LibThreading: Add ability to cancel ongoing BackgroundActions

Handlers of the BackgroundAction are responsible for checking if the
action has been cancelled and returning early.

Userland/Applications/SystemMonitor/ThreadStackWidget.cpp

@@ -61,7 +61,7 @@ private:
 void ThreadStackWidget::refresh()
 {
     Threading::BackgroundAction<Vector<Symbolication::Symbol>>::create(
-        [pid = m_pid, tid = m_tid] {
+        [pid = m_pid, tid = m_tid](auto&) {
             return Symbolication::symbolicate_thread(pid, tid);
         },
 

Userland/Libraries/LibGUI/FileSystemModel.cpp

@@ -587,7 +587,7 @@ bool FileSystemModel::fetch_thumbnail_for(const Node& node)
     auto weak_this = make_weak_ptr();
 
     Threading::BackgroundAction<RefPtr<Gfx::Bitmap>>::create(
-        [path] {
+        [path](auto&) {
             return render_thumbnail(path);
         },
 

Userland/Libraries/LibThreading/BackgroundAction.h

@@ -39,16 +39,26 @@ class BackgroundAction final : public Core::Object
 
 public:
     static NonnullRefPtr<BackgroundAction<Result>> create(
-        Function<Result()> action,
+        Function<Result(BackgroundAction&)> action,
         Function<void(Result)> on_complete = nullptr)
     {
         return adopt_ref(*new BackgroundAction(move(action), move(on_complete)));
     }
 
+    void cancel()
+    {
+        m_cancelled = true;
+    }
+
+    bool is_cancelled() const
+    {
+        return m_cancelled;
+    }
+
     virtual ~BackgroundAction() { }
 
 private:
-    BackgroundAction(Function<Result()> action, Function<void(Result)> on_complete)
+    BackgroundAction(Function<Result(BackgroundAction&)> action, Function<void(Result)> on_complete)
         : Core::Object(&background_thread())
         , m_action(move(action))
         , m_on_complete(move(on_complete))
@@ -56,7 +66,7 @@ private:
         Locker locker(all_actions().lock());
 
         all_actions().resource().enqueue([this] {
-            m_result = m_action();
+            m_result = m_action(*this);
             if (m_on_complete) {
                 Core::EventLoop::current().post_event(*this, make<Core::DeferredInvocationEvent>([this](auto&) {
                     m_on_complete(m_result.release_value());
@@ -69,7 +79,8 @@ private:
         });
     }
 
-    Function<Result()> m_action;
+    bool m_cancelled { false };
+    Function<Result(BackgroundAction&)> m_action;
     Function<void(Result)> m_on_complete;
     Optional<Result> m_result;
 };

Userland/Services/WindowServer/Compositor.cpp

@@ -689,7 +689,7 @@ bool Compositor::set_wallpaper_mode(const String& mode)
 bool Compositor::set_wallpaper(const String& path, Function<void(bool)>&& callback)
 {
     Threading::BackgroundAction<RefPtr<Gfx::Bitmap>>::create(
-        [path] {
+        [path](auto&) {
             return Gfx::Bitmap::load_from_file(path);
         },