LibThread: Introduce a new threading library

This library is meant to provide C++-style wrappers over lower
level APIs such as syscalls and pthread_* functions, as well as
utilities for easily running pieces of logic on different
threads.

Libraries/LibThread/BackgroundAction.h

@@ -0,0 +1,75 @@
+#pragma once
+
+#include <AK/Function.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/Optional.h>
+#include <AK/Queue.h>
+#include <AK/RefCounted.h>
+#include <LibCore/CEventLoop.h>
+#include <LibCore/CLock.h>
+#include <LibCore/CObject.h>
+#include <LibThread/Thread.h>
+
+namespace LibThread {
+
+template<typename Result>
+class BackgroundAction;
+
+class BackgroundActionBase {
+    template<typename Result>
+    friend class BackgroundAction;
+
+private:
+    BackgroundActionBase() {}
+
+    static CLockable<Queue<Function<void()>>>& all_actions();
+    static Thread& background_thread();
+};
+
+template<typename Result>
+class BackgroundAction final : public CObject
+    , public RefCounted<BackgroundAction<Result>>
+    , private BackgroundActionBase {
+
+    C_OBJECT(BackgroundAction);
+
+public:
+    static NonnullRefPtr<BackgroundAction<Result>> create(
+        Function<Result()> action,
+        Function<void(Result)> on_complete = nullptr
+    )
+    {
+        return adopt(*new BackgroundAction(move(action), move(on_complete)));
+    }
+
+    virtual ~BackgroundAction() {}
+
+private:
+
+    BackgroundAction(Function<Result()> action, Function<void(Result)> on_complete)
+        : CObject(background_thread())
+        , m_action(move(action))
+        , m_on_complete(move(on_complete))
+    {
+        LOCKER(all_actions().lock());
+
+        this->ref();
+        all_actions().resource().enqueue([this] {
+            m_result = m_action();
+            if (m_on_complete) {
+                CEventLoop::main().post_event(*this, make<CDeferredInvocationEvent>([this](CObject&) {
+                    m_on_complete(m_result.release_value());
+                    this->deref();
+                }));
+                CEventLoop::main().wake();
+            } else
+                this->deref();
+        });
+    }
+
+    Function<Result()> m_action;
+    Function<void(Result)> m_on_complete;
+    Optional<Result> m_result;
+};
+
+}