source: vbox/trunk/src/VBox/HostServices/SharedClipboard/VBoxSharedClipboardSvc.cpp@ 100017

/* $Id: VBoxSharedClipboardSvc.cpp 100017 2023-05-30 16:06:53Z vboxsync $ */
/** @file
 * Shared Clipboard Service - Host service entry points.
 */

/*
 * Copyright (C) 2006-2023 Oracle and/or its affiliates.
 *
 * This file is part of VirtualBox base platform packages, as
 * available from https://www.virtualbox.org.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation, in version 3 of the
 * License.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, see <https://www.gnu.org/licenses>.
 *
 * SPDX-License-Identifier: GPL-3.0-only
 */


/** @page pg_hostclip       The Shared Clipboard Host Service
 *
 * The shared clipboard host service is the host half of the clibpoard proxying
 * between the host and the guest.  The guest parts live in VBoxClient, VBoxTray
 * and VBoxService depending on the OS, with code shared between host and guest
 * under src/VBox/GuestHost/SharedClipboard/.
 *
 * The service is split into a platform-independent core and platform-specific
 * backends.  The service defines two communication protocols - one to
 * communicate with the clipboard service running on the guest, and one to
 * communicate with the backend.  These will be described in a very skeletal
 * fashion here.
 *
 * r=bird: The "two communication protocols" does not seems to be factual, there
 * is only one protocol, the first one mentioned.  It cannot be backend
 * specific, because the guest/host protocol is platform and backend agnostic in
 * nature.  You may call it versions, but I take a great dislike to "protocol
 * versions" here, as you've just extended the existing protocol with a feature
 * that allows to transfer files and directories too.  See @bugref{9437#c39}.
 *
 *
 * @section sec_hostclip_guest_proto  The guest communication protocol
 *
 * The guest clipboard service communicates with the host service over HGCM
 * (the host is a HGCM service).  HGCM is connection based, so the guest side
 * has to connect before anything else can be done.  (Windows hosts currently
 * only support one simultaneous connection.)  Once it has connected, it can
 * send messages to the host services, some of which will receive immediate
 * replies from the host, others which will block till a reply becomes
 * available.  The latter is because HGCM does't allow the host to initiate
 * communication, it must be guest triggered.  The HGCM service is single
 * threaded, so it doesn't matter if the guest tries to send lots of requests in
 * parallel, the service will process them one at the time.
 *
 * There are currently four messages defined.  The first is
 * VBOX_SHCL_GUEST_FN_MSG_GET / VBOX_SHCL_GUEST_FN_MSG_OLD_GET_WAIT, which waits
 * for a message from the host.  If a host message is sent while the guest is
 * not waiting, it will be queued until the guest requests it.  The host code
 * only supports a single simultaneous GET call from one client guest.
 *
 * The second guest message is VBOX_SHCL_GUEST_FN_REPORT_FORMATS, which tells
 * the host that the guest has new clipboard data available.  The third is
 * VBOX_SHCL_GUEST_FN_DATA_READ, which asks the host to send its clipboard data
 * and waits until it arrives.  The host supports at most one simultaneous
 * VBOX_SHCL_GUEST_FN_DATA_READ call from a guest - if a second call is made
 * before the first has returned, the first will be aborted.
 *
 * The last guest message is VBOX_SHCL_GUEST_FN_DATA_WRITE, which is used to
 * send the contents of the guest clipboard to the host.  This call should be
 * used after the host has requested data from the guest.
 *
 *
 * @section sec_hostclip_backend_proto  The communication protocol with the
 *                                      platform-specific backend
 *
 * The initial protocol implementation (called protocol v0) was very simple,
 * and could only handle simple data (like copied text and so on). It also
 * was limited to two (2) fixed parameters at all times.
 *
 * Since VBox 6.1 a newer protocol (v1) has been established to also support
 * file transfers. This protocol uses a (per-client) message queue instead
 * (see VBOX_SHCL_GUEST_FN_MSG_OLD_GET_WAIT vs. VBOX_SHCL_GUEST_FN_GET_HOST_MSG).
 *
 * To distinguish the old (legacy) or new(er) protocol, the VBOX_SHCL_GUEST_FN_CONNECT
 * message has been introduced. If an older guest does not send this message,
 * an appropriate translation will be done to serve older Guest Additions (< 6.1).
 *
 * The protocol also support out-of-order messages by using so-called "context IDs",
 * which are generated by the host. A context ID consists of a so-called "source event ID"
 * and a so-called "event ID". Each HGCM client has an own, random, source event ID and
 * generates non-deterministic event IDs so that the guest side does not known what
 * comes next; the guest side has to reply with the same conext ID which was sent by
 * the host request.
 *
 * Also see the protocol changelog at VBoxShClSvc.h.
 *
 *
 * @section sec_uri_intro               Transferring files
 *
 * Since VBox x.x.x transferring files via Shared Clipboard is supported.
 * See the VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS define for supported / enabled
 * platforms. This is called "Shared Clipboard transfers".
 *
 * Copying files / directories from guest A to guest B requires the host
 * service to act as a proxy and cache, as we don't allow direct VM-to-VM
 * communication. Copying from / to the host also is taken into account.
 *
 * At the moment a transfer is a all-or-nothing operation, e.g. it either
 * completes or fails completely. There might be callbacks in the future
 * to e.g. skip failing entries.
 *
 * Known limitations:
 *
 * - Support for VRDE (VRDP) is not implemented yet (see #9498).
 * - Unicode support on Windows hosts / guests is not enabled (yet).
 * - Symbolic links / Windows junctions are not allowed.
 * - Windows alternate data streams (ADS) are not allowed.
 * - No support for ACLs yet.
 * - No (maybe never) support for NT4.

 * @section sec_transfer_structure        Transfer handling structure
 *
 * All structures / classes are designed for running on both, on the guest
 * (via VBoxTray / VBoxClient) or on the host (host service) to avoid code
 * duplication where applicable.
 *
 * Per HGCM client there is a so-called "transfer context", which in turn can
 * have one or mulitple so-called "Shared Clipboard transfer" objects. At the
 * moment we only support on concurrent Shared Clipboard transfer per transfer
 * context. It's being used for reading from a source or writing to destination,
 * depening on its direction. An Shared Clipboard transfer can have optional
 * callbacks which might be needed by various implementations. Also, transfers
 * optionally can run in an asynchronous thread to prevent blocking the UI while
 * running.
 *
 * @section sec_transfer_providers        Transfer providers
 *
 * For certain implementations (for example on Windows guests / hosts, using
 * IDataObject and IStream objects) a more flexible approach reqarding reading /
 * writing is needed. For this so-called transfer providers abstract the way of how
 * data is being read / written in the current context (host / guest), while
 * the rest of the code stays the same.
 *
 * @section sec_transfer_protocol         Transfer protocol
 *
 * The host service issues commands which the guest has to respond with an own
 * message to. The protocol itself is designed so that it has primitives to list
 * directories and open/close/read/write file system objects.
 *
 * Note that this is different from the DnD approach, as Shared Clipboard transfers
 * need to be deeper integrated within the host / guest OS (i.e. for progress UI),
 * and this might require non-monolithic / random access APIs to achieve.
 *
 * As there can be multiple file system objects (fs objects) selected for transfer,
 * a transfer can be queried for its root entries, which then contains the top-level
 * elements. Based on these elements, (a) (recursive) listing(s) can be performed
 * to (partially) walk down into directories and query fs object information. The
 * provider provides appropriate interface for this, even if not all implementations
 * might need this mechanism.
 *
 * An Shared Clipboard transfer has three stages:
 *  - 1. Announcement: An Shared Clipboard transfer-compatible format (currently only one format available)
 *          has been announced, the destination side creates a transfer object, which then,
 *          depending on the actual implementation, can be used to tell the OS that
 *          there is transfer (file) data available.
 *          At this point this just acts as a (kind-of) promise to the OS that we
 *          can provide (file) data at some later point in time.
 *
 *  - 2. Initialization: As soon as the OS requests the (file) data, mostly triggered
 *          by the user starting a paste operation (CTRL + V), the transfer get initialized
 *          on the destination side, which in turn lets the source know that a transfer
 *          is going to happen.
 *
 *  - 3. Transfer: At this stage the actual transfer from source to the destination takes
 *          place. How the actual transfer is structurized (e.g. which files / directories
 *          are transferred in which order) depends on the destination implementation. This
 *          is necessary in order to fulfill requirements on the destination side with
 *          regards to ETA calculation or other dependencies.
 *          Both sides can abort or cancel the transfer at any time.
 */


/*********************************************************************************************************************************
*   Header Files                                                                                                                 *
*********************************************************************************************************************************/
#define LOG_GROUP LOG_GROUP_SHARED_CLIPBOARD
#include <VBox/log.h>
#include <VBox/vmm/vmmr3vtable.h> /* must be included before hgcmsvc.h */

#include <VBox/GuestHost/clipboard-helper.h>
#include <VBox/HostServices/Service.h>
#include <VBox/HostServices/VBoxClipboardSvc.h>
#include <VBox/HostServices/VBoxClipboardExt.h>

#include <VBox/AssertGuest.h>
#include <VBox/err.h>
#include <VBox/VMMDev.h>
#include <VBox/vmm/ssm.h>

#include <iprt/mem.h>
#include <iprt/string.h>
#include <iprt/assert.h>
#include <iprt/critsect.h>
#include <iprt/rand.h>

#include "VBoxSharedClipboardSvc-internal.h"
#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
# include "VBoxSharedClipboardSvc-transfers.h"
#endif

using namespace HGCM;


/*********************************************************************************************************************************
*   Defined Constants And Macros                                                                                                 *
*********************************************************************************************************************************/
/** @name The saved state versions for the shared clipboard service.
 *
 * @note We set bit 31 because prior to version 0x80000002 there would be a
 *       structure size rather than a version number.  Setting bit 31 dispells
 *       any possible ambiguity.
 *
 * @{ */
/** The current saved state version. */
#define VBOX_SHCL_SAVED_STATE_VER_CURRENT   VBOX_SHCL_SAVED_STATE_LEGACY_CID
/** Adds the legacy context ID list. */
#define VBOX_SHCL_SAVED_STATE_LEGACY_CID    UINT32_C(0x80000005)
/** Adds the client's POD state and client state flags.
 * @since 6.1 RC1 */
#define VBOX_SHCL_SAVED_STATE_VER_6_1RC1    UINT32_C(0x80000004)
/** First attempt saving state during @bugref{9437} development.
 * @since 6.1 BETA 2   */
#define VBOX_SHCL_SAVED_STATE_VER_6_1B2     UINT32_C(0x80000003)
/** First structured version.
 * @since 3.1 / r53668 */
#define VBOX_SHCL_SAVED_STATE_VER_3_1       UINT32_C(0x80000002)
/** This was just a state memory dump, including pointers and everything.
 * @note This is not supported any more. Sorry.  */
#define VBOX_SHCL_SAVED_STATE_VER_NOT_SUPP  (ARCH_BITS == 64 ? UINT32_C(72) : UINT32_C(48))
/** @} */


/*********************************************************************************************************************************
*   Global Variables                                                                                                             *
*********************************************************************************************************************************/
/** The backend instance data.
 *  Only one backend at a time is supported currently. */
SHCLBACKEND         g_ShClBackend;
PVBOXHGCMSVCHELPERS g_pHelpers;

static RTCRITSECT g_CritSect;               /** @todo r=andy Put this into some instance struct, avoid globals. */
/** Global Shared Clipboard mode. */
static uint32_t g_uMode  = VBOX_SHCL_MODE_OFF;
#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
/** Global Shared Clipboard (file) transfer mode. */
uint32_t g_fTransferMode = VBOX_SHCL_TRANSFER_MODE_F_NONE;
#endif

/** Is the clipboard running in headless mode? */
static bool g_fHeadless = false;

/** Holds the service extension state. */
SHCLEXTSTATE g_ExtState = { 0 };

/** Global map of all connected clients. */
ClipboardClientMap g_mapClients;

/** Global list of all clients which are queued up (deferred return) and ready
 *  to process new commands. The key is the (unique) client ID. */
ClipboardClientQueue g_listClientsDeferred;

/** Host feature mask (VBOX_SHCL_HF_0_XXX) for VBOX_SHCL_GUEST_FN_REPORT_FEATURES
 * and VBOX_SHCL_GUEST_FN_QUERY_FEATURES. */
static uint64_t const g_fHostFeatures0 = VBOX_SHCL_HF_0_CONTEXT_ID
#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
                                       | VBOX_SHCL_HF_0_TRANSFERS
#endif
                                       ;


