LibWeb+WebDriver: Add an IPC-transferable Web::WebDriver::Response class

This is essentially an ErrorOr<JsonValue, Web::WebDriver::Error> class.
Unfortunately, that ErrorOr would not be default-constructible, which is
required for the generated IPC classes. So this is a thin wrapper around
a Variant<JsonValue, Web::WebDriver::Error> to emulate ErrorOr.

Userland/Libraries/LibWeb/WebDriver/Response.cpp

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibIPC/Decoder.h>
+#include <LibIPC/Encoder.h>
+#include <LibWeb/WebDriver/Response.h>
+
+enum class ResponseType : u8 {
+    Success,
+    Error,
+};
+
+namespace Web::WebDriver {
+
+Response::Response(JsonValue&& value)
+    : m_value_or_error(move(value))
+{
+}
+
+Response::Response(Error&& error)
+    : m_value_or_error(move(error))
+{
+}
+
+}
+
+bool IPC::encode(Encoder& encoder, Web::WebDriver::Response const& response)
+{
+    response.visit(
+        [](Empty) { VERIFY_NOT_REACHED(); },
+        [&](JsonValue const& value) {
+            encoder << ResponseType::Success;
+            encoder << value;
+        },
+        [&](Web::WebDriver::Error const& error) {
+            encoder << ResponseType::Error;
+            encoder << error.http_status;
+            encoder << error.error;
+            encoder << error.message;
+            encoder << error.data;
+        });
+
+    return true;
+}
+
+ErrorOr<void> IPC::decode(Decoder& decoder, Web::WebDriver::Response& response)
+{
+    ResponseType type {};
+    TRY(decoder.decode(type));
+
+    switch (type) {
+    case ResponseType::Success: {
+        JsonValue value;
+        TRY(decoder.decode(value));
+
+        response = move(value);
+        break;
+    }
+
+    case ResponseType::Error: {
+        Web::WebDriver::Error error {};
+        TRY(decoder.decode(error.http_status));
+        TRY(decoder.decode(error.error));
+        TRY(decoder.decode(error.message));
+        TRY(decoder.decode(error.data));
+
+        response = move(error);
+        break;
+    }
+    }
+
+    return {};
+}

Userland/Libraries/LibWeb/WebDriver/Response.h

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/JsonValue.h>
+#include <AK/Variant.h>
+#include <LibIPC/Forward.h>
+#include <LibWeb/WebDriver/Error.h>
+
+namespace Web::WebDriver {
+
+// FIXME: Ideally, this could be `using Response = ErrorOr<JsonValue, Error>`, but that won't be
+//        default-constructible, which is a requirement for the generated IPC.
+struct Response {
+    Response() = default;
+    Response(JsonValue&&);
+    Response(Error&&);
+
+    JsonValue& value() { return m_value_or_error.template get<JsonValue>(); }
+    JsonValue const& value() const { return m_value_or_error.template get<JsonValue>(); }
+
+    Error& error() { return m_value_or_error.template get<Error>(); }
+    Error const& error() const { return m_value_or_error.template get<Error>(); }
+
+    bool is_error() const { return m_value_or_error.template has<Error>(); }
+
+    JsonValue release_value() { return move(value()); }
+    Error release_error() { return move(error()); }
+
+    template<typename... Visitors>
+    decltype(auto) visit(Visitors&&... visitors) const
+    {
+        return m_value_or_error.visit(forward<Visitors>(visitors)...);
+    }
+
+private:
+    // Note: Empty is only a possible state until the Response has been decoded by IPC.
+    Variant<Empty, JsonValue, Error> m_value_or_error;
+};
+
+}
+
+namespace IPC {
+
+bool encode(Encoder&, Web::WebDriver::Response const&);
+ErrorOr<void> decode(Decoder&, Web::WebDriver::Response&);
+
+}