Implement client restarting on Android

Restarting the client previously did not work, as the `shell_execute` function on Android uses `fork` which is not supported.

Now, the client is restarted by using an Android intent to restart the main activity. This is triggered by sending a user-defined message from the native code to the SDL main activity thread.

scripts/android/files/java/org/ddnet/client/NativeMain.java

@@ -3,10 +3,13 @@ package org.ddnet.client;
 import android.app.NativeActivity;
 import org.libsdl.app.SDLActivity;
 import android.os.Bundle;
+import android.content.Intent;
 import android.content.pm.ActivityInfo;
 
 public class NativeMain extends SDLActivity {
 
+	private static final int COMMAND_RESTART_APP = SDLActivity.COMMAND_USER + 1;
+
 	@Override
 	protected String[] getLibraries() {
 		return new String[] {
@@ -19,4 +22,24 @@ public class NativeMain extends SDLActivity {
 		setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);
 		super.onCreate(SavedInstanceState);
 	}
+
+	@Override
+	protected boolean onUnhandledMessage(int command, Object param) {
+		if(command == COMMAND_RESTART_APP) {
+			restartApp();
+			return true;
+		}
+        return false;
+    }
+
+	private void restartApp() {
+		Intent restartIntent =
+			Intent.makeRestartActivityTask(
+				getPackageManager().getLaunchIntentForPackage(
+					getPackageName()
+				).getComponent()
+			);
+		restartIntent.setPackage(getPackageName());
+		startActivity(restartIntent);
+	}
 }

src/android/android_main.cpp

@@ -225,3 +225,12 @@ const char *InitAndroid()
 
 	return nullptr;
 }
+
+// See NativeMain.java
+constexpr uint32_t COMMAND_USER = 0x8000;
+constexpr uint32_t COMMAND_RESTART_APP = COMMAND_USER + 1;
+
+void RestartAndroidApp()
+{
+	SDL_AndroidSendMessage(COMMAND_RESTART_APP, 0);
+}

src/android/android_main.h

@@ -20,4 +20,12 @@
  */
 const char *InitAndroid();
 
+/**
+ * Sends an intent to the Android system to restart the app.
+ *
+ * This will restart the main activity in a new task. The current process
+ * must immediately terminate after this function is called.
+ */
+void RestartAndroidApp();
+
 #endif // ANDROID_ANDROID_MAIN_H

src/engine/client/client.cpp

@@ -4594,11 +4594,13 @@ int main(int argc, const char **argv)
 	pClient->Run();
 
 	const bool Restarting = pClient->State() == CClient::STATE_RESTARTING;
+#if !defined(CONF_PLATFORM_ANDROID)
 	char aRestartBinaryPath[IO_MAX_PATH_LENGTH];
 	if(Restarting)
 	{
 		pStorage->GetBinaryPath(PLAT_CLIENT_EXEC, aRestartBinaryPath, sizeof(aRestartBinaryPath));
 	}
+#endif
 
 	std::vector<SWarning> vQuittingWarnings = pClient->QuittingWarnings();
 
@@ -4611,7 +4613,11 @@ int main(int argc, const char **argv)
 
 	if(Restarting)
 	{
+#if defined(CONF_PLATFORM_ANDROID)
+		RestartAndroidApp();
+#else
 		shell_execute(aRestartBinaryPath, EShellExecuteWindowState::FOREGROUND);
+#endif
 	}
 
 	PerformFinalCleanup();