LibDevTools: Implement enough of the protocol to see a tab list

Previously, we could connect to our DevTools server from Firefox, but
could not see any information on Ladybird's opened tabs. This implements
enough of the protocol to see a tab list, but we cannot yet inspect the
tabs.

Libraries/LibDevTools/Actors/RootActor.cpp

@@ -9,6 +9,7 @@
 #include <LibDevTools/Actors/PreferenceActor.h>
 #include <LibDevTools/Actors/ProcessActor.h>
 #include <LibDevTools/Actors/RootActor.h>
+#include <LibDevTools/Actors/TabActor.h>
 #include <LibDevTools/DevToolsDelegate.h>
 #include <LibDevTools/DevToolsServer.h>
 
@@ -88,7 +89,23 @@ void RootActor::handle_message(StringView type, JsonObject const& message)
     }
 
     if (type == "getTab"sv) {
-        response.set("tab"sv, JsonObject {});
+        auto browser_id = message.get_integer<u64>("browserId"sv);
+        if (!browser_id.has_value()) {
+            send_missing_parameter_error("browserId"sv);
+            return;
+        }
+
+        for (auto const& actor : devtools().actor_registry()) {
+            auto const* tab_actor = as_if<TabActor>(*actor.value);
+            if (!tab_actor)
+                continue;
+            if (tab_actor->description().id != *browser_id)
+                continue;
+
+            response.set("tab"sv, tab_actor->serialize_description());
+            break;
+        }
+
         send_message(move(response));
         return;
     }
@@ -119,7 +136,16 @@ void RootActor::handle_message(StringView type, JsonObject const& message)
     }
 
     if (type == "listTabs"sv) {
-        response.set("tabs"sv, JsonArray {});
+        m_has_sent_tab_list_changed_since_last_list_tabs_request = false;
+
+        JsonArray tabs;
+
+        for (auto& tab_description : devtools().delegate().tab_list()) {
+            auto& actor = devtools().register_actor<TabActor>(move(tab_description));
+            tabs.must_append(actor.serialize_description());
+        }
+
+        response.set("tabs"sv, move(tabs));
         send_message(move(response));
         return;
     }
@@ -133,4 +159,17 @@ void RootActor::handle_message(StringView type, JsonObject const& message)
     send_unrecognized_packet_type_error(type);
 }
 
+void RootActor::send_tab_list_changed_message()
+{
+    if (m_has_sent_tab_list_changed_since_last_list_tabs_request)
+        return;
+
+    JsonObject message;
+    message.set("from"sv, name());
+    message.set("type"sv, "tabListChanged"sv);
+    send_message(move(message));
+
+    m_has_sent_tab_list_changed_since_last_list_tabs_request = true;
+}
+
 }

Libraries/LibDevTools/Actors/RootActor.h

@@ -20,8 +20,14 @@ public:
 
     virtual void handle_message(StringView type, JsonObject const&) override;
 
+    void send_tab_list_changed_message();
+
 private:
     RootActor(DevToolsServer&, ByteString name);
+
+    // https://firefox-source-docs.mozilla.org/devtools/backend/protocol.html#the-request-reply-notify-pattern
+    // the root actor sends at most one "tabListChanged" notification after each "listTabs" request.
+    bool m_has_sent_tab_list_changed_since_last_list_tabs_request { false };
 };
 
