SpiceAgent: Add a new spice agent service :^)

A SPICE agent communicates with the host OS to provide nifty features
like clipboard sharing :^)

This patch implements only plain-text clipboard sharing.

See: github.com/freedesktop/spice-protocol/blob/master/spice/vd_agent.h

Userland/Services/SpiceAgent/SpiceAgent.h

@@ -0,0 +1,127 @@
+/*
+ * Copyright (c) 2021, Kyle Pereira <kyle@xylepereira.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "ClipboardServerConnection.h"
+#include <AK/ByteBuffer.h>
+#include <AK/Vector.h>
+#include <LibCore/Notifier.h>
+
+#pragma once
+
+class SpiceAgent {
+public:
+    SpiceAgent(int fd, ClipboardServerConnection&);
+
+    static constexpr u32 AGENT_PROTOCOL = 1;
+    enum class Port {
+        Client = 1,
+        Server
+    };
+
+    struct [[gnu::packed]] ChunkHeader {
+        u32 port {};
+        u32 size {};
+    };
+
+    struct [[gnu::packed]] Message {
+        u32 protocol;
+        u32 type;
+        u64 opaque;
+        u32 size;
+        u8 data[];
+    };
+
+    enum class MessageType {
+        MouseState = 1,       // server -> client
+        MonitorsConfig,       // client -> agent|server
+        Reply,                // agent -> client
+        Clipboard,            // both directions
+        DisplayConfig,        // client -> agent
+        AnnounceCapabilities, // both directions
+        ClipboardGrab,        // both directions
+        ClipboardRequest,     // both directions
+        ClipboardRelease,     // both directions
+        FileTransferStart,
+        FileTransferStatus,
+        FileTransferData,
+        Disconnected,
+        MaxClipboard,
+        VolumeSync,
+        GraphicsDeviceInfo,
+    };
+
+    enum class Capability {
+        MouseState = 0,
+        MonitorsConfig,
+        Reply,
+        Clipboard,
+        DisplayConfig,
+        ClipboardByDemand,
+        ClipboardSelection,
+        SparseMonitorsConfig,
+        GuestLineEndLF,
+        GuestLineEndCRLF,
+        MaxClipboard,
+        AudioVolumeSync,
+        MonitorsConfigPosition,
+        FileTransferDisabled,
+        FileTransferDetailedErrors,
+        GraphicsCardInfo,
+        ClipboardNoReleaseOnRegrab,
+        ClipboardGrabSerial,
+        __End,
+    };
+
+    enum class ClipboardType {
+        None = 0,
+        Text,
+        PNG,
+        BMP,
+        TIFF,
+        JPG,
+        FileList,
+        __Count
+    };
+
+    constexpr static size_t CAPABILITIES_SIZE = ((size_t)Capability::__End + 31) / 32;
+
+    struct [[gnu::packed]] AnnounceCapabilities {
+        u32 request;
+        u32 caps[CAPABILITIES_SIZE];
+
+        static ByteBuffer make_buffer(bool request, const Vector<Capability>& capabilities);
+    };
+
+    struct [[gnu::packed]] ClipboardGrab {
+        u32 types[0];
+
+        static ByteBuffer make_buffer(Vector<ClipboardType> const&);
+    };
+
+    struct [[gnu::packed]] Clipboard {
+        u32 type;
+        u8 data[];
+
+        static ByteBuffer make_buffer(ClipboardType, ReadonlyBytes);
+    };
+
+    struct [[gnu::packed]] ClipboardRequest {
+        u32 type;
+
+        static ByteBuffer make_buffer(ClipboardType);
+    };
+
+private:
+    int m_fd { -1 };
+    RefPtr<Core::Notifier> m_notifier;
+    ClipboardServerConnection& m_clipboard_connection;
+
+    void on_message_received();
+    void send_message(const ByteBuffer& buffer);
+    bool m_just_set_clip { false };
+    void read_n(void* dest, size_t n);
+    static Message* initialize_headers(u8* data, size_t additional_data_size, MessageType type);
+};