Fix local server startup and shutdown blocking the main thread

Co-authored-by: sfan5 <sfan5@live.de>

src/client/game.cpp

@@ -64,6 +64,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "settings.h"
 #include "shader.h"
 #include "sky.h"
+#include "threading/lambda.h"
 #include "translation.h"
 #include "util/basic_macros.h"
 #include "util/directiontables.h"
@@ -1056,12 +1057,6 @@ Game::Game() :
 
 Game::~Game()
 {
-	delete client;
-	delete soundmaker;
-	sound_manager.reset();
-
-	delete server; // deleted first to stop all server threads
-
 	delete hud;
 	delete camera;
 	delete quicktune;
@@ -1314,6 +1309,28 @@ void Game::shutdown()
 			sleep_ms(100);
 		}
 	}
+
+	delete client;
+	delete soundmaker;
+	sound_manager.reset();
+
+	auto stop_thread = runInThread([=] {
+		delete server;
+	}, "ServerStop");
+
+	FpsControl fps_control;
+	fps_control.reset();
+
+	while (stop_thread->isRunning()) {
+		m_rendering_engine->run();
+		f32 dtime;
+		fps_control.limit(device, &dtime);
+		showOverlayMessage(N_("Shutting down..."), dtime, 0, false);
+	}
+
+	stop_thread->rethrow();
+
+	// to be continued in Game::~Game
 }
 
 
@@ -1419,11 +1436,33 @@ bool Game::createSingleplayerServer(const std::string &map_dir,
 
 	server = new Server(map_dir, gamespec, simple_singleplayer_mode, bind_addr,
 			false, nullptr, error_message);
-	server->start();
 
-	copyServerClientCache();
+	auto start_thread = runInThread([=] {
+		server->start();
+		copyServerClientCache();
+	}, "ServerStart");
 
-	return true;
+	input->clear();
+	bool success = true;
+
+	FpsControl fps_control;
+	fps_control.reset();
+
+	while (start_thread->isRunning()) {
+		if (!m_rendering_engine->run() || input->cancelPressed())
+			success = false;
+		f32 dtime;
+		fps_control.limit(device, &dtime);
+
+		if (success)
+			showOverlayMessage(N_("Creating server..."), dtime, 5);
+		else
+			showOverlayMessage(N_("Shutting down..."), dtime, 0, false);
+	}
+
+	start_thread->rethrow();
+
+	return success;
 }
 
 void Game::copyServerClientCache()

src/debug.h

@@ -19,7 +19,6 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 #pragma once
 
-#include <iostream>
 #include <exception>
 #include <cassert>
 #include "gettime.h"

src/threading/lambda.h

@@ -0,0 +1,64 @@
+// Minetest
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#pragma once
+
+#include <exception>
+#include <functional>
+#include <memory>
+#include "debug.h"
+#include "threading/thread.h"
+
+/**
+ * Class returned by `runInThread`.
+ *
+ * Provides the usual thread methods along with `rethrow()`.
+*/
+class LambdaThread : public Thread
+{
+	friend std::unique_ptr<LambdaThread> runInThread(
+		const std::function<void()> &, const std::string &);
+public:
+	/// Re-throw a caught exception, if any. Can only be called after thread exit.
+	void rethrow()
+	{
+		sanity_check(!isRunning());
+		if (m_exptr)
+			std::rethrow_exception(m_exptr);
+	}
+
+private:
+	// hide methods
+	LambdaThread(const std::string &name="") : Thread(name) {}
+	using Thread::start;
+
+	std::function<void()> m_fn;
+	std::exception_ptr m_exptr;
+
+	void *run()
+	{
+		try {
+			m_fn();
+		} catch(...) {
+			m_exptr = std::current_exception();
+		}
+		return nullptr;
+	};
+};
+
+/**
+ * Run a lambda in a separate thread.
+ *
+ * Exceptions will be caught.
+ * @param fn function to run
+ * @param thread_name name for thread
+ * @return thread object of type `LambdaThread`
+*/
+std::unique_ptr<LambdaThread> runInThread(const std::function<void()> &fn,
+	const std::string &thread_name = "")
+{
+	std::unique_ptr<LambdaThread> t(new LambdaThread(thread_name));
+	t->m_fn = fn;
+	t->start();
+	return t;
+}

src/threading/thread.h

@@ -59,6 +59,7 @@ public:
 	Thread(const std::string &name="");
 	virtual ~Thread();
 	DISABLE_CLASS_COPY(Thread)
+	// Note: class cannot be moved since other references exist
 
 	/*
 	 * Begins execution of a new thread at the pure virtual method Thread::run().