[WIP] Android VPN client

cmd/skywirevisormobile/android/README.md

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+# Skywire VPN draft (Android)
+
+## Prerequisites
+This project uses Skywire mobile library (`skywiremob`) to use all the needed Skywire infrastructure. In order to build
+and use this library, one needs to install `gomobile` (https://github.com/golang/mobile).
+
+## Building and Running
+To build the library you need to use `gomobile`. Main `Makefile` already contains target `build-android` which 
+can be used. 
+IMPORTANT: regardless of go modules and other great stuff done by the Go team, in order to 
+use `gomobile` you need to put Skywire code according to `GOPATH`. Otherwise you'll get all kinds of errors.
+The output file (`.aar` for Android) may be used straight from the mobile app code.
+
+## Skywire Mobile API
+- `PrintString(string)`: Logs string argument using info log level. May be useful to use it instead of standard logging features of mobile apps for debugging. 
+All the output strings are prefixed with `GoLog`, so printing logs with this func one may grep all the logs both from `skywiremob` 
+internal and mobile application;
+- `IsPKValid(string) string`: Checks if passed pub key is valid. Returns non-empty string with error in case of failure;
+- `GetMTU() int`: Gets VPN connection MTU;
+- `GetTUNIPPrefix() int`: Gets netmask prefix of TUN IP address;
+- `IsVPNReady() bool`: Checks whether VPN client is ready on the Go side. Once it is, the mobile application is free to start 
+forwarding packets. Starts returning `true` after the `ServerVPN` call;
+- `PrepareVisor() string`: Creates and runs visor instance. Returns non-empty string with error in case of failure;
+- `NextDmsgSocket() int`: Returns file descriptor of the Dmsg socket. There may be more than one socket in use by the dmsg client, so 
+this function should be called repeatedly until next call returns 0.
+- `PrepareVPNClient(string, string) string`: Creates VPN client instance. First string argument is remote VPN server pub key, second one is passcode to 
+authenticate within the server. Returns non-empty string with error in case of failure;
+- `ShakeHands() string`: Requires `PrepareVPNClient` to be called first. Performs handshake between the client and the server. 
+Returns non-empty string with error in case of failure;
+- `TUNIP() string`: Requires `ShakeHands` to be called first. Returns the assigned TUN IP;
+- `TUNGateway() string`: Requires `ShakeHands` to be called first. Returns the assigned TUN gateway;
+- `StopVisor() string`: Stops currently running visor. Returns non-empty string with error in case of failure;
+- `SetMobileAppAddr(string)`: Passes address of the UDP connection opened on the mobile application side;
+- `ServeVPN()`: Starts off the goroutine serving VPN connection. After this call `IsVPNReady` starts returning `true`;
+- `StartListeningUDP() string`: Opens UDP listener on the Go side. Returns non-empty string with error in case of failure;
+- `IsVisorStarting() bool`: Checks if visor is starting. Will get `false` when it's fully functional;
+- `IsVisorRunning() bool`: Checks if visor is running. Will get `true` whn visor is fully functional;
+- `WaitVisorReady() string`: Blocks until visor gets fully initialized. Returns non-empty error string in case of failure;
+- `StopVPNClient`: Stops VPN client without stopping visor itself;
+- `StopListeningUDP`: Closes UDP socket;
+- `VPNBandwidthSent`: Returns amount of bandwidth sent over VPN (bytes);
+- `VPNBandwidthReceived`: Returns amount of bandwidth received over VPN (bytes);
+- `VPNLatency`: Returns latency (ms);
+- `VPNThroughput`: Returns throughput (bytes/s).
+
+
+## Mobile/Go Communication
+API may seem a bit complicated at first. Currently tested for Android devices, should be used with caution on iOS. 
+Mobile app communicates with the Go part via UDP. All the packets are sent to the Go part via UDP and then get resent 
+to the Skywire network. 
+
+To setup the Go side properly you need to call at least:
+- `PrepareVisor` to run the visor;
+- `PrepareVPNClient` to run the VPN client;
+- `ShakeHands` to perform handshake with the server;
+- `StartListeningUDP` to open the UDP listener on the Go side;
+- `ServeVPN` to start forwarding traffic.
+
+All other calls should be done as needed.
+
+### Android
+Consult this page: https://developer.android.com/guide/topics/connectivity/vpn
+
+In the example mobile app communicates with the remote server via `DatagramChannel`. Socket opened to the server gets protected 
+with the `protect` method. We do the same here. But instead of a remote server we open the `DatagramChannel` to the Go part of the app.
+We protect not only the tunnel socket, but also we need to protect all the sockets used for `Dmsg` communication to let traffic go back and forth freely. 

