android: defer vpn permission until activity is resumed (#647)

Right now, we register the launcher in MainActivity.onCreate(), inject this into the ViewModel, then show the launcher in MainView.
There is no guarantee that the activity is in RESUMED when the Composable runs, showing the launcher. This can lead to a silent RESULT_CANCELED on some OEMs.
The fix is to add a lifecycle-aware wrapper that defers the launch.

Updates tailscale/tailscale#15419

Signed-off-by: kari-ts <[email protected]>

Author: kari-ts

android/src/main/java/com/tailscale/ipn/ui/localapi/Client.kt

@@ -14,9 +14,6 @@ import com.tailscale.ipn.ui.model.StableNodeID
 import com.tailscale.ipn.ui.model.Tailcfg
 import com.tailscale.ipn.ui.util.InputStreamAdapter
 import com.tailscale.ipn.util.TSLog
-import java.nio.charset.Charset
-import kotlin.reflect.KType
-import kotlin.reflect.typeOf
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
@@ -26,6 +23,9 @@ import kotlinx.serialization.json.Json
 import kotlinx.serialization.json.decodeFromStream
 import kotlinx.serialization.serializer
 import libtailscale.FilePart
+import java.nio.charset.Charset
+import kotlin.reflect.KType
+import kotlin.reflect.typeOf
 
 private object Endpoint {
   const val DEBUG = "debug"

android/src/main/java/com/tailscale/ipn/ui/view/MainView.kt

@@ -70,6 +70,9 @@ import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.tooling.preview.Preview
 import androidx.compose.ui.unit.dp
+import androidx.lifecycle.Lifecycle
+import androidx.lifecycle.compose.LocalLifecycleOwner
+import androidx.lifecycle.repeatOnLifecycle
 import com.google.accompanist.permissions.ExperimentalPermissionsApi
 import com.tailscale.ipn.App
 import com.tailscale.ipn.R
@@ -207,8 +210,8 @@ fun MainView(
               Ipn.State.Running -> {
 
                 PromptPermissionsIfNecessary()
-
-                viewModel.showVPNPermissionLauncherIfUnauthorized()
+                viewModel.maybeRequestVpnPermission()
+                LaunchVpnPermissionIfNeeded(viewModel)
 
                 if (showKeyExpiry) {
                   ExpiryNotification(netmap = netmap, action = { viewModel.login() })
@@ -253,6 +256,21 @@ fun MainView(
   }
 }
 
+@Composable
+fun LaunchVpnPermissionIfNeeded(viewModel: MainViewModel) {
+  val lifecycleOwner = LocalLifecycleOwner.current
+  val shouldRequest by viewModel.requestVpnPermission.collectAsState()
+
+  LaunchedEffect(shouldRequest) {
+    if (!shouldRequest) return@LaunchedEffect
+
+    // Defer showing permission launcher until activity is resumed to avoid silent RESULT_CANCELED
+    lifecycleOwner.lifecycle.repeatOnLifecycle(Lifecycle.State.RESUMED) {
+      viewModel.showVPNPermissionLauncherIfUnauthorized()
+    }
+  }
+}
+
 @Composable
 fun ExitNodeStatus(navAction: () -> Unit, viewModel: MainViewModel) {
   val nodeState by viewModel.nodeState.collectAsState()

android/src/main/java/com/tailscale/ipn/ui/viewModel/MainViewModel.kt

@@ -25,7 +25,6 @@ import com.tailscale.ipn.ui.util.PeerCategorizer
 import com.tailscale.ipn.ui.util.PeerSet
 import com.tailscale.ipn.ui.util.TimeUtil
 import com.tailscale.ipn.ui.util.set
-import java.time.Duration
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.FlowPreview
 import kotlinx.coroutines.Job
@@ -34,6 +33,7 @@ import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.debounce
 import kotlinx.coroutines.launch
+import java.time.Duration
 
 class MainViewModelFactory(private val vpnViewModel: VpnViewModel) : ViewModelProvider.Factory {
   @Suppress("UNCHECKED_CAST")
@@ -60,6 +60,8 @@ class MainViewModel(private val vpnViewModel: VpnViewModel) : IpnViewModel() {
 
   // Permission to prepare VPN
   private var vpnPermissionLauncher: ActivityResultLauncher<Intent>? = null
+  private val _requestVpnPermission = MutableStateFlow(false)
+  val requestVpnPermission: StateFlow<Boolean> = _requestVpnPermission
 
   // The list of peers
   private val _peers = MutableStateFlow<List<PeerSet>>(emptyList())
@@ -187,6 +189,10 @@ class MainViewModel(private val vpnViewModel: VpnViewModel) : IpnViewModel() {
     }
   }
 
+  fun maybeRequestVpnPermission() {
+    _requestVpnPermission.value = true
+  }
+
   fun showVPNPermissionLauncherIfUnauthorized() {
     val vpnIntent = VpnService.prepare(App.get())
     if (vpnIntent != null) {
@@ -195,6 +201,7 @@ class MainViewModel(private val vpnViewModel: VpnViewModel) : IpnViewModel() {
       vpnViewModel.setVpnPrepared(true)
       startVPN()
     }
+    _requestVpnPermission.value = false // reset
   }
 
   fun toggleVpn(desiredState: Boolean) {