/**
 * Returns the current Shared Clipboard service mode.
 *
 * @returns Current Shared Clipboard service mode.
 */
uint32_t ShClSvcGetMode(void)
{
    return g_uMode;
}

/**
 * Returns the Shared Clipboard backend in use.
 *
 * @returns Pointer to backend instance.
 */
PSHCLBACKEND ShClSvcGetBackend(void)
{
    return &g_ShClBackend;
}

/**
 * Getter for headless setting. Also needed by testcase.
 *
 * @returns Whether service currently running in headless mode or not.
 */
bool ShClSvcGetHeadless(void)
{
    return g_fHeadless;
}

static int shClSvcModeSet(uint32_t uMode)
{
    int rc = VERR_NOT_SUPPORTED;

    switch (uMode)
    {
        case VBOX_SHCL_MODE_OFF:
            RT_FALL_THROUGH();
        case VBOX_SHCL_MODE_HOST_TO_GUEST:
            RT_FALL_THROUGH();
        case VBOX_SHCL_MODE_GUEST_TO_HOST:
            RT_FALL_THROUGH();
        case VBOX_SHCL_MODE_BIDIRECTIONAL:
        {
            g_uMode = uMode;

            rc = VINF_SUCCESS;
            break;
        }

        default:
        {
            g_uMode = VBOX_SHCL_MODE_OFF;
            break;
        }
    }

    LogFlowFuncLeaveRC(rc);
    return rc;
}

/**
 * Takes the global Shared Clipboard service lock.
 *
 * @returns \c true if locking was successful, or \c false if not.
 */
bool ShClSvcLock(void)
{
    return RT_SUCCESS(RTCritSectEnter(&g_CritSect));
}

/**
 * Unlocks the formerly locked global Shared Clipboard service lock.
 */
void ShClSvcUnlock(void)
{
    int rc2 = RTCritSectLeave(&g_CritSect);
    AssertRC(rc2);
}

/**
 * Resets a client's state message queue.
 *
 * @param   pClient             Pointer to the client data structure to reset message queue for.
 * @note    Caller enters pClient->CritSect.
 */
void shClSvcMsgQueueReset(PSHCLCLIENT pClient)
{
    Assert(RTCritSectIsOwner(&pClient->CritSect));
    LogFlowFuncEnter();

    while (!RTListIsEmpty(&pClient->MsgQueue))
    {
        PSHCLCLIENTMSG pMsg = RTListRemoveFirst(&pClient->MsgQueue, SHCLCLIENTMSG, ListEntry);
        shClSvcMsgFree(pClient, pMsg);
    }
    pClient->cMsgAllocated = 0;

    while (!RTListIsEmpty(&pClient->Legacy.lstCID))
    {
        PSHCLCLIENTLEGACYCID pCID = RTListRemoveFirst(&pClient->Legacy.lstCID, SHCLCLIENTLEGACYCID, Node);
        RTMemFree(pCID);
    }
    pClient->Legacy.cCID = 0;
}

/**
 * Allocates a new clipboard message.
 *
 * @returns Allocated clipboard message, or NULL on failure.
 * @param   pClient     The client which is target of this message.
 * @param   idMsg       The message ID (VBOX_SHCL_HOST_MSG_XXX) to use
 * @param   cParms      The number of parameters the message takes.
 */
PSHCLCLIENTMSG shClSvcMsgAlloc(PSHCLCLIENT pClient, uint32_t idMsg, uint32_t cParms)
{
    RT_NOREF(pClient);
    PSHCLCLIENTMSG pMsg = (PSHCLCLIENTMSG)RTMemAllocZ(RT_UOFFSETOF_DYN(SHCLCLIENTMSG, aParms[cParms]));
    if (pMsg)
    {
        uint32_t cAllocated = ASMAtomicIncU32(&pClient->cMsgAllocated);
        if (cAllocated <= 4096)
        {
            RTListInit(&pMsg->ListEntry);
            pMsg->cParms = cParms;
            pMsg->idMsg  = idMsg;
            return pMsg;
        }
        AssertMsgFailed(("Too many messages allocated for client %u! (%u)\n", pClient->State.uClientID, cAllocated));
        ASMAtomicDecU32(&pClient->cMsgAllocated);
        RTMemFree(pMsg);
    }
    return NULL;
}

/**
 * Frees a formerly allocated clipboard message.
 *
 * @param   pClient     The client which was the target of this message.
 * @param   pMsg        Clipboard message to free.
 */
void shClSvcMsgFree(PSHCLCLIENT pClient, PSHCLCLIENTMSG pMsg)
{
    RT_NOREF(pClient);
    /** @todo r=bird: Do accounting. */
    if (pMsg)
    {
        pMsg->idMsg = UINT32_C(0xdeadface);
        RTMemFree(pMsg);

        uint32_t cAllocated = ASMAtomicDecU32(&pClient->cMsgAllocated);
        Assert(cAllocated < UINT32_MAX / 2);
        RT_NOREF(cAllocated);
    }
}

/**
 * Sets the VBOX_SHCL_GUEST_FN_MSG_PEEK_WAIT and VBOX_SHCL_GUEST_FN_MSG_PEEK_NOWAIT
 * return parameters.
 *
 * @param   pMsg        Message to set return parameters to.
 * @param   paDstParms  The peek parameter vector.
 * @param   cDstParms   The number of peek parameters (at least two).
 * @remarks ASSUMES the parameters has been cleared by clientMsgPeek.
 */
static void shClSvcMsgSetPeekReturn(PSHCLCLIENTMSG pMsg, PVBOXHGCMSVCPARM paDstParms, uint32_t cDstParms)
{
    Assert(cDstParms >= 2);
    if (paDstParms[0].type == VBOX_HGCM_SVC_PARM_32BIT)
        paDstParms[0].u.uint32 = pMsg->idMsg;
    else
        paDstParms[0].u.uint64 = pMsg->idMsg;
    paDstParms[1].u.uint32 = pMsg->cParms;

    uint32_t i = RT_MIN(cDstParms, pMsg->cParms + 2);
    while (i-- > 2)
        switch (pMsg->aParms[i - 2].type)
        {
            case VBOX_HGCM_SVC_PARM_32BIT: paDstParms[i].u.uint32 = ~(uint32_t)sizeof(uint32_t); break;
            case VBOX_HGCM_SVC_PARM_64BIT: paDstParms[i].u.uint32 = ~(uint32_t)sizeof(uint64_t); break;
            case VBOX_HGCM_SVC_PARM_PTR:   paDstParms[i].u.uint32 = pMsg->aParms[i - 2].u.pointer.size; break;
        }
}

/**
 * Sets the VBOX_SHCL_GUEST_FN_MSG_OLD_GET_WAIT return parameters.
 *
 * @returns VBox status code.
 * @param   pMsg        The message which parameters to return to the guest.
 * @param   paDstParms  The peek parameter vector.
 * @param   cDstParms   The number of peek parameters should be exactly two
 */
static int shClSvcMsgSetOldWaitReturn(PSHCLCLIENTMSG pMsg, PVBOXHGCMSVCPARM paDstParms, uint32_t cDstParms)
{
    /*
     * Assert sanity.
     */
    AssertPtr(pMsg);
    AssertPtrReturn(paDstParms, VERR_INVALID_POINTER);
    AssertReturn(cDstParms >= 2, VERR_INVALID_PARAMETER);

    Assert(pMsg->cParms == 2);
    Assert(pMsg->aParms[0].u.uint32 == pMsg->idMsg);
    switch (pMsg->idMsg)
    {
        case VBOX_SHCL_HOST_MSG_READ_DATA:
        case VBOX_SHCL_HOST_MSG_FORMATS_REPORT:
            break;
        default:
            AssertFailed();
    }

    /*
     * Set the parameters.
     */
    if (pMsg->cParms > 0)
        paDstParms[0] = pMsg->aParms[0];
    if (pMsg->cParms > 1)
        paDstParms[1] = pMsg->aParms[1];
    return VINF_SUCCESS;
}

/**
 * Adds a new message to a client'S message queue.
 *
 * @param   pClient             Pointer to the client data structure to add new message to.
 * @param   pMsg                Pointer to message to add. The queue then owns the pointer.
 * @param   fAppend             Whether to append or prepend the message to the queue.
 *
 * @note    Caller must enter critical section.
 */
void shClSvcMsgAdd(PSHCLCLIENT pClient, PSHCLCLIENTMSG pMsg, bool fAppend)
{
    Assert(RTCritSectIsOwner(&pClient->CritSect));
    AssertPtr(pMsg);

    LogFlowFunc(("idMsg=%s (%RU32) cParms=%RU32 fAppend=%RTbool\n",
                 ShClHostMsgToStr(pMsg->idMsg), pMsg->idMsg, pMsg->cParms, fAppend));

    if (fAppend)
        RTListAppend(&pClient->MsgQueue, &pMsg->ListEntry);
    else
        RTListPrepend(&pClient->MsgQueue, &pMsg->ListEntry);
}


/**
 * Appends a message to the client's queue and wake it up.
 *
 * @returns VBox status code, though the message is consumed regardless of what
 *          is returned.
 * @param   pClient             The client to queue the message on.
 * @param   pMsg                The message to queue.  Ownership is always
 *                              transfered to the queue.
 *
 * @note    Caller must enter critical section.
 */
int shClSvcMsgAddAndWakeupClient(PSHCLCLIENT pClient, PSHCLCLIENTMSG pMsg)
{
    Assert(RTCritSectIsOwner(&pClient->CritSect));
    AssertPtr(pMsg);
    AssertPtr(pClient);
    LogFlowFunc(("idMsg=%s (%u) cParms=%u\n", ShClHostMsgToStr(pMsg->idMsg), pMsg->idMsg, pMsg->cParms));

    RTListAppend(&pClient->MsgQueue, &pMsg->ListEntry);
    return shClSvcClientWakeup(pClient);
}

/**
 * Initializes a Shared Clipboard client.
 *
 * @param   pClient             Client to initialize.
 * @param   uClientID           HGCM client ID to assign client to.
 */
int shClSvcClientInit(PSHCLCLIENT pClient, uint32_t uClientID)
{
    AssertPtrReturn(pClient, VERR_INVALID_POINTER);

    /* Assign the client ID. */
    pClient->State.uClientID = uClientID;

    RTListInit(&pClient->MsgQueue);
    pClient->cMsgAllocated = 0;

    RTListInit(&pClient->Legacy.lstCID);
    pClient->Legacy.cCID = 0;

    LogFlowFunc(("[Client %RU32]\n", pClient->State.uClientID));

    int rc = RTCritSectInit(&pClient->CritSect);
    if (RT_SUCCESS(rc))
    {
        /* Create the client's own event source. */
        rc = ShClEventSourceCreate(&pClient->EventSrc, 0 /* ID, ignored */);
        if (RT_SUCCESS(rc))
        {
            LogFlowFunc(("[Client %RU32] Using event source %RU32\n", uClientID, pClient->EventSrc.uID));

            /* Reset the client state. */
            shclSvcClientStateReset(&pClient->State);

            /* (Re-)initialize the client state. */
            rc = shClSvcClientStateInit(&pClient->State, uClientID);

#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
            if (RT_SUCCESS(rc))
                rc = ShClTransferCtxInit(&pClient->Transfers.Ctx);
#endif
        }
    }

    LogFlowFuncLeaveRC(rc);
    return rc;
}

/**
 * Destroys a Shared Clipboard client.
 *
 * @param   pClient             Client to destroy.
 */
void shClSvcClientDestroy(PSHCLCLIENT pClient)
{
    AssertPtrReturnVoid(pClient);

    LogFlowFunc(("[Client %RU32]\n", pClient->State.uClientID));

    /* Make sure to send a quit message to the guest so that it can terminate gracefully. */
    RTCritSectEnter(&pClient->CritSect);
    if (pClient->Pending.uType)
    {
        if (pClient->Pending.cParms > 1)
            HGCMSvcSetU32(&pClient->Pending.paParms[0], VBOX_SHCL_HOST_MSG_QUIT);
        if (pClient->Pending.cParms > 2)
            HGCMSvcSetU32(&pClient->Pending.paParms[1], 0);
        g_pHelpers->pfnCallComplete(pClient->Pending.hHandle, VINF_SUCCESS);
        pClient->Pending.uType   = 0;
        pClient->Pending.cParms  = 0;
        pClient->Pending.hHandle = NULL;
        pClient->Pending.paParms = NULL;
    }
    RTCritSectLeave(&pClient->CritSect);

