android: lazily initialize App to avoid starting Tailscale when unnecessary

For example, when viewing the QuickSettings tile, there's no need
to start Tailscale's backend.

Fixes tailscale/corp#19860

Signed-off-by: Percy Wegmann <percy@tailscale.com>

android/src/main/java/com/tailscale/ipn/App.kt

@@ -70,7 +70,7 @@ class App : Application(), libtailscale.AppContext {
             .addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
             .addCapability(NetworkCapabilities.NET_CAPABILITY_NOT_VPN)
             .build()
-    lateinit var appInstance: App
+    private lateinit var appInstance: App
 
     @JvmStatic
     fun startActivityForResult(act: Activity, intent: Intent?, request: Int) {
@@ -78,8 +78,13 @@ class App : Application(), libtailscale.AppContext {
       f.startActivityForResult(intent, request)
     }
 
+    /**
+     * Initializes the app (if necessary) and returns the singleton app instance. Always use this
+     * function to obtain an App reference to make sure the app initializes.
+     */
     @JvmStatic
     fun getApplication(): App {
+      appInstance.initOnce()
       return appInstance
     }
   }
@@ -98,6 +103,24 @@ class App : Application(), libtailscale.AppContext {
 
   override fun onCreate() {
     super.onCreate()
+    appInstance = this
+  }
+
+  override fun onTerminate() {
+    super.onTerminate()
+    Notifier.stop()
+    applicationScope.cancel()
+  }
+
+  var initialized = false
+
+  @Synchronized
+  private fun initOnce() {
+    if (initialized) {
+      return
+    }
+    initialized = true
+
     val dataDir = this.filesDir.absolutePath
 
     // Set this to enable direct mode for taildrop whereby downloads will be saved directly
@@ -105,7 +128,6 @@ class App : Application(), libtailscale.AppContext {
     // an app local directory "Taildrop" if we cannot create that.  This mode does not support
     // user notifications for incoming files.
     val directFileDir = this.prepareDownloadsFolder()
-
     app = Libtailscale.start(dataDir, directFileDir.absolutePath, this)
     Request.setApp(app)
     Notifier.setApp(app)
@@ -116,18 +138,11 @@ class App : Application(), libtailscale.AppContext {
         STATUS_CHANNEL_ID, "VPN Status", NotificationManagerCompat.IMPORTANCE_LOW)
     createNotificationChannel(
         FILE_CHANNEL_ID, "File transfers", NotificationManagerCompat.IMPORTANCE_DEFAULT)
-    appInstance = this
     applicationScope.launch {
       Notifier.connStatus.collect { connStatus -> updateConnStatus(connStatus) }
     }
   }
 
-  override fun onTerminate() {
-    super.onTerminate()
-    Notifier.stop()
-    applicationScope.cancel()
-  }
-
   fun setWantRunning(wantRunning: Boolean) {
     val callback: (Result<Ipn.Prefs>) -> Unit = { result ->
       result.fold(
@@ -181,7 +196,6 @@ class App : Application(), libtailscale.AppContext {
     startService(intent)
   }
 
-
   // encryptToPref a byte array of data using the Jetpack Security
   // library and writes it to a global encrypted preference store.
   @Throws(IOException::class, GeneralSecurityException::class)
@@ -215,26 +229,17 @@ class App : Application(), libtailscale.AppContext {
     QuickToggleService.setReady(this, ready)
     Log.d("App", "Set Tile Ready: $ready")
     val action = if (ready) IPNReceiver.INTENT_DISCONNECT_VPN else IPNReceiver.INTENT_CONNECT_VPN
-    val intent = Intent(this, IPNReceiver::class.java).apply {
-    this.action = action
-    }
-    val pendingIntent : PendingIntent = PendingIntent.getBroadcast(
-      this,
-      0,
-      intent,
-      PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE
-    )
+    val intent = Intent(this, IPNReceiver::class.java).apply { this.action = action }
+    val pendingIntent: PendingIntent =
+        PendingIntent.getBroadcast(
+            this, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE)
     if (ready) {
       startVPN()
     }
-    val notificationMessage = if (ready) getString(R.string.connected) else getString(R.string.not_connected)
+    val notificationMessage =
+        if (ready) getString(R.string.connected) else getString(R.string.not_connected)
     notify(
-    "Tailscale",
-    notificationMessage,
-    STATUS_CHANNEL_ID,
-    pendingIntent,
-    STATUS_NOTIFICATION_ID
-)
+        "Tailscale", notificationMessage, STATUS_CHANNEL_ID, pendingIntent, STATUS_NOTIFICATION_ID)
   }
 
