LibThreading: Introduce MutexProtected generic synchronization primitive

MutexProtected mirrors the identically-named Kernel primitive and can be
used to synchronize access to any object that might not be thread safe
on its own. Synchronization is done with a simple mutex, so access to a
MutexProtected object is potentially blocking.

Mutex now has an internal nesting variable which is there to harden it
against lock-unlock ordering issues (e.g. double unlocking).

Userland/Libraries/LibThreading/MutexProtected.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2022, kleines Filmröllchen <malu.bertsch@gmail.com>.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Concepts.h>
+#include <AK/Noncopyable.h>
+#include <LibThreading/Mutex.h>
+
+namespace Threading {
+
+template<typename T>
+class MutexProtected {
+    AK_MAKE_NONCOPYABLE(MutexProtected);
+    AK_MAKE_NONMOVABLE(MutexProtected);
+    using ProtectedType = T;
+
+public:
+    ALWAYS_INLINE MutexProtected() = default;
+    ALWAYS_INLINE MutexProtected(T&& value)
+        : m_value(move(value))
+    {
+    }
+    ALWAYS_INLINE explicit MutexProtected(T& value)
+        : m_value(value)
+    {
+    }
+
+    template<typename Callback>
+    decltype(auto) with_locked(Callback callback)
+    {
+        auto lock = this->lock();
+        // This allows users to get a copy, but if we don't allow references through &m_value, it's even more complex.
+        return callback(m_value);
+    }
+
+    template<VoidFunction<T> Callback>
+    void for_each_locked(Callback callback)
+    {
+        with_locked([&](auto& value) {
+            for (auto& item : value)
+                callback(item);
+        });
+    }
+
+private:
+    [[nodiscard]] ALWAYS_INLINE MutexLocker lock() { return MutexLocker(m_lock); }
+
+    T m_value;
+    Mutex m_lock {};
+};
+
+}