 }

Libraries/LibDevTools/Actors/TabActor.cpp

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/JsonObject.h>
+#include <LibDevTools/Actors/TabActor.h>
+#include <LibDevTools/DevToolsServer.h>
+
+namespace DevTools {
+
+NonnullRefPtr<TabActor> TabActor::create(DevToolsServer& devtools, ByteString name, TabDescription description)
+{
+    return adopt_ref(*new TabActor(devtools, move(name), move(description)));
+}
+
+TabActor::TabActor(DevToolsServer& devtools, ByteString name, TabDescription description)
+    : Actor(devtools, move(name))
+    , m_description(move(description))
+{
+}
+
+TabActor::~TabActor() = default;
+
+void TabActor::handle_message(StringView type, JsonObject const&)
+{
+    JsonObject response;
+    response.set("from"sv, name());
+
+    if (type == "getFavicon"sv) {
+        // FIXME: Firefox DevTools wants a favicon URL here, but supplying a URL seems to prevent this tab from being
+        //        listed on the about:debugging page. Both Servo and Firefox itself supply `null` here.
+        response.set("favicon"sv, JsonValue {});
+        send_message(move(response));
+        return;
+    }
+
+    send_unrecognized_packet_type_error(type);
+}
+
+JsonObject TabActor::serialize_description() const
+{
+    JsonObject traits;
+    traits.set("watcher"sv, true);
+    traits.set("supportsReloadDescriptor"sv, true);
+
+    // FIXME: We are using the tab's ID multiple times here. This is likely not correct, as both Firefox and Servo
+    //        provide different IDs for browserId, browsingContextID, and outerWindowID.
+    JsonObject description;
+    description.set("actor"sv, name());
+    description.set("title"sv, m_description.title);
+    description.set("url"sv, m_description.url);
+    description.set("browserId"sv, m_description.id);
+    description.set("browsingContextID"sv, m_description.id);
+    description.set("outerWindowID"sv, m_description.id);
+    description.set("traits"sv, move(traits));
+    return description;
+}
+
+}

Libraries/LibDevTools/Actors/TabActor.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <LibDevTools/Actor.h>
+
+namespace DevTools {
+
+struct TabDescription {
+    u64 id { 0 };
+    ByteString title;
+    ByteString url;
+};
+
+class TabActor final : public Actor {
+public:
+    static constexpr auto base_name = "tab"sv;
+
+    static NonnullRefPtr<TabActor> create(DevToolsServer&, ByteString name, TabDescription);
+    virtual ~TabActor() override;
+
+    virtual void handle_message(StringView type, JsonObject const&) override;
+
+    TabDescription const& description() const { return m_description; }
+    JsonObject serialize_description() const;
+
+private:
+    TabActor(DevToolsServer&, ByteString name, TabDescription);
+
+    TabDescription m_description;
+};
+
+}

Libraries/LibDevTools/CMakeLists.txt

@@ -4,6 +4,7 @@ set(SOURCES
     Actors/PreferenceActor.cpp
     Actors/ProcessActor.cpp
     Actors/RootActor.cpp
+    Actors/TabActor.cpp
     Connection.cpp
     DevToolsServer.cpp
 )

Libraries/LibDevTools/DevToolsDelegate.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+#include <AK/Vector.h>
+#include <LibDevTools/Actors/TabActor.h>
 #include <LibDevTools/Forward.h>
 
 namespace DevTools {
@@ -13,6 +15,8 @@ namespace DevTools {
 class DevToolsDelegate {
 public:
     virtual ~DevToolsDelegate() = default;
+
+    virtual Vector<TabDescription> tab_list() const { return {}; }
 };
 
 }

Libraries/LibDevTools/DevToolsServer.cpp

@@ -13,6 +13,7 @@
 #include <LibDevTools/Actors/DeviceActor.h>
 #include <LibDevTools/Actors/PreferenceActor.h>
 #include <LibDevTools/Actors/ProcessActor.h>
+#include <LibDevTools/Actors/TabActor.h>
 #include <LibDevTools/Connection.h>
 #include <LibDevTools/DevToolsServer.h>
 
@@ -43,6 +44,18 @@ DevToolsServer::DevToolsServer(DevToolsDelegate& delegate, NonnullRefPtr<Core::T
 
 DevToolsServer::~DevToolsServer() = default;
 
+void DevToolsServer::refresh_tab_list()
+{
+    if (!m_root_actor)
+        return;
+
+    m_actor_registry.remove_all_matching([](auto const&, auto const& actor) {
+        return is<TabActor>(*actor);
+    });
+
+    m_root_actor->send_tab_list_changed_message();
+}
+
 ErrorOr<void> DevToolsServer::on_new_client()
 {
     if (m_connection)

Libraries/LibDevTools/DevToolsServer.h

@@ -45,6 +45,8 @@ public:
         return actor;
     }
 
+    void refresh_tab_list();
+
 private:
     explicit DevToolsServer(DevToolsDelegate&, NonnullRefPtr<Core::TCPServer>);
 

Libraries/LibDevTools/Forward.h

@@ -16,7 +16,9 @@ class DevToolsServer;
 class PreferenceActor;
 class ProcessActor;
 class RootActor;
+class TabActor;
 
 struct ProcessDescription;
+struct TabDescription;
 
 }