android,cmd/tailscale: implement taildrop receive for Android < 10

Fixes tailscale/tailscale#2720
Fixes tailscale/tailscale#2296

Signed-off-by: Elias Naur <mail@eliasnaur.com>

android/src/main/AndroidManifest.xml

@@ -3,6 +3,7 @@
 	package="com.tailscale.ipn">
 	<uses-permission android:name="android.permission.INTERNET" />
 	<uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+	<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" android:maxSdkVersion="28"/>
 
 	<!-- Disable input emulation on ChromeOS -->
 	<uses-feature android:name="android.hardware.type.pc" android:required="false"/>

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

@@ -6,6 +6,7 @@ package com.tailscale.ipn;
 
 import android.app.Application;
 import android.app.Activity;
+import android.app.DownloadManager;
 import android.app.Fragment;
 import android.app.FragmentTransaction;
 import android.app.NotificationChannel;
@@ -31,6 +32,7 @@ import android.os.Environment;
 import android.os.Handler;
 import android.os.Looper;
 
+import android.Manifest;
 import android.webkit.MimeTypeMap;
 
 import java.io.IOException;
@@ -42,6 +44,8 @@ import java.security.GeneralSecurityException;
 import androidx.core.app.NotificationCompat;
 import androidx.core.app.NotificationManagerCompat;
 
+import androidx.core.content.ContextCompat;
+
 import androidx.security.crypto.EncryptedSharedPreferences;
 import androidx.security.crypto.MasterKey;
 
@@ -234,15 +238,33 @@ public class App extends Application {
 		return null;
 	}
 
+	void requestWriteStoragePermission(Activity act) {
+		if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q || Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {
+			// We can write files without permission.
+			return;
+		}
+		if (ContextCompat.checkSelfPermission(act, Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED) {
+			return;
+		}
+		act.requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, IPNActivity.WRITE_STORAGE_RESULT);
+	}
+
 	String insertMedia(String name, String mimeType) throws IOException {
-		ContentResolver resolver = getContentResolver();
-		ContentValues contentValues = new ContentValues();
-		contentValues.put(MediaStore.MediaColumns.DISPLAY_NAME, name);
-		if (!"".equals(mimeType)) {
-			contentValues.put(MediaStore.MediaColumns.MIME_TYPE, mimeType);
+		if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+			ContentResolver resolver = getContentResolver();
+			ContentValues contentValues = new ContentValues();
+			contentValues.put(MediaStore.MediaColumns.DISPLAY_NAME, name);
+			if (!"".equals(mimeType)) {
+				contentValues.put(MediaStore.MediaColumns.MIME_TYPE, mimeType);
+			}
+			Uri root = MediaStore.Files.getContentUri("external");
+			return resolver.insert(root, contentValues).toString();
+		} else {
+			File dir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);
+			dir.mkdirs();
+			File f = new File(dir, name);
+			return Uri.fromFile(f).toString();
 		}
-		Uri root = MediaStore.Files.getContentUri("external");
-		return resolver.insert(root, contentValues).toString();
 	}
 
 	int openUri(String uri, String mode) throws IOException {
@@ -256,8 +278,14 @@ public class App extends Application {
 	}
 
 	public void notifyFile(String uri, String msg) {
-		Intent fileIntent = new Intent(Intent.ACTION_VIEW, Uri.parse(uri));
-		PendingIntent pending = PendingIntent.getActivity(this, 0, fileIntent, PendingIntent.FLAG_UPDATE_CURRENT);
+		Intent viewIntent;
+		if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+			viewIntent = new Intent(Intent.ACTION_VIEW, Uri.parse(uri));
+		} else {
+			// uri is a file:// which is not allowed to be shared outside the app.
+			viewIntent = new Intent(DownloadManager.ACTION_VIEW_DOWNLOADS);
+		}
+		PendingIntent pending = PendingIntent.getActivity(this, 0, viewIntent, PendingIntent.FLAG_UPDATE_CURRENT);
 		NotificationCompat.Builder builder = new NotificationCompat.Builder(this, FILE_CHANNEL_ID)
 			.setSmallIcon(R.drawable.ic_notification)
 			.setContentTitle("File received")
@@ -283,4 +311,5 @@ public class App extends Application {
 	static native void onVPNPrepared();
 	private static native void onConnectivityChanged(boolean connected);
 	static native void onShareIntent(int nfiles, int[] types, String[] mimes, String[] items, String[] names, long[] sizes);
+	static native void onWriteStorageGranted();
 }

