hostinfo: cache device model to speed up init

This was causing a relatively consistent ~10ms of delay on Linux.

Signed-off-by: Kyle Carberry <kyle@carberry.com>

hostinfo/hostinfo.go

@@ -52,7 +52,7 @@ func New() *tailcfg.Hostinfo {
 		GoArchVar:       lazyGoArchVar.Get(),
 		GoVersion:       runtime.Version(),
 		Machine:         condCall(unameMachine),
-		DeviceModel:     deviceModel(),
+		DeviceModel:     deviceModelCached(),
 		Cloud:           string(cloudenv.Get()),
 		NoLogsNoSupport: envknob.NoLogsNoSupport(),
 		AllowsUpdate:    envknob.AllowsRemoteUpdate(),
@@ -68,6 +68,7 @@ var (
 	distroVersion  func() string
 	distroCodeName func() string
 	unameMachine   func() string
+	deviceModel    func() string
 )
 
 func condCall[T any](fn func() T) T {
@@ -176,6 +177,20 @@ var (
 // SetDeviceModel sets the device model for use in Hostinfo updates.
 func SetDeviceModel(model string) { deviceModelAtomic.Store(model) }
 
+func deviceModelCached() string {
+	if v, _ := deviceModelAtomic.Load().(string); v != "" {
+		return v
+	}
+	if deviceModel == nil {
+		return ""
+	}
+	v := deviceModel()
+	if v != "" {
+		deviceModelAtomic.Store(v)
+	}
+	return v
+}
+
 // SetOSVersion sets the OS version.
 func SetOSVersion(v string) { osVersionAtomic.Store(v) }
 
@@ -193,11 +208,6 @@ func SetPackage(v string) { packagingType.Store(v) }
 // and "k8s-operator".
 func SetApp(v string) { appType.Store(v) }
 
-func deviceModel() string {
-	s, _ := deviceModelAtomic.Load().(string)
-	return s
-}
-
 // FirewallMode returns the firewall mode for the app.
 // It is empty if unset.
 func FirewallMode() string {

hostinfo/hostinfo_linux.go

@@ -22,9 +22,7 @@ func init() {
 	distroName = distroNameLinux
 	distroVersion = distroVersionLinux
 	distroCodeName = distroCodeNameLinux
-	if v := linuxDeviceModel(); v != "" {
-		SetDeviceModel(v)
-	}
+	deviceModel = deviceModelLinux
 }
 
 var (
@@ -50,7 +48,7 @@ func distroCodeNameLinux() string {
 	return lazyVersionMeta.Get().DistroCodeName
 }
 
-func linuxDeviceModel() string {
+func deviceModelLinux() string {
 	for _, path := range []string{
 		// First try the Synology-specific location.
 		// Example: "DS916+-j"