Ladybird: Allow posting events to the Qt event loop from other threads

Previously, a QTimer was used to start processing of our event queue in
the main Qt event loop. Unfortunately, QTimers are not thread-safe, and
disallow starting of a timer from a different thread than it was
created in.

Instead, use a dummy QObject to post a custom QEvent to the main loop
from whatever thread we like, and process our event queue when it is
received by our dummy object.

Ladybird/CMakeLists.txt

@@ -84,6 +84,7 @@ set(SOURCES
     BrowserWindow.cpp
     ConsoleWidget.cpp
     EventLoopImplementationQt.cpp
+    EventLoopImplementationQtEventTarget.cpp
     HelperProcess.cpp
     InspectorWidget.cpp
     LocationEdit.cpp

Ladybird/EventLoopImplementationQt.cpp

@@ -5,6 +5,7 @@
  */
 
 #include "EventLoopImplementationQt.h"
+#include "EventLoopImplementationQtEventTarget.h"
 #include <AK/IDAllocator.h>
 #include <LibCore/Event.h>
 #include <LibCore/Notifier.h>
@@ -149,16 +150,21 @@ void EventLoopManagerQt::unregister_notifier(Core::Notifier& notifier)
 
 void EventLoopManagerQt::did_post_event()
 {
-    m_process_core_events_timer.start();
+    QCoreApplication::postEvent(m_main_thread_event_target.ptr(), new QtEventLoopManagerEvent(QtEventLoopManagerEvent::process_event_queue_event_type()));
+}
+
+bool EventLoopManagerQt::event_target_received_event(Badge<EventLoopImplementationQtEventTarget>, QEvent* event)
+{
+    if (event->type() == QtEventLoopManagerEvent::process_event_queue_event_type()) {
+        Core::ThreadEventQueue::current().process();
+        return true;
+    }
+    return false;
 }
 
 EventLoopManagerQt::EventLoopManagerQt()
+    : m_main_thread_event_target(make<EventLoopImplementationQtEventTarget>())
 {
-    m_process_core_events_timer.setSingleShot(true);
-    m_process_core_events_timer.setInterval(0);
-    QObject::connect(&m_process_core_events_timer, &QTimer::timeout, [] {
-        Core::ThreadEventQueue::current().process();
-    });
 }
 
 EventLoopManagerQt::~EventLoopManagerQt() = default;

Ladybird/EventLoopImplementationQt.h

@@ -6,16 +6,20 @@
 
 #pragma once
 
+#include <AK/Badge.h>
 #include <AK/HashMap.h>
 #include <AK/NonnullOwnPtr.h>
 #include <AK/OwnPtr.h>
 #include <LibCore/EventLoopImplementation.h>
+#include <QEvent>
 #include <QEventLoop>
 #include <QSocketNotifier>
 #include <QTimer>
 
 namespace Ladybird {
 
+class EventLoopImplementationQtEventTarget;
+
 class EventLoopManagerQt final : public Core::EventLoopManager {
 public:
     EventLoopManagerQt();
@@ -29,13 +33,28 @@ public:
     virtual void unregister_notifier(Core::Notifier&) override;
 
     virtual void did_post_event() override;
+    static bool event_target_received_event(Badge<EventLoopImplementationQtEventTarget>, QEvent* event);
 
     // FIXME: These APIs only exist for obscure use-cases inside SerenityOS. Try to get rid of them.
     virtual int register_signal(int, Function<void(int)>) override { return 0; }
     virtual void unregister_signal(int) override { }
 
 private:
-    QTimer m_process_core_events_timer;
+    NonnullOwnPtr<EventLoopImplementationQtEventTarget> m_main_thread_event_target;
+};
+
+class QtEventLoopManagerEvent final : public QEvent {
+public:
+    static QEvent::Type process_event_queue_event_type()
+    {
+        static auto const type = static_cast<QEvent::Type>(QEvent::registerEventType());
+        return type;
+    }
+
+    QtEventLoopManagerEvent(QEvent::Type type)
+        : QEvent(type)
+    {
+    }
 };
 
 class EventLoopImplementationQt final : public Core::EventLoopImplementation {

Ladybird/EventLoopImplementationQtEventTarget.cpp

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2023, Gregory Bertilson <zaggy1024@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "EventLoopImplementationQtEventTarget.h"
+
+namespace Ladybird {
+
+bool EventLoopImplementationQtEventTarget::event(QEvent* event)
+{
+    return EventLoopManagerQt::event_target_received_event({}, event);
+}
+
+}

Ladybird/EventLoopImplementationQtEventTarget.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2023, Gregory Bertilson <zaggy1024@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <QEvent>
+
+#include "EventLoopImplementationQt.h"
+
+namespace Ladybird {
+
+class EventLoopImplementationQtEventTarget final : public QObject {
+    Q_OBJECT
+
+public:
+    virtual bool event(QEvent* event) override;
+};
+
+}

Ladybird/WebContent/CMakeLists.txt

@@ -9,6 +9,7 @@ set(WEBCONTENT_SOURCES
     ../AudioCodecPluginQt.cpp
     ../AudioThread.cpp
     ../EventLoopImplementationQt.cpp
+    ../EventLoopImplementationQtEventTarget.cpp
     ../FontPlugin.cpp
     ../HelperProcess.cpp
     ../ImageCodecPlugin.cpp