cmd/skywirevisormobile/android/app/build.gradle

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+/*
+ * Copyright 2015 The Go Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style
+ * license that can be found in the LICENSE file.
+ */
+apply plugin: 'com.android.application'
+
+repositories {
+    flatDir {
+        dirs '.'
+    }
+    maven { url 'https://jitpack.io' }
+}
+
+android {
+    compileSdkVersion 29
+
+    defaultConfig {
+        applicationId "com.skywire.skycoin.vpn"
+        minSdkVersion 21
+        targetSdkVersion 29
+        versionCode 1
+        versionName "1.0"
+    }
+    buildTypes {
+        release {
+            minifyEnabled false
+        }
+    }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+}
+
+
+dependencies {
+    // Appcompat.
+    implementation "androidx.appcompat:appcompat:1.2.0"
+    implementation 'com.google.android.material:material:1.2.1'
+    implementation "androidx.preference:preference:1.1.1"
+    implementation "androidx.recyclerview:recyclerview:1.1.0"
+    implementation "androidx.viewpager2:viewpager2:1.0.0"
+
+    // Skywire lib.
+    implementation(name:'skywire', ext:'aar')
+
+    // RxJava.
+    implementation 'io.reactivex.rxjava3:rxandroid:3.0.0'
+    implementation 'io.reactivex.rxjava3:rxjava:3.0.0'
+
+    // Retrofit.
+    implementation 'com.google.code.gson:gson:2.8.5'
+    implementation 'com.squareup.retrofit2:retrofit:2.9.0'
+    implementation 'com.squareup.retrofit2:converter-gson:2.9.0'
+    implementation 'com.squareup.retrofit2:adapter-rxjava3:2.9.0'
+    implementation 'com.squareup.retrofit2:converter-scalars:2.9.0'
+
+    // MPAndroidChart.
+    implementation 'com.github.PhilJay:MPAndroidChart:v3.1.0'
+}

cmd/skywirevisormobile/android/app/src/main/AndroidManifest.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="com.skywire.skycoin.vpn" >
+
+    <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+    <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
+
+    <application
+        android:name=".App"
+        android:allowBackup="true"
+        android:label="@string/general_app_name"
+        android:icon="@mipmap/ic_launcher"
+        android:theme="@style/AppTheme"
+    >
+
+        <receiver
+            android:directBootAware="true"
+            android:name=".Receiver"
+            android:enabled="true">
+            <intent-filter android:priority="2000000000">
+                <action android:name="android.intent.action.BOOT_COMPLETED" />
+            </intent-filter>
+        </receiver>
+
+        <service android:name=".vpn.SkywireVPNService"
+            android:description="@string/general_service_description"
+            android:permission="android.permission.BIND_VPN_SERVICE">
+            <intent-filter>
+                <action android:name="android.net.VpnService"/>
+            </intent-filter>
+        </service>
+
+        <activity
+            android:name=".activities.index.IndexActivity"
+            android:configChanges="keyboardHidden"
+            android:launchMode="singleTask"
+            android:label="@string/general_app_name" >
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+
+        <activity
+            android:name=".activities.apps.AppsActivity"
+            android:configChanges="keyboardHidden"
+            android:label="@string/tmp_select_apps_title" >
+        </activity>
+
+    </application>
+
+</manifest>

cmd/skywirevisormobile/android/app/src/main/java/com/skywire/skycoin/vpn/App.java