    ShClEventSourceDestroy(&pClient->EventSrc);

    shClSvcClientStateDestroy(&pClient->State);

    PSHCLCLIENTLEGACYCID pCidIter, pCidIterNext;
    RTListForEachSafe(&pClient->Legacy.lstCID, pCidIter, pCidIterNext, SHCLCLIENTLEGACYCID, Node)
    {
        RTMemFree(pCidIter);
    }

    int rc2 = RTCritSectDelete(&pClient->CritSect);
    AssertRC(rc2);

    ClipboardClientMap::iterator itClient = g_mapClients.find(pClient->State.uClientID);
    if (itClient != g_mapClients.end())
        g_mapClients.erase(itClient);
    else
        AssertFailed();

    LogFlowFuncLeave();
}

void shClSvcClientLock(PSHCLCLIENT pClient)
{
    int rc2 = RTCritSectEnter(&pClient->CritSect);
    AssertRC(rc2);
}

void shClSvcClientUnlock(PSHCLCLIENT pClient)
{
    int rc2 = RTCritSectLeave(&pClient->CritSect);
    AssertRC(rc2);
}

/**
 * Resets a Shared Clipboard client.
 *
 * @param   pClient             Client to reset.
 */
void shClSvcClientReset(PSHCLCLIENT pClient)
{
    if (!pClient)
        return;

    LogFlowFunc(("[Client %RU32]\n", pClient->State.uClientID));
    RTCritSectEnter(&pClient->CritSect);

    /* Reset message queue. */
    shClSvcMsgQueueReset(pClient);

    /* Reset event source. */
    ShClEventSourceReset(&pClient->EventSrc);

    /* Reset pending state. */
    RT_ZERO(pClient->Pending);

#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
    shClSvcClientTransfersReset(pClient);
#endif

    shclSvcClientStateReset(&pClient->State);

    RTCritSectLeave(&pClient->CritSect);
}

static int shClSvcClientNegogiateChunkSize(PSHCLCLIENT pClient, VBOXHGCMCALLHANDLE hCall,
                                           uint32_t cParms, VBOXHGCMSVCPARM paParms[])
{
    /*
     * Validate the request.
     */
    ASSERT_GUEST_RETURN(cParms == VBOX_SHCL_CPARMS_NEGOTIATE_CHUNK_SIZE, VERR_WRONG_PARAMETER_COUNT);
    ASSERT_GUEST_RETURN(paParms[0].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE);
    uint32_t const cbClientMaxChunkSize = paParms[0].u.uint32;
    ASSERT_GUEST_RETURN(paParms[1].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE);
    uint32_t const cbClientChunkSize    = paParms[1].u.uint32;

    uint32_t const cbHostMaxChunkSize = VBOX_SHCL_MAX_CHUNK_SIZE; /** @todo Make this configurable. */

    /*
     * Do the work.
     */
    if (cbClientChunkSize == 0) /* Does the client want us to choose? */
    {
        paParms[0].u.uint32 = cbHostMaxChunkSize;                                     /* Maximum */
        paParms[1].u.uint32 = RT_MIN(pClient->State.cbChunkSize, cbHostMaxChunkSize); /* Preferred */

    }
    else /* The client told us what it supports, so update and report back. */
    {
        paParms[0].u.uint32 = RT_MIN(cbClientMaxChunkSize, cbHostMaxChunkSize);         /* Maximum */
        paParms[1].u.uint32 = RT_MIN(cbClientMaxChunkSize, pClient->State.cbChunkSize); /* Preferred */
    }

    int rc = g_pHelpers->pfnCallComplete(hCall, VINF_SUCCESS);
    if (RT_SUCCESS(rc))
    {
        Log(("[Client %RU32] chunk size: %#RU32, max: %#RU32\n",
             pClient->State.uClientID, paParms[1].u.uint32, paParms[0].u.uint32));
    }
    else
        LogFunc(("pfnCallComplete -> %Rrc\n", rc));

    return VINF_HGCM_ASYNC_EXECUTE;
}

/**
 * Implements VBOX_SHCL_GUEST_FN_REPORT_FEATURES.
 *
 * @returns VBox status code.
 * @retval  VINF_HGCM_ASYNC_EXECUTE on success (we complete the message here).
 * @retval  VERR_ACCESS_DENIED if not master
 * @retval  VERR_INVALID_PARAMETER if bit 63 in the 2nd parameter isn't set.
 * @retval  VERR_WRONG_PARAMETER_COUNT
 *
 * @param   pClient     The client state.
 * @param   hCall       The client's call handle.
 * @param   cParms      Number of parameters.
 * @param   paParms     Array of parameters.
 */
static int shClSvcClientReportFeatures(PSHCLCLIENT pClient, VBOXHGCMCALLHANDLE hCall,
                                       uint32_t cParms, VBOXHGCMSVCPARM paParms[])
{
    /*
     * Validate the request.
     */
    ASSERT_GUEST_RETURN(cParms == 2, VERR_WRONG_PARAMETER_COUNT);
    ASSERT_GUEST_RETURN(paParms[0].type == VBOX_HGCM_SVC_PARM_64BIT, VERR_WRONG_PARAMETER_TYPE);
    uint64_t const fFeatures0 = paParms[0].u.uint64;
    ASSERT_GUEST_RETURN(paParms[1].type == VBOX_HGCM_SVC_PARM_64BIT, VERR_WRONG_PARAMETER_TYPE);
    uint64_t const fFeatures1 = paParms[1].u.uint64;
    ASSERT_GUEST_RETURN(fFeatures1 & VBOX_SHCL_GF_1_MUST_BE_ONE, VERR_INVALID_PARAMETER);

    /*
     * Do the work.
     */
    paParms[0].u.uint64 = g_fHostFeatures0;
    paParms[1].u.uint64 = 0;

    int rc = g_pHelpers->pfnCallComplete(hCall, VINF_SUCCESS);
    if (RT_SUCCESS(rc))
    {
        pClient->State.fGuestFeatures0 = fFeatures0;
        pClient->State.fGuestFeatures1 = fFeatures1;
        LogRel2(("Shared Clipboard: Guest reported the following features: %#RX64\n",
                 pClient->State.fGuestFeatures0)); /* Note: fFeatures1 not used yet. */
        if (pClient->State.fGuestFeatures0 & VBOX_SHCL_GF_0_TRANSFERS)
            LogRel2(("Shared Clipboard: Guest supports file transfers\n"));
    }
    else
        LogFunc(("pfnCallComplete -> %Rrc\n", rc));

    return VINF_HGCM_ASYNC_EXECUTE;
}

/**
 * Implements VBOX_SHCL_GUEST_FN_QUERY_FEATURES.
 *
 * @returns VBox status code.
 * @retval  VINF_HGCM_ASYNC_EXECUTE on success (we complete the message here).
 * @retval  VERR_WRONG_PARAMETER_COUNT
 *
 * @param   hCall       The client's call handle.
 * @param   cParms      Number of parameters.
 * @param   paParms     Array of parameters.
 */
static int shClSvcClientQueryFeatures(VBOXHGCMCALLHANDLE hCall, uint32_t cParms, VBOXHGCMSVCPARM paParms[])
{
    /*
     * Validate the request.
     */
    ASSERT_GUEST_RETURN(cParms == 2, VERR_WRONG_PARAMETER_COUNT);
    ASSERT_GUEST_RETURN(paParms[0].type == VBOX_HGCM_SVC_PARM_64BIT, VERR_WRONG_PARAMETER_TYPE);
    ASSERT_GUEST_RETURN(paParms[1].type == VBOX_HGCM_SVC_PARM_64BIT, VERR_WRONG_PARAMETER_TYPE);
    ASSERT_GUEST(paParms[1].u.uint64 & RT_BIT_64(63));

    /*
     * Do the work.
     */
    paParms[0].u.uint64 = g_fHostFeatures0;
    paParms[1].u.uint64 = 0;
    int rc = g_pHelpers->pfnCallComplete(hCall, VINF_SUCCESS);
    if (RT_FAILURE(rc))
        LogFunc(("pfnCallComplete -> %Rrc\n", rc));

    return VINF_HGCM_ASYNC_EXECUTE;
}

/**
 * Implements VBOX_SHCL_GUEST_FN_MSG_PEEK_WAIT and VBOX_SHCL_GUEST_FN_MSG_PEEK_NOWAIT.
 *
 * @returns VBox status code.
 * @retval  VINF_SUCCESS if a message was pending and is being returned.
 * @retval  VERR_TRY_AGAIN if no message pending and not blocking.
 * @retval  VERR_RESOURCE_BUSY if another read already made a waiting call.
 * @retval  VINF_HGCM_ASYNC_EXECUTE if message wait is pending.
 *
 * @param   pClient     The client state.
 * @param   hCall       The client's call handle.
 * @param   cParms      Number of parameters.
 * @param   paParms     Array of parameters.
 * @param   fWait       Set if we should wait for a message, clear if to return
 *                      immediately.
 *
 * @note    Caller takes and leave the client's critical section.
 */
static int shClSvcClientMsgPeek(PSHCLCLIENT pClient, VBOXHGCMCALLHANDLE hCall, uint32_t cParms, VBOXHGCMSVCPARM paParms[], bool fWait)
{
    /*
     * Validate the request.
     */
    ASSERT_GUEST_MSG_RETURN(cParms >= 2, ("cParms=%u!\n", cParms), VERR_WRONG_PARAMETER_COUNT);

    uint64_t idRestoreCheck = 0;
    uint32_t i              = 0;
    if (paParms[i].type == VBOX_HGCM_SVC_PARM_64BIT)
    {
        idRestoreCheck = paParms[0].u.uint64;
        paParms[0].u.uint64 = 0;
        i++;
    }
    for (; i < cParms; i++)
    {
        ASSERT_GUEST_MSG_RETURN(paParms[i].type == VBOX_HGCM_SVC_PARM_32BIT, ("#%u type=%u\n", i, paParms[i].type),
                                VERR_WRONG_PARAMETER_TYPE);
        paParms[i].u.uint32 = 0;
    }

    /*
     * Check restore session ID.
     */
    if (idRestoreCheck != 0)
    {
        uint64_t idRestore = g_pHelpers->pfnGetVMMDevSessionId(g_pHelpers);
        if (idRestoreCheck != idRestore)
        {
            paParms[0].u.uint64 = idRestore;
            LogFlowFunc(("[Client %RU32] VBOX_SHCL_GUEST_FN_MSG_PEEK_XXX -> VERR_VM_RESTORED (%#RX64 -> %#RX64)\n",
                         pClient->State.uClientID, idRestoreCheck, idRestore));
            return VERR_VM_RESTORED;
        }
        Assert(!g_pHelpers->pfnIsCallRestored(hCall));
    }

    /*
     * Return information about the first message if one is pending in the list.
     */
    PSHCLCLIENTMSG pFirstMsg = RTListGetFirst(&pClient->MsgQueue, SHCLCLIENTMSG, ListEntry);
    if (pFirstMsg)
    {
        shClSvcMsgSetPeekReturn(pFirstMsg, paParms, cParms);
        LogFlowFunc(("[Client %RU32] VBOX_SHCL_GUEST_FN_MSG_PEEK_XXX -> VINF_SUCCESS (idMsg=%s (%u), cParms=%u)\n",
                     pClient->State.uClientID, ShClHostMsgToStr(pFirstMsg->idMsg), pFirstMsg->idMsg, pFirstMsg->cParms));
        return VINF_SUCCESS;
    }

    /*
     * If we cannot wait, fail the call.
     */
    if (!fWait)
    {
        LogFlowFunc(("[Client %RU32] GUEST_MSG_PEEK_NOWAIT -> VERR_TRY_AGAIN\n", pClient->State.uClientID));
        return VERR_TRY_AGAIN;
    }

    /*
     * Wait for the host to queue a message for this client.
     */
    ASSERT_GUEST_MSG_RETURN(pClient->Pending.uType == 0, ("Already pending! (idClient=%RU32)\n",
                                                           pClient->State.uClientID), VERR_RESOURCE_BUSY);
    pClient->Pending.hHandle = hCall;
    pClient->Pending.cParms  = cParms;
    pClient->Pending.paParms = paParms;
    pClient->Pending.uType   = VBOX_SHCL_GUEST_FN_MSG_PEEK_WAIT;
    LogFlowFunc(("[Client %RU32] Is now in pending mode...\n", pClient->State.uClientID));
    return VINF_HGCM_ASYNC_EXECUTE;
}