android/src/main/java/com/tailscale/ipn/IPNActivity.java

@@ -12,6 +12,7 @@ import android.database.Cursor;
 import android.os.Bundle;
 import android.provider.OpenableColumns;
 import android.net.Uri;
+import android.content.pm.PackageManager;
 
 import java.util.List;
 import java.util.ArrayList;
@@ -19,6 +20,8 @@ import java.util.ArrayList;
 import org.gioui.GioView;
 
 public final class IPNActivity extends Activity {
+	final static int WRITE_STORAGE_RESULT = 1000;
+
 	private GioView view;
 
 	@Override public void onCreate(Bundle state) {
@@ -89,6 +92,15 @@ public final class IPNActivity extends Activity {
 		App.onShareIntent(nfiles, types, mimes, items, names, sizes);
 	}
 
+	@Override public void onRequestPermissionsResult(int reqCode, String[] perms, int[] grants) {
+		switch (reqCode) {
+		case WRITE_STORAGE_RESULT:
+			if (grants.length > 0 && grants[0] == PackageManager.PERMISSION_GRANTED) {
+				App.onWriteStorageGranted();
+			}
+		}
+	}
+
 	@Override public void onDestroy() {
 		view.destroy();
 		super.onDestroy();

cmd/tailscale/callbacks.go

@@ -39,6 +39,9 @@ var (
 
 	// onFileShare receives file sharing intents.
 	onFileShare = make(chan []File, 1)
+
+	// onWriteStorageGranted is notified when we are granted WRITE_STORAGE_PERMISSION.
+	onWriteStorageGranted = make(chan struct{}, 1)
 )
 
 const (
@@ -58,6 +61,14 @@ func Java_com_tailscale_ipn_App_onVPNPrepared(env *C.JNIEnv, class C.jclass) {
 	notifyVPNPrepared()
 }
 
+//export Java_com_tailscale_ipn_App_onWriteStorageGranted
+func Java_com_tailscale_ipn_App_onWriteStorageGranted(env *C.JNIEnv, class C.jclass) {
+	select {
+	case onWriteStorageGranted <- struct{}{}:
+	default:
+	}
+}
+
 func notifyVPNPrepared() {
 	select {
 	case onVPNPrepared <- struct{}{}:

cmd/tailscale/main.go

@@ -385,6 +385,8 @@ func (a *App) runBackend() error {
 			a.targetsLoaded <- FileTargets{targets, err}
 			waitingFiles = n.FilesWaiting != nil
 			processFiles()
+		case <-onWriteStorageGranted:
+			processFiles()
 		case <-alarmChan:
 			if m := state.NetworkMap; m != nil && service != 0 {
 				alarm(a.notifyExpiry(service, m.Expiry))
@@ -864,6 +866,11 @@ func (a *App) runUI() error {
 				if err := a.callVoidMethod(a.appCtx, "startVPN", "()V"); err != nil {
 					return err
 				}
+				if activity != 0 {
+					if err := a.callVoidMethod(a.appCtx, "requestWriteStoragePermission", "(Landroid/app/Activity;)V", jni.Value(activity)); err != nil {
+						return err
+					}
+				}
 			}
 		case <-onVPNRevoked:
 			ui.ShowMessage("VPN access denied or another VPN service is always-on")