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+package com.skywire.skycoin.vpn;
+
+import android.app.Activity;
+import android.app.Application;
+import android.app.NotificationChannel;
+import android.app.NotificationManager;
+import android.content.Context;
+import android.os.Build;
+import android.os.Bundle;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
+import com.skywire.skycoin.vpn.helpers.HelperFunctions;
+import com.skywire.skycoin.vpn.helpers.Notifications;
+import com.skywire.skycoin.vpn.vpn.VPNCoordinator;
+
+import io.reactivex.rxjava3.plugins.RxJavaPlugins;
+
+/**
+ * Class for the main app instance.
+ */
+public class App extends Application {
+    /**
+     * Class used internally to know when there are activities being displayed.
+     */
+    private static class ActivityLifecycleCallback implements Application.ActivityLifecycleCallbacks {
+
+        // How many activities are being shown.
+        private static int foregroundActivities = 0;
+
+        // Functions for knowing when activities start and stop being shown.
+        @Override
+        public void onActivityResumed(@NonNull final Activity activity) { foregroundActivities++; }
+        @Override
+        public void onActivityStopped(@NonNull final Activity activity) { foregroundActivities--; }
+
+        /**
+         * Returns if there is at least one activity being displayed.
+         */
+        public static boolean isApplicationInForeground() { return foregroundActivities > 0; }
+
+        // Other functions needed by the interface.
+        @Override
+        public void onActivityPaused(@NonNull Activity activity) { }
+        @Override
+        public void onActivitySaveInstanceState(@NonNull Activity activity, @NonNull Bundle outState) { }
+        @Override
+        public void onActivityDestroyed(@NonNull Activity activity) { }
+        @Override
+        public void onActivityCreated(@NonNull Activity activity, @Nullable Bundle savedInstanceState) { }
+        @Override
+        public void onActivityStarted(@NonNull Activity activity) { }
+    }
+
+    /**
+     * Reference to the current app instance.
+     */
+    private static Context appContext;
+
+    @Override
+    public void onCreate() {
+        super.onCreate();
+        // Save the current app instance.
+        appContext = this;
+
+        // Ensure the singleton is initialized early.
+        VPNCoordinator.getInstance();
+
+        // Create the notification channels, but only on API 26+ because
+        // the NotificationChannel class is new and not in the support library
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
+            // Channel for the VPN service state updates.
+            NotificationChannel stateChannel = new NotificationChannel(
+                Notifications.NOTIFICATION_CHANNEL_ID,
+                getString(R.string.general_app_name),
+                NotificationManager.IMPORTANCE_DEFAULT
+            );
+            stateChannel.setDescription(getString(R.string.general_notification_channel_description));
+            stateChannel.setSound(null,null);
+            NotificationManager notificationManager = getSystemService(NotificationManager.class);
+            notificationManager.createNotificationChannel(stateChannel);
+
+            // Channel for alerts.
+            NotificationChannel alertsChannel = new NotificationChannel(
+                    Notifications.ALERT_NOTIFICATION_CHANNEL_ID,
+                    getString(R.string.general_alert_notification_name),
+                    NotificationManager.IMPORTANCE_HIGH
+            );
+            alertsChannel.setDescription(getString(R.string.general_alert_notification_channel_description));
+            notificationManager.createNotificationChannel(alertsChannel);
+        }
+
+        // Code for precessing errors which were not caught by the normal error management
+        // procedures RxJava has. This prevents the app to be closed by unexpected errors, mainly
+        // code trying to report events in closed observables.
+        RxJavaPlugins.setErrorHandler(throwable -> {
+            HelperFunctions.logError("ERROR INSIDE RX: ", throwable);
+        });
+
+        // Detect when activities are started and stopped.
+        registerActivityLifecycleCallbacks(new ActivityLifecycleCallback());
+    }
+
+    /**
+     * Gets the current app context.
+     */
+    public static Context getContext(){
+        return appContext;
+    }
+
+    /**
+     * Gets if the UI is being displayed.
+     */
+    public static boolean displayingUI(){
+        return ActivityLifecycleCallback.isApplicationInForeground();
+    }
+}

cmd/skywirevisormobile/android/app/src/main/java/com/skywire/skycoin/vpn/Receiver.java

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+package com.skywire.skycoin.vpn;
+
+import android.content.BroadcastReceiver;
+import android.content.Context;
+import android.content.Intent;
+
+import com.skywire.skycoin.vpn.vpn.VPNCoordinator;
+import com.skywire.skycoin.vpn.vpn.VPNGeneralPersistentData;
+
+/**
+ * Class for receiving the system boot event broadcast.
+ */
+public class Receiver extends BroadcastReceiver {
+    public void onReceive(Context context, Intent intent) {
+        if (intent.getAction().equals(Intent.ACTION_BOOT_COMPLETED)) {
+            // If the option for starting the service automatically after booting the OS is active
+            // and the service is not currently running, start the service.
+            if (VPNGeneralPersistentData.getStartOnBoot() && !VPNCoordinator.getInstance().isServiceRunning()) {
+                VPNCoordinator.getInstance().activateAutostart();
+            }
+        }
+    }
+}