/**
 * Implements VBOX_SHCL_GUEST_FN_MSG_OLD_GET_WAIT.
 *
 * @returns VBox status code.
 * @retval  VINF_SUCCESS if a message was pending and is being returned.
 * @retval  VINF_HGCM_ASYNC_EXECUTE if message wait is pending.
 *
 * @param   pClient     The client state.
 * @param   hCall       The client's call handle.
 * @param   cParms      Number of parameters.
 * @param   paParms     Array of parameters.
 *
 * @note    Caller takes and leave the client's critical section.
 */
static int shClSvcClientMsgOldGet(PSHCLCLIENT pClient, VBOXHGCMCALLHANDLE hCall, uint32_t cParms, VBOXHGCMSVCPARM paParms[])
{
    /*
     * Validate input.
     */
    ASSERT_GUEST_RETURN(cParms == VBOX_SHCL_CPARMS_GET_HOST_MSG_OLD, VERR_WRONG_PARAMETER_COUNT);
    ASSERT_GUEST_RETURN(paParms[0].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE); /* id32Msg */
    ASSERT_GUEST_RETURN(paParms[1].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE); /* f32Formats */

    paParms[0].u.uint32 = 0;
    paParms[1].u.uint32 = 0;

    /*
     * If there is a message pending we can return immediately.
     */
    int rc;
    PSHCLCLIENTMSG pFirstMsg = RTListGetFirst(&pClient->MsgQueue, SHCLCLIENTMSG, ListEntry);
    if (pFirstMsg)
    {
        LogFlowFunc(("[Client %RU32] uMsg=%s (%RU32), cParms=%RU32\n", pClient->State.uClientID,
                     ShClHostMsgToStr(pFirstMsg->idMsg), pFirstMsg->idMsg, pFirstMsg->cParms));

        rc = shClSvcMsgSetOldWaitReturn(pFirstMsg, paParms, cParms);
        AssertPtr(g_pHelpers);
        rc = g_pHelpers->pfnCallComplete(hCall, rc);
        if (rc != VERR_CANCELLED)
        {
            RTListNodeRemove(&pFirstMsg->ListEntry);
            shClSvcMsgFree(pClient, pFirstMsg);

            rc = VINF_HGCM_ASYNC_EXECUTE; /* The caller must not complete it. */
        }
    }
    /*
     * Otherwise we must wait.
     */
    else
    {
        ASSERT_GUEST_MSG_RETURN(pClient->Pending.uType == 0, ("Already pending! (idClient=%RU32)\n", pClient->State.uClientID),
                                VERR_RESOURCE_BUSY);

        pClient->Pending.hHandle = hCall;
        pClient->Pending.cParms  = cParms;
        pClient->Pending.paParms = paParms;
        pClient->Pending.uType   = VBOX_SHCL_GUEST_FN_MSG_OLD_GET_WAIT;

        rc = VINF_HGCM_ASYNC_EXECUTE; /* The caller must not complete it. */

        LogFlowFunc(("[Client %RU32] Is now in pending mode...\n", pClient->State.uClientID));
    }

    LogFlowFunc(("[Client %RU32] rc=%Rrc\n", pClient->State.uClientID, rc));
    return rc;
}

/**
 * Implements VBOX_SHCL_GUEST_FN_MSG_GET.
 *
 * @returns VBox status code.
 * @retval  VINF_SUCCESS if message retrieved and removed from the pending queue.
 * @retval  VERR_TRY_AGAIN if no message pending.
 * @retval  VERR_BUFFER_OVERFLOW if a parmeter buffer is too small.  The buffer
 *          size was updated to reflect the required size, though this isn't yet
 *          forwarded to the guest.  (The guest is better of using peek with
 *          parameter count + 2 parameters to get the sizes.)
 * @retval  VERR_MISMATCH if the incoming message ID does not match the pending.
 * @retval  VINF_HGCM_ASYNC_EXECUTE if message was completed already.
 *
 * @param   pClient      The client state.
 * @param   hCall        The client's call handle.
 * @param   cParms       Number of parameters.
 * @param   paParms      Array of parameters.
 *
 * @note    Called from within pClient->CritSect.
 */
static int shClSvcClientMsgGet(PSHCLCLIENT pClient, VBOXHGCMCALLHANDLE hCall, uint32_t cParms, VBOXHGCMSVCPARM paParms[])
{
    /*
     * Validate the request.
     */
    uint32_t const idMsgExpected = cParms > 0 && paParms[0].type == VBOX_HGCM_SVC_PARM_32BIT ? paParms[0].u.uint32
                                 : cParms > 0 && paParms[0].type == VBOX_HGCM_SVC_PARM_64BIT ? paParms[0].u.uint64
                                 : UINT32_MAX;

    /*
     * Return information about the first message if one is pending in the list.
     */
    PSHCLCLIENTMSG pFirstMsg = RTListGetFirst(&pClient->MsgQueue, SHCLCLIENTMSG, ListEntry);
    if (pFirstMsg)
    {
        LogFlowFunc(("First message is: %s (%u), cParms=%RU32\n", ShClHostMsgToStr(pFirstMsg->idMsg), pFirstMsg->idMsg, pFirstMsg->cParms));

        ASSERT_GUEST_MSG_RETURN(pFirstMsg->idMsg == idMsgExpected || idMsgExpected == UINT32_MAX,
                                ("idMsg=%u (%s) cParms=%u, caller expected %u (%s) and %u\n",
                                 pFirstMsg->idMsg, ShClHostMsgToStr(pFirstMsg->idMsg), pFirstMsg->cParms,
                                 idMsgExpected, ShClHostMsgToStr(idMsgExpected), cParms),
                                VERR_MISMATCH);
        ASSERT_GUEST_MSG_RETURN(pFirstMsg->cParms == cParms,
                                ("idMsg=%u (%s) cParms=%u, caller expected %u (%s) and %u\n",
                                 pFirstMsg->idMsg, ShClHostMsgToStr(pFirstMsg->idMsg), pFirstMsg->cParms,
                                 idMsgExpected, ShClHostMsgToStr(idMsgExpected), cParms),
                                VERR_WRONG_PARAMETER_COUNT);

        /* Check the parameter types. */
        for (uint32_t i = 0; i < cParms; i++)
            ASSERT_GUEST_MSG_RETURN(pFirstMsg->aParms[i].type == paParms[i].type,
                                    ("param #%u: type %u, caller expected %u (idMsg=%u %s)\n", i, pFirstMsg->aParms[i].type,
                                     paParms[i].type, pFirstMsg->idMsg, ShClHostMsgToStr(pFirstMsg->idMsg)),
                                    VERR_WRONG_PARAMETER_TYPE);
        /*
         * Copy out the parameters.
         *
         * No assertions on buffer overflows, and keep going till the end so we can
         * communicate all the required buffer sizes.
         */
        int rc = VINF_SUCCESS;
        for (uint32_t i = 0; i < cParms; i++)
            switch (pFirstMsg->aParms[i].type)
            {
                case VBOX_HGCM_SVC_PARM_32BIT:
                    paParms[i].u.uint32 = pFirstMsg->aParms[i].u.uint32;
                    break;

                case VBOX_HGCM_SVC_PARM_64BIT:
                    paParms[i].u.uint64 = pFirstMsg->aParms[i].u.uint64;
                    break;

                case VBOX_HGCM_SVC_PARM_PTR:
                {
                    uint32_t const cbSrc = pFirstMsg->aParms[i].u.pointer.size;
                    uint32_t const cbDst = paParms[i].u.pointer.size;
                    paParms[i].u.pointer.size = cbSrc; /** @todo Check if this is safe in other layers...
                                                        * Update: Safe, yes, but VMMDevHGCM doesn't pass it along. */
                    if (cbSrc <= cbDst)
                        memcpy(paParms[i].u.pointer.addr, pFirstMsg->aParms[i].u.pointer.addr, cbSrc);
                    else
                    {
                        AssertMsgFailed(("#%u: cbSrc=%RU32 is bigger than cbDst=%RU32\n", i, cbSrc, cbDst));
                        rc = VERR_BUFFER_OVERFLOW;
                    }
                    break;
                }

                default:
                    AssertMsgFailed(("#%u: %u\n", i, pFirstMsg->aParms[i].type));
                    rc = VERR_INTERNAL_ERROR;
                    break;
            }
        if (RT_SUCCESS(rc))
        {
            /*
             * Complete the message and remove the pending message unless the
             * guest raced us and cancelled this call in the meantime.
             */
            AssertPtr(g_pHelpers);
            rc = g_pHelpers->pfnCallComplete(hCall, rc);

            LogFlowFunc(("[Client %RU32] pfnCallComplete -> %Rrc\n", pClient->State.uClientID, rc));

            if (rc != VERR_CANCELLED)
            {
                RTListNodeRemove(&pFirstMsg->ListEntry);
                shClSvcMsgFree(pClient, pFirstMsg);
            }

            return VINF_HGCM_ASYNC_EXECUTE; /* The caller must not complete it. */
        }

        LogFlowFunc(("[Client %RU32] Returning %Rrc\n", pClient->State.uClientID, rc));
        return rc;
    }

    paParms[0].u.uint32 = 0;
    paParms[1].u.uint32 = 0;
    LogFlowFunc(("[Client %RU32] -> VERR_TRY_AGAIN\n", pClient->State.uClientID));
    return VERR_TRY_AGAIN;
}

/**
 * Implements VBOX_SHCL_GUEST_FN_MSG_GET.
 *
 * @returns VBox status code.
 * @retval  VINF_SUCCESS if message retrieved and removed from the pending queue.
 * @retval  VERR_TRY_AGAIN if no message pending.
 * @retval  VERR_MISMATCH if the incoming message ID does not match the pending.
 * @retval  VINF_HGCM_ASYNC_EXECUTE if message was completed already.
 *
 * @param   pClient      The client state.
 * @param   cParms       Number of parameters.
 *
 * @note    Called from within pClient->CritSect.
 */
static int shClSvcClientMsgCancel(PSHCLCLIENT pClient, uint32_t cParms)
{
    /*
     * Validate the request.
     */
    ASSERT_GUEST_MSG_RETURN(cParms == 0, ("cParms=%u!\n", cParms), VERR_WRONG_PARAMETER_COUNT);

    /*
     * Execute.
     */
    if (pClient->Pending.uType != 0)
    {
        LogFlowFunc(("[Client %RU32] Cancelling waiting thread, isPending=%d, pendingNumParms=%RU32, m_idSession=%x\n",
                     pClient->State.uClientID, pClient->Pending.uType, pClient->Pending.cParms, pClient->State.uSessionID));

        /*
         * The PEEK call is simple: At least two parameters, all set to zero before sleeping.
         */
        int rcComplete;
        if (pClient->Pending.uType == VBOX_SHCL_GUEST_FN_MSG_PEEK_WAIT)
        {
            Assert(pClient->Pending.cParms >= 2);
            if (pClient->Pending.paParms[0].type == VBOX_HGCM_SVC_PARM_64BIT)
                HGCMSvcSetU64(&pClient->Pending.paParms[0], VBOX_SHCL_HOST_MSG_CANCELED);
            else
                HGCMSvcSetU32(&pClient->Pending.paParms[0], VBOX_SHCL_HOST_MSG_CANCELED);
            rcComplete = VINF_TRY_AGAIN;
        }
        /*
         * The MSG_OLD call is complicated, though we're
         * generally here to wake up someone who is peeking and have two parameters.
         * If there aren't two parameters, fail the call.
         */
        else
        {
            Assert(pClient->Pending.uType == VBOX_SHCL_GUEST_FN_MSG_OLD_GET_WAIT);
            if (pClient->Pending.cParms > 0)
                HGCMSvcSetU32(&pClient->Pending.paParms[0], VBOX_SHCL_HOST_MSG_CANCELED);
            if (pClient->Pending.cParms > 1)
                HGCMSvcSetU32(&pClient->Pending.paParms[1], 0);
            rcComplete = pClient->Pending.cParms == 2 ? VINF_SUCCESS : VERR_TRY_AGAIN;
        }

        g_pHelpers->pfnCallComplete(pClient->Pending.hHandle, rcComplete);

        pClient->Pending.hHandle    = NULL;
        pClient->Pending.paParms    = NULL;
        pClient->Pending.cParms     = 0;
        pClient->Pending.uType      = 0;
        return VINF_SUCCESS;
    }
    return VWRN_NOT_FOUND;
}


