SpiceAgent: Let's start rewriting the messaging system :^)

The old message system was very dependent on syscalls, and the overall
structure made it hard to implement new features.

The new message system is pretty expandible, where each message has its
own dedicated class. As well as this, we now use Core::File and
AK::Stream for reading and writing messages.

Using AK::Stream also allows us to change the actual data source
(in this case, Core::File) without having to update a whole lot of code
in the future.

Userland/Services/SpiceAgent/Message.h

@@ -0,0 +1,101 @@
+/*
+ * Copyright (c) 2023, Caoimhe Byrne <caoimhebyrne06@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <AK/Vector.h>
+
+namespace SpiceAgent {
+
+static constexpr u32 AGENT_PROTOCOL = 1;
+
+// Used to communicate what the client/or server is capable of.
+// Not a lot of documentation is available for all of these, but the headers contain some information:
+// https://gitlab.freedesktop.org/spice/spice-protocol/-/blob/master/spice/vd_agent.h
+enum class Capability : u32 {
+    MouseState = 0,
+    MonitorsConfig,
+    Reply,
+    Clipboard,
+    DisplayConfig,
+    ClipboardByDemand,
+    ClipboardSelection,
+    SparseMonitorsConfig,
+    GuestLineEndLF,
+    GuestLineEndCRLF,
+    MaxClipboard,
+    AudioVolumeSync,
+    MonitorsConfigPosition,
+    FileTransferDisabled,
+    FileTransferDetailedErrors,
+    GraphicsCardInfo,
+    ClipboardNoReleaseOnRegrab,
+    ClipboardGrabSerial
+};
+
+class Message {
+public:
+    // The spice protocol headers contain a bit of documentation about these, but nothing major:
+    // https://gitlab.freedesktop.org/spice/spice-protocol/-/blob/master/spice/vd_agent.h
+    enum class Type : u32 {
+        MouseState = 1,
+        MonitorsConfig,
+        Reply,
+        Clipboard,
+        DisplayConfig,
+        AnnounceCapabilities,
+        ClipboardGrab,
+        ClipboardRequest,
+        ClipboardRelease,
+        FileTransferStart,
+        FileTransferStatus,
+        FileTransferData,
+        Disconnected,
+        MaxClipboard,
+        VolumeSync,
+        GraphicsDeviceInfo
+    };
+
+    Message(Type type)
+        : m_type(type)
+    {
+    }
+
+    Type type() { return m_type; }
+
+    virtual ErrorOr<String> debug_description() = 0;
+    virtual ~Message() = default;
+
+private:
+    Type m_type;
+};
+
+// Sent to the server to tell it what we are capable of.
+// See the Capabilities enum to see the available capabilities.
+class AnnounceCapabilitiesMessage : public Message {
+public:
+    AnnounceCapabilitiesMessage(bool is_request, Vector<Capability> capabilities = {})
+        : Message(Type::AnnounceCapabilities)
+        , m_is_request(is_request)
+        , m_capabilities(move(capabilities))
+    {
+    }
+
+    static ErrorOr<AnnounceCapabilitiesMessage> read_from_stream(AK::Stream& stream);
+
+    ErrorOr<void> write_to_stream(AK::Stream& stream);
+    ErrorOr<String> debug_description() override;
+
+    bool is_request() const& { return m_is_request; }
+    Vector<Capability> const& capabilities() { return m_capabilities; }
+
+private:
+    bool m_is_request { false };
+    Vector<Capability> m_capabilities;
+};
+
+}