LibCore: Move event queueing to a per-thread event queue

Instead of juggling events between individual instances of
Core::EventLoop, move queueing and processing to a separate per-thread
queue (ThreadEventQueue).

Userland/Libraries/LibCore/ThreadEventQueue.cpp

@@ -0,0 +1,123 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Vector.h>
+#include <LibCore/DeferredInvocationContext.h>
+#include <LibCore/Object.h>
+#include <LibCore/Promise.h>
+#include <LibCore/ThreadEventQueue.h>
+#include <LibThreading/Mutex.h>
+
+namespace Core {
+
+struct ThreadEventQueue::Private {
+    struct QueuedEvent {
+        AK_MAKE_NONCOPYABLE(QueuedEvent);
+
+    public:
+        QueuedEvent(Object& receiver, NonnullOwnPtr<Event> event)
+            : receiver(receiver)
+            , event(move(event))
+        {
+        }
+
+        QueuedEvent(QueuedEvent&& other)
+            : receiver(other.receiver)
+            , event(move(other.event))
+        {
+        }
+
+        ~QueuedEvent() = default;
+
+        WeakPtr<Object> receiver;
+        NonnullOwnPtr<Event> event;
+    };
+
+    Threading::Mutex mutex;
+    Vector<QueuedEvent, 128> queued_events;
+    Vector<NonnullRefPtr<Promise<NonnullRefPtr<Object>>>, 16> pending_promises;
+    bool warned_promise_count { false };
+};
+
+static thread_local ThreadEventQueue* s_current_thread_event_queue;
+
+ThreadEventQueue& ThreadEventQueue::current()
+{
+    if (!s_current_thread_event_queue) {
+        // FIXME: Don't leak these.
+        s_current_thread_event_queue = new ThreadEventQueue;
+    }
+    return *s_current_thread_event_queue;
+}
+
+ThreadEventQueue::ThreadEventQueue()
+    : m_private(make<Private>())
+{
+}
+
+ThreadEventQueue::~ThreadEventQueue() = default;
+
+void ThreadEventQueue::post_event(Core::Object& receiver, NonnullOwnPtr<Core::Event> event)
+{
+    Threading::MutexLocker lock(m_private->mutex);
+    m_private->queued_events.empend(receiver, move(event));
+}
+
+void ThreadEventQueue::add_job(NonnullRefPtr<Promise<NonnullRefPtr<Object>>> promise)
+{
+    Threading::MutexLocker lock(m_private->mutex);
+    m_private->pending_promises.append(move(promise));
+}
+
+size_t ThreadEventQueue::process()
+{
+    decltype(m_private->queued_events) events;
+    {
+        Threading::MutexLocker locker(m_private->mutex);
+        events = move(m_private->queued_events);
+        m_private->pending_promises.remove_all_matching([](auto& job) { return job->is_resolved() || job->is_canceled(); });
+    }
+
+    size_t processed_events = 0;
+    for (size_t i = 0; i < events.size(); ++i) {
+        auto& queued_event = events.at(i);
+        auto receiver = queued_event.receiver.strong_ref();
+        auto& event = *queued_event.event;
+
+        if (!receiver) {
+            switch (event.type()) {
+            case Event::Quit:
+                VERIFY_NOT_REACHED();
+            default:
+                dbgln("ThreadEventQueue::process: Event of type {} with no receiver", event.type());
+                break;
+            }
+        } else if (event.type() == Event::Type::DeferredInvoke) {
+            static_cast<DeferredInvocationEvent&>(event).m_invokee();
+        } else {
+            NonnullRefPtr<Object> protector(*receiver);
+            receiver->dispatch_event(event);
+        }
+        ++processed_events;
+    }
+
+    {
+        Threading::MutexLocker locker(m_private->mutex);
+        if (m_private->pending_promises.size() > 30 && !m_private->warned_promise_count) {
+            m_private->warned_promise_count = true;
+            dbgln("ThreadEventQueue::process: Job queue wasn't designed for this load ({} promises)", m_private->pending_promises.size());
+        }
+    }
+    return processed_events;
+}
+
+bool ThreadEventQueue::has_pending_events() const
+{
+    Threading::MutexLocker locker(m_private->mutex);
+    return !m_private->queued_events.is_empty();
+}
+
+}