/**
 * Wakes up a pending client (i.e. waiting for new messages).
 *
 * @returns VBox status code.
 * @retval  VINF_NO_CHANGE if the client is not in pending mode.
 *
 * @param   pClient             Client to wake up.
 * @note    Caller must enter pClient->CritSect.
 */
int shClSvcClientWakeup(PSHCLCLIENT pClient)
{
    Assert(RTCritSectIsOwner(&pClient->CritSect));
    int rc = VINF_NO_CHANGE;

    if (pClient->Pending.uType != 0)
    {
        LogFunc(("[Client %RU32] Waking up ...\n", pClient->State.uClientID));

        PSHCLCLIENTMSG pFirstMsg = RTListGetFirst(&pClient->MsgQueue, SHCLCLIENTMSG, ListEntry);
        AssertReturn(pFirstMsg, VERR_INTERNAL_ERROR);

        LogFunc(("[Client %RU32] Current host message is %s (%RU32), cParms=%RU32\n",
                 pClient->State.uClientID, ShClHostMsgToStr(pFirstMsg->idMsg), pFirstMsg->idMsg, pFirstMsg->cParms));

        if (pClient->Pending.uType == VBOX_SHCL_GUEST_FN_MSG_PEEK_WAIT)
            shClSvcMsgSetPeekReturn(pFirstMsg, pClient->Pending.paParms, pClient->Pending.cParms);
        else if (pClient->Pending.uType == VBOX_SHCL_GUEST_FN_MSG_OLD_GET_WAIT) /* Legacy, Guest Additions < 6.1. */
            shClSvcMsgSetOldWaitReturn(pFirstMsg, pClient->Pending.paParms, pClient->Pending.cParms);
        else
            AssertMsgFailedReturn(("pClient->Pending.uType=%u\n", pClient->Pending.uType), VERR_INTERNAL_ERROR_3);

        rc = g_pHelpers->pfnCallComplete(pClient->Pending.hHandle, VINF_SUCCESS);

        if (   rc != VERR_CANCELLED
            && pClient->Pending.uType == VBOX_SHCL_GUEST_FN_MSG_OLD_GET_WAIT)
        {
            RTListNodeRemove(&pFirstMsg->ListEntry);
            shClSvcMsgFree(pClient, pFirstMsg);
        }

        pClient->Pending.hHandle = NULL;
        pClient->Pending.paParms = NULL;
        pClient->Pending.cParms  = 0;
        pClient->Pending.uType   = 0;
    }
    else
        LogFunc(("[Client %RU32] Not in pending state, skipping wakeup\n", pClient->State.uClientID));

    return rc;
}

/**
 * Requests to read clipboard data from the guest.
 *
 * @returns VBox status code.
 * @param   pClient             Client to request to read data form.
 * @param   fFormats            The formats being requested, OR'ed together (VBOX_SHCL_FMT_XXX).
 * @param   ppEvent             Where to return the event for waiting for new data on success. Optional.
 *                              Must be released by the caller with ShClEventRelease().
 */
int ShClSvcGuestDataRequest(PSHCLCLIENT pClient, SHCLFORMATS fFormats, PSHCLEVENT *ppEvent)
{
    AssertPtrReturn(pClient, VERR_INVALID_POINTER);

    LogFlowFunc(("fFormats=%#x\n", fFormats));

    int rc = VERR_NOT_SUPPORTED;

    /* Generate a separate message for every (valid) format we support. */
    while (fFormats)
    {
        /* Pick the next format to get from the mask: */
        /** @todo Make format reporting precedence configurable? */
        SHCLFORMAT fFormat;
        if (fFormats & VBOX_SHCL_FMT_UNICODETEXT)
            fFormat = VBOX_SHCL_FMT_UNICODETEXT;
        else if (fFormats & VBOX_SHCL_FMT_BITMAP)
            fFormat = VBOX_SHCL_FMT_BITMAP;
        else if (fFormats & VBOX_SHCL_FMT_HTML)
            fFormat = VBOX_SHCL_FMT_HTML;
        else
            AssertMsgFailedBreak(("%#x\n", fFormats));

        /* Remove it from the mask. */
        fFormats &= ~fFormat;

#ifdef LOG_ENABLED
        char *pszFmt = ShClFormatsToStrA(fFormat);
        AssertPtrReturn(pszFmt, VERR_NO_MEMORY);
        LogRel2(("Shared Clipboard: Requesting guest clipboard data in format '%s'\n", pszFmt));
        RTStrFree(pszFmt);
#endif
        /*
         * Allocate messages, one for each format.
         */
        PSHCLCLIENTMSG pMsg = shClSvcMsgAlloc(pClient,
                                              pClient->State.fGuestFeatures0 & VBOX_SHCL_GF_0_CONTEXT_ID
                                              ? VBOX_SHCL_HOST_MSG_READ_DATA_CID : VBOX_SHCL_HOST_MSG_READ_DATA,
                                              2);
        if (pMsg)
        {
            /*
             * Enter the critical section and generate an event.
             */
            RTCritSectEnter(&pClient->CritSect);

            PSHCLEVENT pEvent;
            rc = ShClEventSourceGenerateAndRegisterEvent(&pClient->EventSrc, &pEvent);
            if (RT_SUCCESS(rc))
            {
                LogFlowFunc(("fFormats=%#x -> fFormat=%#x, idEvent=%#x\n", fFormats, fFormat, pEvent->idEvent));

                const uint64_t uCID = VBOX_SHCL_CONTEXTID_MAKE(pClient->State.uSessionID, pClient->EventSrc.uID, pEvent->idEvent);

                rc = VINF_SUCCESS;

                /* Save the context ID in our legacy cruft if we have to deal with old(er) Guest Additions (< 6.1). */
                if (!(pClient->State.fGuestFeatures0 & VBOX_SHCL_GF_0_CONTEXT_ID))
                {
                    AssertStmt(pClient->Legacy.cCID < 4096, rc = VERR_TOO_MUCH_DATA);
                    if (RT_SUCCESS(rc))
                    {
                        PSHCLCLIENTLEGACYCID pCID = (PSHCLCLIENTLEGACYCID)RTMemAlloc(sizeof(SHCLCLIENTLEGACYCID));
                        if (pCID)
                        {
                            pCID->uCID    = uCID;
                            pCID->enmType = 0; /* Not used yet. */
                            pCID->uFormat = fFormat;
                            RTListAppend(&pClient->Legacy.lstCID, &pCID->Node);
                            pClient->Legacy.cCID++;
                        }
                        else
                            rc = VERR_NO_MEMORY;
                    }
                }

                if (RT_SUCCESS(rc))
                {
                    /*
                     * Format the message.
                     */
                    if (pMsg->idMsg == VBOX_SHCL_HOST_MSG_READ_DATA_CID)
                        HGCMSvcSetU64(&pMsg->aParms[0], uCID);
                    else
                        HGCMSvcSetU32(&pMsg->aParms[0], VBOX_SHCL_HOST_MSG_READ_DATA);
                    HGCMSvcSetU32(&pMsg->aParms[1], fFormat);

                    shClSvcMsgAdd(pClient, pMsg, true /* fAppend */);

                    /* Return event handle to the caller if requested. */
                    if (ppEvent)
                    {
                        *ppEvent = pEvent;
                    }

                    shClSvcClientWakeup(pClient);
                }

                /* Remove event from list if caller did not request event handle or in case
                 * of failure (in this case caller should not release event). */
                if (   RT_FAILURE(rc)
                    || !ppEvent)
                {
                    ShClEventRelease(pEvent);
                }
            }
            else
                rc = VERR_SHCLPB_MAX_EVENTS_REACHED;

            RTCritSectLeave(&pClient->CritSect);

            if (RT_FAILURE(rc))
                shClSvcMsgFree(pClient, pMsg);
        }
        else
            rc = VERR_NO_MEMORY;

        if (RT_FAILURE(rc))
            break;
    }

    if (RT_FAILURE(rc))
        LogRel(("Shared Clipboard: Requesting data in formats %#x from guest failed with %Rrc\n", fFormats, rc));

    LogFlowFuncLeaveRC(rc);
    return rc;
}

/**
 * Signals the host that clipboard data from the guest has been received.
 *
 * @returns VBox status code. Returns VERR_NOT_FOUND when related event ID was not found.
 * @param   pClient             Client the guest clipboard data was received from.
 * @param   pCmdCtx             Client command context.
 * @param   uFormat             Clipboard format of data received.
 * @param   pvData              Pointer to clipboard data received.  This can be
 *                              NULL if @a cbData is zero.
 * @param   cbData              Size (in bytes) of clipboard data received.
 *                              This can be zero.
 */
int ShClSvcGuestDataSignal(PSHCLCLIENT pClient, PSHCLCLIENTCMDCTX pCmdCtx, SHCLFORMAT uFormat, void *pvData, uint32_t cbData)
{
    LogFlowFuncEnter();
    RT_NOREF(uFormat);

    /*
     * Validate input.
     */
    AssertPtrReturn(pClient, VERR_INVALID_POINTER);
    AssertPtrReturn(pCmdCtx, VERR_INVALID_POINTER);
    if (cbData > 0)
        AssertPtrReturn(pvData, VERR_INVALID_POINTER);

    const SHCLEVENTID idEvent = VBOX_SHCL_CONTEXTID_GET_EVENT(pCmdCtx->uContextID);
    AssertMsgReturn(idEvent != NIL_SHCLEVENTID, ("NIL event in context ID %#RX64\n", pCmdCtx->uContextID), VERR_WRONG_ORDER);

    PSHCLEVENT pEvent = ShClEventSourceGetFromId(&pClient->EventSrc, idEvent);
    AssertMsgReturn(pEvent != NULL, ("Event %#x not found\n", idEvent), VERR_NOT_FOUND);

    /*
     * Make a copy of the data so we can attach it to the signal.
     *
     * Note! We still signal the waiter should we run out of memory,
     *       because otherwise it will be stuck waiting.
     */
    int rc = VINF_SUCCESS;
    PSHCLEVENTPAYLOAD pPayload = NULL;
    if (cbData > 0)
        rc = ShClPayloadAlloc(idEvent, pvData, cbData, &pPayload);

    /*
     * Signal the event.
     */
    int rc2 = ShClEventSignal(pEvent, pPayload);
    if (RT_FAILURE(rc2))
    {
        rc = rc2;
        ShClPayloadFree(pPayload);
        LogRel(("Shared Clipboard: Signalling of guest clipboard data to the host failed: %Rrc\n", rc));
    }

    LogFlowFuncLeaveRC(rc);
    return rc;
}

/**
 * Reports available VBox clipboard formats to the guest.
 *
 * @note    Host backend callers must check if it's active (use
 *          ShClSvcIsBackendActive) before calling to prevent mixing up the
 *          VRDE clipboard.
 *
 * @returns VBox status code.
 * @param   pClient             Client to report clipboard formats to.
 * @param   fFormats            The formats to report (VBOX_SHCL_FMT_XXX), zero
 *                              is okay (empty the clipboard).
 */
int ShClSvcHostReportFormats(PSHCLCLIENT pClient, SHCLFORMATS fFormats)
{
    /*
     * Check if the service mode allows this operation and whether the guest is
     * supposed to be reading from the host. Otherwise, silently ignore reporting
     * formats and return VINF_SUCCESS in order to do not trigger client
     * termination in svcConnect().
     */
    uint32_t uMode = ShClSvcGetMode();
    if (   uMode == VBOX_SHCL_MODE_BIDIRECTIONAL
        || uMode == VBOX_SHCL_MODE_HOST_TO_GUEST)
    { /* likely */ }
    else
        return VINF_SUCCESS;

    AssertPtrReturn(pClient, VERR_INVALID_POINTER);

    LogFlowFunc(("fFormats=%#x\n", fFormats));

