Fix local server startup and shutdown blocking the main thread

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

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().