalBackend) SetControlClientStatus(c controlclient.Client, st control
 	}
 	b.mu.Lock()
 	defer b.mu.Unlock()
+	b.setControlClientStatusLocked(c, st)
+}
 
+// setControlClientStatusLocked is the locked version of SetControlClientStatus.
+//
+// b.mu must be held.
+func (b *LocalBackend) setControlClientStatusLocked(c controlclient.Client, st controlclient.Status) {
 	if b.cc != c {
 		b.logf("Ignoring SetControlClientStatus from old client")
 		return
@@ -2414,6 +2421,14 @@ func (b *LocalBackend) initOnce() {
 	b.extHost.Init()
 }
 
+func (b *LocalBackend) controlDebugFlags() []string {
+	debugFlags := controlDebugFlags
+	if b.sys.IsNetstackRouter() {
+		return append([]string{"netstack"}, debugFlags...)
+	}
+	return debugFlags
+}
+
 // Start applies the configuration specified in opts, and starts the
 // state machine.
 //
@@ -2570,14 +2585,18 @@ func (b *LocalBackend) startLocked(opts ipn.Options) error {
 		persistv = new(persist.Persist)
 	}
 
-	discoPublic := b.MagicConn().DiscoPublicKey()
-
-	isNetstack := b.sys.IsNetstackRouter()
-	debugFlags := controlDebugFlags
-	if isNetstack {
-		debugFlags = append([]string{"netstack"}, debugFlags...)
+	if envknob.Bool("TS_USE_CACHED_NETMAP") {
+		if nm, ok := b.loadDiskCacheLocked(); ok {
+			logf("loaded netmap from disk cache; %d peers", len(nm.Peers))
+			b.setControlClientStatusLocked(nil, controlclient.Status{
+				NetMap:   nm,
+				LoggedIn: true, // sure
+			})
+		}
 	}
 
+	discoPublic := b.MagicConn().DiscoPublicKey()
+
 	var ccShutdownCbs []func()
 	ccShutdown := func() {
 		for _, cb := range ccShutdownCbs {
@@ -2603,7 +2622,7 @@ func (b *LocalBackend) startLocked(opts ipn.Options) error {
 		Hostinfo:             b.hostInfoWithServicesLocked(),
 		HTTPTestClient:       httpTestClient,
 		DiscoPublicKey:       discoPublic,
-		DebugFlags:           debugFlags,
+		DebugFlags:           b.controlDebugFlags(),
 		HealthTracker:        b.health,
 		PolicyClient:         b.sys.PolicyClientOrDefault(),
 		Pinger:               b,
@@ -2619,7 +2638,7 @@ func (b *LocalBackend) startLocked(opts ipn.Options) error {
 
 		// Don't warn about broken Linux IP forwarding when
 		// netstack is being used.
-		SkipIPForwardingCheck: isNetstack,
+		SkipIPForwardingCheck: b.sys.IsNetstackRouter(),
 	})
 	if err != nil {
 		return err
@@ -6248,6 +6267,9 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	var login string
 	if nm != nil {
 		login = cmp.Or(profileFromView(nm.UserProfiles[nm.User()]).LoginName, "<missing-profile>")
+		if err := b.writeNetmapToDiskLocked(nm); err != nil {
+			b.logf("write netmap to cache: %v", err)
+		}
 	}
 	b.currentNode().SetNetMap(nm)
 	if ms, ok := b.sys.MagicSock.GetOK(); ok {