#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
    bool fSkipTransfers = false;
    if (!(g_fTransferMode & VBOX_SHCL_TRANSFER_MODE_F_ENABLED))
    {
        LogRel2(("Shared Clipboard: File transfers are disabled, skipping reporting those to the guest\n"));
        fSkipTransfers = true;
    }
    else if (!(pClient->State.fGuestFeatures0 & VBOX_SHCL_GF_0_TRANSFERS))
    {
        LogRel2(("Shared Clipboard: File transfers not supported by installed Guest Addtions, skipping reporting those to the guest\n"));
        fSkipTransfers = true;
    }

    if (fSkipTransfers)
        fFormats &= ~VBOX_SHCL_FMT_URI_LIST;
#endif

#ifdef LOG_ENABLED
    char *pszFmts = ShClFormatsToStrA(fFormats);
    AssertPtrReturn(pszFmts, VERR_NO_MEMORY);
    LogRel2(("Shared Clipboard: Reporting formats '%s' to guest\n", pszFmts));
    RTStrFree(pszFmts);
#endif

    /*
     * Allocate a message, populate parameters and post it to the client.
     */
    int rc;
    PSHCLCLIENTMSG pMsg = shClSvcMsgAlloc(pClient, VBOX_SHCL_HOST_MSG_FORMATS_REPORT, 2);
    if (pMsg)
    {
        HGCMSvcSetU32(&pMsg->aParms[0], VBOX_SHCL_HOST_MSG_FORMATS_REPORT);
        HGCMSvcSetU32(&pMsg->aParms[1], fFormats);

        RTCritSectEnter(&pClient->CritSect);
        shClSvcMsgAddAndWakeupClient(pClient, pMsg);
        RTCritSectLeave(&pClient->CritSect);

#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
        /* Create a transfer locally and also tell the guest to create a transfer on the guest side. */
        if (   !fSkipTransfers
            && fFormats & VBOX_SHCL_FMT_URI_LIST) /* Only start a transfer if we supply an URI list. */
        {
            rc = shClSvcTransferStart(pClient, SHCLTRANSFERDIR_TO_REMOTE, SHCLSOURCE_LOCAL,
                                      NULL /* pTransfer */);
            if (RT_SUCCESS(rc))
                rc = shClSvcSetSource(pClient, SHCLSOURCE_LOCAL);

            if (RT_FAILURE(rc))
                LogRel(("Shared Clipboard: Initializing host write transfer failed with %Rrc\n", rc));
        }
        else
#endif
            rc = VINF_SUCCESS;
    }
    else
        rc = VERR_NO_MEMORY;

    if (RT_FAILURE(rc))
        LogRel(("Shared Clipboard: Reporting formats %#x to guest failed with %Rrc\n", fFormats, rc));

    LogFlowFuncLeaveRC(rc);
    return rc;
}


/**
 * Handles the VBOX_SHCL_GUEST_FN_REPORT_FORMATS message from the guest.
 */
static int shClSvcClientReportFormats(PSHCLCLIENT pClient, uint32_t cParms, VBOXHGCMSVCPARM paParms[])
{
    /*
     * Check if the service mode allows this operation and whether the guest is
     * supposed to be reading from the host.
     */
    uint32_t uMode = ShClSvcGetMode();
    if (   uMode == VBOX_SHCL_MODE_BIDIRECTIONAL
        || uMode == VBOX_SHCL_MODE_GUEST_TO_HOST)
    { /* likely */ }
    else
        return VERR_ACCESS_DENIED;

    /*
     * Digest parameters.
     */
    ASSERT_GUEST_RETURN(   cParms == VBOX_SHCL_CPARMS_REPORT_FORMATS
                        || (   cParms == VBOX_SHCL_CPARMS_REPORT_FORMATS_61B
                            && (pClient->State.fGuestFeatures0 & VBOX_SHCL_GF_0_CONTEXT_ID)),
                        VERR_WRONG_PARAMETER_COUNT);

    uintptr_t iParm = 0;
    if (cParms == VBOX_SHCL_CPARMS_REPORT_FORMATS_61B)
    {
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_64BIT, VERR_WRONG_PARAMETER_TYPE);
        /* no defined value, so just ignore it */
        iParm++;
    }
    ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE);
    uint32_t const fFormats = paParms[iParm].u.uint32;
    iParm++;
    if (cParms == VBOX_SHCL_CPARMS_REPORT_FORMATS_61B)
    {
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE);
        ASSERT_GUEST_RETURN(paParms[iParm].u.uint32 == 0, VERR_INVALID_FLAGS);
        iParm++;
    }
    Assert(iParm == cParms);

    /*
     * Report the formats.
     *
     * We ignore empty reports if the guest isn't the clipboard owner, this
     * prevents a freshly booted guest with an empty clibpoard from clearing
     * the host clipboard on startup.  Likewise, when a guest shutdown it will
     * typically issue an empty report in case it's the owner, we don't want
     * that to clear host content either.
     */
    int rc;
    if (!fFormats && pClient->State.enmSource != SHCLSOURCE_REMOTE)
        rc = VINF_SUCCESS;
    else
    {
        rc = shClSvcSetSource(pClient, SHCLSOURCE_REMOTE);
        if (RT_SUCCESS(rc))
        {
            rc = RTCritSectEnter(&g_CritSect);
            if (RT_SUCCESS(rc))
            {
                if (g_ExtState.pfnExtension)
                {
                    SHCLEXTPARMS parms;
                    RT_ZERO(parms);
                    parms.uFormat = fFormats;

                    g_ExtState.pfnExtension(g_ExtState.pvExtension, VBOX_CLIPBOARD_EXT_FN_FORMAT_ANNOUNCE, &parms, sizeof(parms));
                }
                else
                {
#ifdef LOG_ENABLED
                    char *pszFmts = ShClFormatsToStrA(fFormats);
                    if (pszFmts)
                    {
                        LogRel2(("Shared Clipboard: Guest reported formats '%s' to host\n", pszFmts));
                        RTStrFree(pszFmts);
                    }
#endif
                    rc = ShClBackendReportFormats(&g_ShClBackend, pClient, fFormats);
                    if (RT_FAILURE(rc))
                        LogRel(("Shared Clipboard: Reporting guest clipboard formats to the host failed with %Rrc\n", rc));
                }

                RTCritSectLeave(&g_CritSect);
            }
            else
                LogRel2(("Shared Clipboard: Unable to take internal lock while receiving guest clipboard announcement: %Rrc\n", rc));
        }
    }

    return rc;
}

/**
 * Called when the guest wants to read host clipboard data.
 * Handles the VBOX_SHCL_GUEST_FN_DATA_READ message.
 *
 * @returns VBox status code.
 * @retval  VINF_BUFFER_OVERFLOW if the guest supplied a smaller buffer than needed in order to read the host clipboard data.
 * @param   pClient             Client that wants to read host clipboard data.
 * @param   cParms              Number of HGCM parameters supplied in \a paParms.
 * @param   paParms             Array of HGCM parameters.
 */
static int shClSvcClientReadData(PSHCLCLIENT pClient, uint32_t cParms, VBOXHGCMSVCPARM paParms[])
{
    LogFlowFuncEnter();

    /*
     * Check if the service mode allows this operation and whether the guest is
     * supposed to be reading from the host.
     */
    uint32_t uMode = ShClSvcGetMode();
    if (   uMode == VBOX_SHCL_MODE_BIDIRECTIONAL
        || uMode == VBOX_SHCL_MODE_HOST_TO_GUEST)
    { /* likely */ }
    else
        return VERR_ACCESS_DENIED;

    /*
     * Digest parameters.
     *
     * We are dragging some legacy here from the 6.1 dev cycle, a 5 parameter
     * variant which prepends a 64-bit context ID (RAZ as meaning not defined),
     * a 32-bit flag (MBZ, no defined meaning) and switches the last two parameters.
     */
    ASSERT_GUEST_RETURN(   cParms == VBOX_SHCL_CPARMS_DATA_READ
                        || (    cParms == VBOX_SHCL_CPARMS_DATA_READ_61B
                            &&  (pClient->State.fGuestFeatures0 & VBOX_SHCL_GF_0_CONTEXT_ID)),
                        VERR_WRONG_PARAMETER_COUNT);

    uintptr_t iParm = 0;
    SHCLCLIENTCMDCTX cmdCtx;
    RT_ZERO(cmdCtx);
    if (cParms == VBOX_SHCL_CPARMS_DATA_READ_61B)
    {
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_64BIT, VERR_WRONG_PARAMETER_TYPE);
        /* This has no defined meaning and was never used, however the guest passed stuff, so ignore it and leave idContext=0. */
        iParm++;
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE);
        ASSERT_GUEST_RETURN(paParms[iParm].u.uint32 == 0, VERR_INVALID_FLAGS);
        iParm++;
    }

    SHCLFORMAT  uFormat = VBOX_SHCL_FMT_NONE;
    uint32_t    cbData  = 0;
    void       *pvData  = NULL;

    ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE);
    uFormat = paParms[iParm].u.uint32;
    iParm++;
    if (cParms != VBOX_SHCL_CPARMS_DATA_READ_61B)
    {
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_PTR, VERR_WRONG_PARAMETER_TYPE); /* Data buffer */
        pvData = paParms[iParm].u.pointer.addr;
        cbData = paParms[iParm].u.pointer.size;
        iParm++;
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE); /*cbDataReturned*/
        iParm++;
    }
    else
    {
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE); /*cbDataReturned*/
        iParm++;
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_PTR, VERR_WRONG_PARAMETER_TYPE); /* Data buffer */
        pvData = paParms[iParm].u.pointer.addr;
        cbData = paParms[iParm].u.pointer.size;
        iParm++;
    }
    Assert(iParm == cParms);

    /*
     * For some reason we need to do this (makes absolutely no sense to bird).
     */
    /** @todo r=bird: I really don't get why you need the State.POD.uFormat
     *        member.  I'm sure there is a reason.  Incomplete code? */
    if (!(pClient->State.fGuestFeatures0 & VBOX_SHCL_GF_0_CONTEXT_ID))
    {
        if (pClient->State.POD.uFormat == VBOX_SHCL_FMT_NONE)
            pClient->State.POD.uFormat = uFormat;
    }

#ifdef LOG_ENABLED
    char *pszFmt = ShClFormatsToStrA(uFormat);
    AssertPtrReturn(pszFmt, VERR_NO_MEMORY);
    LogRel2(("Shared Clipboard: Guest wants to read %RU32 bytes host clipboard data in format '%s'\n", cbData, pszFmt));
    RTStrFree(pszFmt);
#endif

    /*
     * Do the reading.
     */
    uint32_t cbActual = 0;

    int rc = RTCritSectEnter(&g_CritSect);
    AssertRCReturn(rc, rc);

    /* If there is a service extension active, try reading data from it first. */
    if (g_ExtState.pfnExtension)
    {
        SHCLEXTPARMS parms;
        RT_ZERO(parms);

        parms.uFormat  = uFormat;
        parms.u.pvData = pvData;
        parms.cbData   = cbData;

        g_ExtState.fReadingData = true;

        /* Read clipboard data from the extension. */
        rc = g_ExtState.pfnExtension(g_ExtState.pvExtension, VBOX_CLIPBOARD_EXT_FN_DATA_READ, &parms, sizeof(parms));

        LogRel2(("Shared Clipboard: Read extension clipboard data (fDelayedAnnouncement=%RTbool, fDelayedFormats=%#x, max %RU32 bytes), got %RU32 bytes: rc=%Rrc\n",
                 g_ExtState.fDelayedAnnouncement, g_ExtState.fDelayedFormats, cbData, parms.cbData, rc));

        /* Did the extension send the clipboard formats yet?
         * Otherwise, do this now. */
        if (g_ExtState.fDelayedAnnouncement)
        {
            int rc2 = ShClSvcHostReportFormats(pClient, g_ExtState.fDelayedFormats);
            AssertRC(rc2);

            g_ExtState.fDelayedAnnouncement = false;
            g_ExtState.fDelayedFormats = 0;
        }

        g_ExtState.fReadingData = false;

        if (RT_SUCCESS(rc))
            cbActual = parms.cbData;
    }
    else
    {
        rc = ShClBackendReadData(&g_ShClBackend, pClient, &cmdCtx, uFormat, pvData, cbData, &cbActual);
        if (RT_SUCCESS(rc))
            LogRel2(("Shared Clipboard: Read host clipboard data (max %RU32 bytes), got %RU32 bytes\n", cbData, cbActual));
        else
            LogRel(("Shared Clipboard: Reading host clipboard data failed with %Rrc\n", rc));
    }

    if (RT_SUCCESS(rc))
    {
        /* Return the actual size required to fullfil the request. */
        if (cParms != VBOX_SHCL_CPARMS_DATA_READ_61B)
            HGCMSvcSetU32(&paParms[2], cbActual);
        else
            HGCMSvcSetU32(&paParms[3], cbActual);

