LibWeb: Implement basic support for MessageChannel and MessagePort

This patch adds a basic initial implementation of these API's.

Since LibWeb currently doesn't support workers, this implementation of
messaging doesn't bother with serializing and deserializing messages.

Userland/Libraries/LibWeb/HTML/MessagePort.cpp

@@ -0,0 +1,126 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/MessagePortWrapper.h>
+#include <LibWeb/Bindings/ScriptExecutionContext.h>
+#include <LibWeb/DOM/EventDispatcher.h>
+#include <LibWeb/HTML/EventHandler.h>
+#include <LibWeb/HTML/EventLoop/EventLoop.h>
+#include <LibWeb/HTML/EventNames.h>
+#include <LibWeb/HTML/MessageEvent.h>
+#include <LibWeb/HTML/MessagePort.h>
+
+namespace Web::HTML {
+
+MessagePort::MessagePort(Bindings::ScriptExecutionContext& context)
+    : DOM::EventTarget(context)
+{
+}
+
+MessagePort::~MessagePort()
+{
+}
+
+void MessagePort::disentangle()
+{
+    m_remote_port->m_remote_port = nullptr;
+    m_remote_port = nullptr;
+}
+
+// https://html.spec.whatwg.org/multipage/web-messaging.html#entangle
+void MessagePort::entangle_with(Badge<MessageChannel>, MessagePort& remote_port)
+{
+    if (m_remote_port == &remote_port)
+        return;
+
+    // 1. If one of the ports is already entangled, then disentangle it and the port that it was entangled with.
+    if (is_entangled())
+        disentangle();
+    if (remote_port.is_entangled())
+        remote_port.disentangle();
+
+    // 2. Associate the two ports to be entangled, so that they form the two parts of a new channel.
+    //    (There is no MessageChannel object that represents this channel.)
+    remote_port.m_remote_port = *this;
+    m_remote_port = &remote_port;
+}
+
+// https://html.spec.whatwg.org/multipage/web-messaging.html#dom-messageport-postmessage
+void MessagePort::post_message(JS::Value message)
+{
+    // 1. Let targetPort be the port with which this MessagePort is entangled, if any; otherwise let it be null.
+    auto* target_port = m_remote_port.ptr();
+
+    // FIXME: 2. Let options be «[ "transfer" → transfer ]».
+
+    // 3. Run the message port post message steps providing targetPort, message and options.
+
+    // https://html.spec.whatwg.org/multipage/web-messaging.html#message-port-post-message-steps
+
+    // FIXME: 1. Let transfer be options["transfer"].
+
+    // FIXME: 2. If transfer contains this MessagePort, then throw a "DataCloneError" DOMException.
+
+    // 3. Let doomed be false.
+    bool doomed = false;
+
+    // FIXME: 4. If targetPort is not null and transfer contains targetPort, then set doomed to true and optionally report to a developer console that the target port was posted to itself, causing the communication channel to be lost.
+
+    // FIXME: 5. Let serializeWithTransferResult be StructuredSerializeWithTransfer(message, transfer). Rethrow any exceptions.
+
+    // 6. If targetPort is null, or if doomed is true, then return.
+    if (!target_port || doomed)
+        return;
+
+    // FIXME: 7. Add a task that runs the following steps to the port message queue of targetPort:
+
+    // FIXME: This is an ad-hoc hack implementation instead, since we don't currently
+    //        have serialization and deserialization of messages.
+    main_thread_event_loop().task_queue().add(HTML::Task::create(HTML::Task::Source::PostedMessage, nullptr, [strong_port = NonnullRefPtr { *target_port }, message]() mutable {
+        strong_port->dispatch_event(MessageEvent::create(HTML::EventNames::message, message.to_string_without_side_effects(), "<origin>"));
+    }));
+}
+
+bool MessagePort::dispatch_event(NonnullRefPtr<DOM::Event> event)
+{
+    return DOM::EventDispatcher::dispatch(*this, move(event));
+}
+
+JS::Object* MessagePort::create_wrapper(JS::GlobalObject& global_object)
+{
+    return wrap(global_object, *this);
+}
+
+void MessagePort::start()
+{
+    // FIXME: Message ports are supposed to be disabled by default.
+}
+
+// https://html.spec.whatwg.org/multipage/web-messaging.html#dom-messageport-close
+void MessagePort::close()
+{
+    // 1. Set this MessagePort object's [[Detached]] internal slot value to true.
+    m_detached = true;
+
+    // 2. If this MessagePort object is entangled, disentangle it.
+    if (is_entangled())
+        disentangle();
+}
+
+#undef __ENUMERATE
+#define __ENUMERATE(attribute_name, event_name)                      \
+    void MessagePort::set_##attribute_name(HTML::EventHandler value) \
+    {                                                                \
+        set_event_handler_attribute(event_name, move(value));        \
+    }                                                                \
+    HTML::EventHandler MessagePort::attribute_name()                 \
+    {                                                                \
+        return event_handler_attribute(event_name);                  \
+    }
+ENUMERATE_MESSAGE_PORT_EVENT_HANDLERS(__ENUMERATE)
+#undef __ENUMERATE
+
+}