   fun getHostname(): String {
@@ -337,7 +342,8 @@ class App : Application(), libtailscale.AppContext {
     }
     val pending: PendingIntent =
         PendingIntent.getActivity(this, 0, viewIntent, PendingIntent.FLAG_UPDATE_CURRENT)
-    notify(getString(R.string.file_notification), msg, FILE_CHANNEL_ID, pending, FILE_NOTIFICATION_ID)
+    notify(
+        getString(R.string.file_notification), msg, FILE_CHANNEL_ID, pending, FILE_NOTIFICATION_ID)
   }
 
   fun createNotificationChannel(id: String?, name: String?, importance: Int) {
@@ -346,7 +352,13 @@ class App : Application(), libtailscale.AppContext {
     nm.createNotificationChannel(channel)
   }
 
-  fun notify(title: String?, message: String?, channel: String, intent: PendingIntent?, notificationID: Int) {
+  fun notify(
+      title: String?,
+      message: String?,
+      channel: String,
+      intent: PendingIntent?,
+      notificationID: Int
+  ) {
     val builder: NotificationCompat.Builder =
         NotificationCompat.Builder(this, channel)
             .setSmallIcon(R.drawable.ic_notification)

android/src/main/java/com/tailscale/ipn/IPNService.kt

@@ -8,8 +8,6 @@ import android.content.pm.PackageManager
 import android.net.VpnService
 import android.os.Build
 import android.system.OsConstants
-import androidx.core.app.NotificationCompat
-import androidx.core.app.NotificationManagerCompat
 import libtailscale.Libtailscale
 import java.util.UUID
 
@@ -20,8 +18,13 @@ open class IPNService : VpnService(), libtailscale.IPNService {
     return randomID
   }
 
+  override fun onCreate() {
+    super.onCreate()
+    // grab app to make sure it initializes
+    App.getApplication()
+  }
+
   override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
-    val app = applicationContext as App
     if (intent != null && "android.net.VpnService" == intent.action) {
       // Start VPN and connect to it due to Always-on VPN
       val i = Intent(IPNReceiver.INTENT_CONNECT_VPN)
@@ -30,15 +33,14 @@ open class IPNService : VpnService(), libtailscale.IPNService {
       sendBroadcast(i)
     }
     Libtailscale.requestVPN(this)
-    app.setWantRunning(true)
+    App.getApplication().setWantRunning(true)
     return START_STICKY
   }
 
   override public fun close() {
     stopForeground(true)
     Libtailscale.serviceDisconnect(this)
-    val app = applicationContext as App
-    app.setWantRunning(false)
+    App.getApplication().setWantRunning(false)
   }
 
   override fun onDestroy() {

android/src/main/java/com/tailscale/ipn/MainActivity.kt

@@ -11,12 +11,10 @@ import android.content.RestrictionsManager
 import android.content.pm.ActivityInfo
 import android.content.res.Configuration.SCREENLAYOUT_SIZE_LARGE
 import android.content.res.Configuration.SCREENLAYOUT_SIZE_MASK
-import android.net.Uri
 import android.net.VpnService
 import android.os.Bundle
 import android.provider.Settings
 import android.util.Log
-import android.widget.Toast
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.result.ActivityResultLauncher
@@ -100,6 +98,9 @@ class MainActivity : ComponentActivity() {
   override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)
 
+    // grab app to make sure it initializes
+    App.getApplication()
+
     // (jonathan) TODO: Force the app to be portrait on small screens until we have
     // proper landscape layout support
     if (!isLandscapeCapable()) {

android/src/main/java/com/tailscale/ipn/ui/util/AndroidTVUtil.kt

@@ -14,8 +14,10 @@ import com.tailscale.ipn.ui.util.AndroidTVUtil.isAndroidTV
 
 object AndroidTVUtil {
   fun isAndroidTV(): Boolean {
-    return (App.appInstance.packageManager.hasSystemFeature(PackageManager.FEATURE_TELEVISION) ||
-        App.appInstance.packageManager.hasSystemFeature(PackageManager.FEATURE_LEANBACK))
+    return (App.getApplication()
+        .packageManager
+        .hasSystemFeature(PackageManager.FEATURE_TELEVISION) ||
+        App.getApplication().packageManager.hasSystemFeature(PackageManager.FEATURE_LEANBACK))
   }
 }