        /* If the data to return exceeds the buffer the guest supplies, tell it (and let it try again). */
        if (cbActual >= cbData)
            rc = VINF_BUFFER_OVERFLOW;
    }

    RTCritSectLeave(&g_CritSect);

    LogFlowFuncLeaveRC(rc);
    return rc;
}

/**
 * Called when the guest writes clipboard data to the host.
 * Handles the VBOX_SHCL_GUEST_FN_DATA_WRITE message.
 *
 * @returns VBox status code.
 * @param   pClient             Client that wants to read host clipboard data.
 * @param   cParms              Number of HGCM parameters supplied in \a paParms.
 * @param   paParms             Array of HGCM parameters.
 */
static int shClSvcClientWriteData(PSHCLCLIENT pClient, uint32_t cParms, VBOXHGCMSVCPARM paParms[])
{
    LogFlowFuncEnter();

    /*
     * Check if the service mode allows this operation and whether the guest is
     * supposed to be reading from the host.
     */
    uint32_t uMode = ShClSvcGetMode();
    if (   uMode == VBOX_SHCL_MODE_BIDIRECTIONAL
        || uMode == VBOX_SHCL_MODE_GUEST_TO_HOST)
    { /* likely */ }
    else
        return VERR_ACCESS_DENIED;

    const bool fReportsContextID = RT_BOOL(pClient->State.fGuestFeatures0 & VBOX_SHCL_GF_0_CONTEXT_ID);

    /*
     * Digest parameters.
     *
     * There are 3 different format here, formatunately no parameters have been
     * switch around so it's plain sailing compared to the DATA_READ message.
     */
    ASSERT_GUEST_RETURN(fReportsContextID
                        ? cParms == VBOX_SHCL_CPARMS_DATA_WRITE || cParms == VBOX_SHCL_CPARMS_DATA_WRITE_61B
                        : cParms == VBOX_SHCL_CPARMS_DATA_WRITE_OLD,
                        VERR_WRONG_PARAMETER_COUNT);

    uintptr_t iParm = 0;
    SHCLCLIENTCMDCTX cmdCtx;
    RT_ZERO(cmdCtx);
    if (cParms > VBOX_SHCL_CPARMS_DATA_WRITE_OLD)
    {
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_64BIT, VERR_WRONG_PARAMETER_TYPE);
        cmdCtx.uContextID = paParms[iParm].u.uint64;
        iParm++;
    }
    else
    {
        /* Older Guest Additions (< 6.1) did not supply a context ID.
         * We dig it out from our saved context ID list then a bit down below. */
    }

    if (cParms == VBOX_SHCL_CPARMS_DATA_WRITE_61B)
    {
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE);
        ASSERT_GUEST_RETURN(paParms[iParm].u.uint32 == 0, VERR_INVALID_FLAGS);
        iParm++;
    }

    SHCLFORMAT  uFormat = VBOX_SHCL_FMT_NONE;
    uint32_t    cbData  = 0;
    void       *pvData  = NULL;

    ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE); /* Format bit. */
    uFormat = paParms[iParm].u.uint32;
    iParm++;
    if (cParms == VBOX_SHCL_CPARMS_DATA_WRITE_61B)
    {
        ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_32BIT, VERR_WRONG_PARAMETER_TYPE); /* "cbData" - duplicates buffer size. */
        iParm++;
    }
    ASSERT_GUEST_RETURN(paParms[iParm].type == VBOX_HGCM_SVC_PARM_PTR, VERR_WRONG_PARAMETER_TYPE); /* Data buffer */
    pvData = paParms[iParm].u.pointer.addr;
    cbData = paParms[iParm].u.pointer.size;
    iParm++;
    Assert(iParm == cParms);

    /*
     * Handle / check context ID.
     */
    if (!fReportsContextID) /* Do we have to deal with old(er) GAs (< 6.1) which don't support context IDs? Dig out the context ID then. */
    {
        PSHCLCLIENTLEGACYCID pCID = NULL;
        PSHCLCLIENTLEGACYCID pCIDIter;
        RTListForEach(&pClient->Legacy.lstCID, pCIDIter, SHCLCLIENTLEGACYCID, Node) /* Slow, but does the job for now. */
        {
            if (pCIDIter->uFormat == uFormat)
            {
                pCID = pCIDIter;
                break;
            }
        }

        ASSERT_GUEST_MSG_RETURN(pCID != NULL, ("Context ID for format %#x not found\n", uFormat), VERR_INVALID_CONTEXT);
        cmdCtx.uContextID = pCID->uCID;

        /* Not needed anymore; clean up. */
        Assert(pClient->Legacy.cCID);
        pClient->Legacy.cCID--;
        RTListNodeRemove(&pCID->Node);
        RTMemFree(pCID);
    }

    uint64_t const idCtxExpected = VBOX_SHCL_CONTEXTID_MAKE(pClient->State.uSessionID, pClient->EventSrc.uID,
                                                            VBOX_SHCL_CONTEXTID_GET_EVENT(cmdCtx.uContextID));
    ASSERT_GUEST_MSG_RETURN(cmdCtx.uContextID == idCtxExpected,
                            ("Wrong context ID: %#RX64, expected %#RX64\n", cmdCtx.uContextID, idCtxExpected),
                            VERR_INVALID_CONTEXT);

    /*
     * For some reason we need to do this (makes absolutely no sense to bird).
     */
    /** @todo r=bird: I really don't get why you need the State.POD.uFormat
     *        member.  I'm sure there is a reason.  Incomplete code? */
    if (!(pClient->State.fGuestFeatures0 & VBOX_SHCL_GF_0_CONTEXT_ID))
    {
        if (pClient->State.POD.uFormat == VBOX_SHCL_FMT_NONE)
            pClient->State.POD.uFormat = uFormat;
    }

#ifdef LOG_ENABLED
    char *pszFmt = ShClFormatsToStrA(uFormat);
    if (pszFmt)
    {
        LogRel2(("Shared Clipboard: Guest writes %RU32 bytes clipboard data in format '%s' to host\n", cbData, pszFmt));
        RTStrFree(pszFmt);
    }
#endif

    /*
     * Write the data to the active host side clipboard.
     */
    int rc = RTCritSectEnter(&g_CritSect);
    AssertRCReturn(rc, rc);

    if (g_ExtState.pfnExtension)
    {
        SHCLEXTPARMS parms;
        RT_ZERO(parms);
        parms.uFormat   = uFormat;
        parms.u.pvData  = pvData;
        parms.cbData    = cbData;

        g_ExtState.pfnExtension(g_ExtState.pvExtension, VBOX_CLIPBOARD_EXT_FN_DATA_WRITE, &parms, sizeof(parms));
        rc = VINF_SUCCESS;
    }
    else
    {
        /* Let the backend implementation know. */
        rc = ShClBackendWriteData(&g_ShClBackend, pClient, &cmdCtx, uFormat, pvData, cbData);
        if (RT_FAILURE(rc))
            LogRel(("Shared Clipboard: Writing guest clipboard data to the host failed with %Rrc\n", rc));

        int rc2; /* Don't return internals back to the guest. */
        rc2 = ShClSvcGuestDataSignal(pClient, &cmdCtx, uFormat, pvData, cbData); /* To complete pending events, if any. */
        if (RT_FAILURE(rc2))
            LogRel(("Shared Clipboard: Signalling host about guest clipboard data failed with %Rrc\n", rc2));
        AssertRC(rc2);
    }

    RTCritSectLeave(&g_CritSect);

    LogFlowFuncLeaveRC(rc);
    return rc;
}

/**
 * Gets an error from HGCM service parameters.
 *
 * @returns VBox status code.
 * @param   cParms              Number of HGCM parameters supplied in \a paParms.
 * @param   paParms             Array of HGCM parameters.
 * @param   pRc                 Where to store the received error code.
 */
static int shClSvcClientError(uint32_t cParms, VBOXHGCMSVCPARM paParms[], int *pRc)
{
    AssertPtrReturn(paParms, VERR_INVALID_PARAMETER);
    AssertPtrReturn(pRc,     VERR_INVALID_PARAMETER);

    int rc;

    if (cParms == VBOX_SHCL_CPARMS_ERROR)
    {
        rc = HGCMSvcGetU32(&paParms[1], (uint32_t *)pRc); /** @todo int vs. uint32_t !!! */
    }
    else
        rc = VERR_INVALID_PARAMETER;

    LogFlowFuncLeaveRC(rc);
    return rc;
}

/**
 * Sets the transfer source type of a Shared Clipboard client.
 *
 * @returns VBox status code.
 * @param   pClient             Client to set transfer source type for.
 * @param   enmSource           Source type to set.
 */
int shClSvcSetSource(PSHCLCLIENT pClient, SHCLSOURCE enmSource)
{
    if (!pClient) /* If no client connected (anymore), bail out. */
        return VINF_SUCCESS;

    int rc = VINF_SUCCESS;

    if (ShClSvcLock())
    {
        pClient->State.enmSource = enmSource;

        LogFlowFunc(("Source of client %RU32 is now %RU32\n", pClient->State.uClientID, pClient->State.enmSource));

        ShClSvcUnlock();
    }

    LogFlowFuncLeaveRC(rc);
    return rc;
}

static int svcInit(VBOXHGCMSVCFNTABLE *pTable)
{
    int rc = RTCritSectInit(&g_CritSect);

    if (RT_SUCCESS(rc))
    {
        shClSvcModeSet(VBOX_SHCL_MODE_OFF);

        rc = ShClBackendInit(ShClSvcGetBackend(), pTable);

        /* Clean up on failure, because 'svnUnload' will not be called
         * if the 'svcInit' returns an error.
         */
        if (RT_FAILURE(rc))
        {
            RTCritSectDelete(&g_CritSect);
        }
    }

    return rc;
}

static DECLCALLBACK(int) svcUnload(void *)
{
    LogFlowFuncEnter();

    ShClBackendDestroy(ShClSvcGetBackend());

    RTCritSectDelete(&g_CritSect);

    return VINF_SUCCESS;
}

static DECLCALLBACK(int) svcDisconnect(void *, uint32_t u32ClientID, void *pvClient)
{
    LogFunc(("u32ClientID=%RU32\n", u32ClientID));

    PSHCLCLIENT pClient = (PSHCLCLIENT)pvClient;
    AssertPtr(pClient);

    /* In order to communicate with guest service, HGCM VRDP clipboard extension
     * needs to know its connection client ID. Currently, in svcConnect() we always
     * cache ID of the first ever connected client. When client disconnects,
     * we need to forget its ID and let svcConnect() to pick up the next ID when a new
     * connection will be requested by guest service (see #10115). */
    if (g_ExtState.uClientID == u32ClientID)
    {
        g_ExtState.uClientID = 0;
    }

#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
    shClSvcClientTransfersReset(pClient);
#endif

    ShClBackendDisconnect(&g_ShClBackend, pClient);

    shClSvcClientDestroy(pClient);

    return VINF_SUCCESS;
}

static DECLCALLBACK(int) svcConnect(void *, uint32_t u32ClientID, void *pvClient, uint32_t fRequestor, bool fRestoring)
{
    RT_NOREF(fRequestor, fRestoring);

    PSHCLCLIENT pClient = (PSHCLCLIENT)pvClient;
    AssertPtr(pvClient);

    int rc = shClSvcClientInit(pClient, u32ClientID);
    if (RT_SUCCESS(rc))
    {
        /* Assign weak pointer to client map. */
        /** @todo r=bird: The g_mapClients is only there for looking up
         *        g_ExtState.uClientID (unserialized btw), so why not use store the
         *        pClient value directly in g_ExtState instead of the ID?  It cannot
         *        crash any worse that racing map insertion/removal. */
        g_mapClients[u32ClientID] = pClient; /** @todo Handle OOM / collisions? */
        rc = ShClBackendConnect(&g_ShClBackend, pClient, ShClSvcGetHeadless());
        if (RT_SUCCESS(rc))
        {
            /* Sync the host clipboard content with the client. */
            rc = ShClBackendSync(&g_ShClBackend, pClient);
            if (RT_SUCCESS(rc))
            {
                /* For now we ASSUME that the first client that connects is in charge for
                   communicating with the service extension. */
                /** @todo This isn't optimal, but only the guest really knows which client is in
                 *        focus on the console. See @bugref{10115} for details. */
                if (g_ExtState.uClientID == 0)
                    g_ExtState.uClientID = u32ClientID;

                /* The sync could return VINF_NO_CHANGE if nothing has changed on the host, but
                   older Guest Additions didn't use RT_SUCCESS to but == VINF_SUCCESS to check for
                   success.  So just return VINF_SUCCESS here to not break older Guest Additions. */
                LogFunc(("Successfully connected client %#x%s\n",
                         u32ClientID, g_ExtState.uClientID == u32ClientID ? " - Use by ExtState too" : ""));
                return VINF_SUCCESS;
            }

            LogFunc(("ShClBackendSync failed: %Rrc\n", rc));
            ShClBackendDisconnect(&g_ShClBackend, pClient);
        }
        else
            LogFunc(("ShClBackendConnect failed: %Rrc\n", rc));
        shClSvcClientDestroy(pClient);
    }
    else
        LogFunc(("shClSvcClientInit failed: %Rrc\n", rc));
    LogFlowFuncLeaveRC(rc);
    return rc;
}

static DECLCALLBACK(void) svcCall(void *,
                                  VBOXHGCMCALLHANDLE callHandle,
                                  uint32_t u32ClientID,
                                  void *pvClient,
                                  uint32_t u32Function,
                                  uint32_t cParms,
                                  VBOXHGCMSVCPARM paParms[],
                                  uint64_t tsArrival)
{
    RT_NOREF(u32ClientID, pvClient, tsArrival);
    PSHCLCLIENT pClient = (PSHCLCLIENT)pvClient;
    AssertPtr(pClient);

#ifdef LOG_ENABLED
    Log2Func(("u32ClientID=%RU32, fn=%RU32 (%s), cParms=%RU32, paParms=%p\n",
              u32ClientID, u32Function, ShClGuestMsgToStr(u32Function), cParms, paParms));
    for (uint32_t i = 0; i < cParms; i++)
    {
        switch (paParms[i].type)
        {
            case VBOX_HGCM_SVC_PARM_32BIT:
                Log3Func(("    paParms[%RU32]: type uint32_t - value %RU32\n", i, paParms[i].u.uint32));
                break;
            case VBOX_HGCM_SVC_PARM_64BIT:
                Log3Func(("    paParms[%RU32]: type uint64_t - value %RU64\n", i, paParms[i].u.uint64));
                break;
            case VBOX_HGCM_SVC_PARM_PTR:
                Log3Func(("    paParms[%RU32]: type ptr - value 0x%p (%RU32 bytes)\n",
                          i, paParms[i].u.pointer.addr, paParms[i].u.pointer.size));
                break;
            case VBOX_HGCM_SVC_PARM_PAGES:
                Log3Func(("    paParms[%RU32]: type pages - cb=%RU32, cPages=%RU16\n",
                          i, paParms[i].u.Pages.cb, paParms[i].u.Pages.cPages));
                break;
            default:
                AssertFailed();
        }
    }
    Log2Func(("Client state: fFlags=0x%x, fGuestFeatures0=0x%x, fGuestFeatures1=0x%x\n",
              pClient->State.fFlags, pClient->State.fGuestFeatures0, pClient->State.fGuestFeatures1));
#endif

    int rc;
    switch (u32Function)
    {
        case VBOX_SHCL_GUEST_FN_MSG_OLD_GET_WAIT:
            RTCritSectEnter(&pClient->CritSect);
            rc = shClSvcClientMsgOldGet(pClient, callHandle, cParms, paParms);
            RTCritSectLeave(&pClient->CritSect);
            break;

        case VBOX_SHCL_GUEST_FN_CONNECT:
            LogRel(("Shared Clipboard: 6.1.0 beta or rc Guest Additions detected. Please upgrade!\n"));
            rc = VERR_NOT_IMPLEMENTED;
            break;

        case VBOX_SHCL_GUEST_FN_NEGOTIATE_CHUNK_SIZE:
            rc = shClSvcClientNegogiateChunkSize(pClient, callHandle, cParms, paParms);
            break;

        case VBOX_SHCL_GUEST_FN_REPORT_FEATURES:
            rc = shClSvcClientReportFeatures(pClient, callHandle, cParms, paParms);
            break;

        case VBOX_SHCL_GUEST_FN_QUERY_FEATURES:
            rc = shClSvcClientQueryFeatures(callHandle, cParms, paParms);
            break;

        case VBOX_SHCL_GUEST_FN_MSG_PEEK_NOWAIT:
            RTCritSectEnter(&pClient->CritSect);
            rc = shClSvcClientMsgPeek(pClient, callHandle, cParms, paParms, false /*fWait*/);
            RTCritSectLeave(&pClient->CritSect);
            break;

        case VBOX_SHCL_GUEST_FN_MSG_PEEK_WAIT:
            RTCritSectEnter(&pClient->CritSect);
            rc = shClSvcClientMsgPeek(pClient, callHandle, cParms, paParms, true /*fWait*/);
            RTCritSectLeave(&pClient->CritSect);
            break;

        case VBOX_SHCL_GUEST_FN_MSG_GET:
            RTCritSectEnter(&pClient->CritSect);
            rc = shClSvcClientMsgGet(pClient, callHandle, cParms, paParms);
            RTCritSectLeave(&pClient->CritSect);
            break;

        case VBOX_SHCL_GUEST_FN_MSG_CANCEL:
            RTCritSectEnter(&pClient->CritSect);
            rc = shClSvcClientMsgCancel(pClient, cParms);
            RTCritSectLeave(&pClient->CritSect);
            break;

        case VBOX_SHCL_GUEST_FN_REPORT_FORMATS:
            rc = shClSvcClientReportFormats(pClient, cParms, paParms);
            break;

        case VBOX_SHCL_GUEST_FN_DATA_READ:
            rc = shClSvcClientReadData(pClient, cParms, paParms);
            break;

        case VBOX_SHCL_GUEST_FN_DATA_WRITE:
            rc = shClSvcClientWriteData(pClient, cParms, paParms);
            break;

        case VBOX_SHCL_GUEST_FN_ERROR:
        {
            int rcGuest;
            rc = shClSvcClientError(cParms,paParms, &rcGuest);
            if (RT_SUCCESS(rc))
            {
                LogRel(("Shared Clipboard: Error reported from guest side: %Rrc\n", rcGuest));

                shClSvcClientLock(pClient);

                /* Reset message queue. */
                shClSvcMsgQueueReset(pClient);

#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
                shClSvcClientTransfersReset(pClient);
#endif
                shClSvcClientUnlock(pClient);
            }
            break;
        }

        default:
        {
#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
            if (   u32Function <= VBOX_SHCL_GUEST_FN_LAST
                && (pClient->State.fGuestFeatures0 &  VBOX_SHCL_GF_0_CONTEXT_ID) )
            {
                if (g_fTransferMode & VBOX_SHCL_TRANSFER_MODE_F_ENABLED)
                    rc = shClSvcTransferHandler(pClient, callHandle, u32Function, cParms, paParms, tsArrival);
                else
                {
                    LogRel2(("Shared Clipboard: File transfers are disabled for this VM\n"));
                    rc = VERR_ACCESS_DENIED;
                }
            }
            else
#endif
            {
                LogRel2(("Shared Clipboard: Unknown guest function: %u (%#x)\n", u32Function, u32Function));
                rc = VERR_NOT_IMPLEMENTED;
            }
            break;
        }
    }

    LogFlowFunc(("[Client %RU32] rc=%Rrc\n", pClient->State.uClientID, rc));

    if (rc != VINF_HGCM_ASYNC_EXECUTE)
        g_pHelpers->pfnCallComplete(callHandle, rc);
}

/**
 * Initializes a Shared Clipboard service's client state.
 *
 * @returns VBox status code.
 * @param   pClientState        Client state to initialize.
 * @param   uClientID           Client ID (HGCM) to use for this client state.
 */
int shClSvcClientStateInit(PSHCLCLIENTSTATE pClientState, uint32_t uClientID)
{
    LogFlowFuncEnter();

    shclSvcClientStateReset(pClientState);

    /* Register the client. */
    pClientState->uClientID    = uClientID;

    return VINF_SUCCESS;
}

/**
 * Destroys a Shared Clipboard service's client state.
 *
 * @returns VBox status code.
 * @param   pClientState        Client state to destroy.
 */
int shClSvcClientStateDestroy(PSHCLCLIENTSTATE pClientState)
{
    RT_NOREF(pClientState);

    LogFlowFuncEnter();

    return VINF_SUCCESS;
}

/**
 * Resets a Shared Clipboard service's client state.
 *
 * @param   pClientState    Client state to reset.
 */
void shclSvcClientStateReset(PSHCLCLIENTSTATE pClientState)
{
    LogFlowFuncEnter();

    pClientState->fGuestFeatures0 = VBOX_SHCL_GF_NONE;
    pClientState->fGuestFeatures1 = VBOX_SHCL_GF_NONE;

    pClientState->cbChunkSize     = VBOX_SHCL_DEFAULT_CHUNK_SIZE; /** @todo Make this configurable. */
    pClientState->enmSource       = SHCLSOURCE_INVALID;
    pClientState->fFlags          = SHCLCLIENTSTATE_FLAGS_NONE;

    pClientState->POD.enmDir             = SHCLTRANSFERDIR_UNKNOWN;
    pClientState->POD.uFormat            = VBOX_SHCL_FMT_NONE;
    pClientState->POD.cbToReadWriteTotal = 0;
    pClientState->POD.cbReadWritten      = 0;

#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
    pClientState->Transfers.enmTransferDir = SHCLTRANSFERDIR_UNKNOWN;
#endif
}

/*
 * We differentiate between a function handler for the guest and one for the host.
 */
static DECLCALLBACK(int) svcHostCall(void *,
                                     uint32_t u32Function,
                                     uint32_t cParms,
                                     VBOXHGCMSVCPARM paParms[])
{
    int rc = VINF_SUCCESS;

    LogFlowFunc(("u32Function=%RU32 (%s), cParms=%RU32, paParms=%p\n",
                 u32Function, ShClHostFunctionToStr(u32Function), cParms, paParms));

    switch (u32Function)
    {
        case VBOX_SHCL_HOST_FN_SET_MODE:
        {
            if (cParms != 1)
            {
                rc = VERR_INVALID_PARAMETER;
            }
            else
            {
                uint32_t u32Mode = VBOX_SHCL_MODE_OFF;

                rc = HGCMSvcGetU32(&paParms[0], &u32Mode);
                if (RT_SUCCESS(rc))
                    rc = shClSvcModeSet(u32Mode);
            }

            break;
        }

#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
        case VBOX_SHCL_HOST_FN_SET_TRANSFER_MODE:
        {
            if (cParms != 1)
            {
                rc = VERR_INVALID_PARAMETER;
            }
            else
            {
                uint32_t fTransferMode;
                rc = HGCMSvcGetU32(&paParms[0], &fTransferMode);
                if (RT_SUCCESS(rc))
                    rc = shClSvcTransferModeSet(fTransferMode);
            }
            break;
        }
#endif
        case VBOX_SHCL_HOST_FN_SET_HEADLESS:
        {
            if (cParms != 1)
            {
                rc = VERR_INVALID_PARAMETER;
            }
            else
            {
                uint32_t uHeadless;
                rc = HGCMSvcGetU32(&paParms[0], &uHeadless);
                if (RT_SUCCESS(rc))
                {
                    g_fHeadless = RT_BOOL(uHeadless);
                    LogRel(("Shared Clipboard: Service running in %s mode\n", g_fHeadless ? "headless" : "normal"));
                }
            }
            break;
        }

        default:
        {
#ifdef VBOX_WITH_SHARED_CLIPBOARD_TRANSFERS
            rc = shClSvcTransferHostHandler(u32Function, cParms, paParms);
#else
            rc = VERR_NOT_IMPLEMENTED;
#endif
            break;
        }
    }

    LogFlowFuncLeaveRC(rc);
    return rc;
}

#ifndef UNIT_TEST

/**
 * SSM descriptor table for the SHCLCLIENTLEGACYCID structure.
 *
 * @note Saving the ListEntry attribute is not necessary, as this gets used on runtime only.
 */
static SSMFIELD const s_aShClSSMClientLegacyCID[] =
{
    SSMFIELD_ENTRY(SHCLCLIENTLEGACYCID, uCID),
    SSMFIELD_ENTRY(SHCLCLIENTLEGACYCID, enmType),
    SSMFIELD_ENTRY(SHCLCLIENTLEGACYCID, uFormat),
    SSMFIELD_ENTRY_TERM()
};

/**
 * SSM descriptor table for the SHCLCLIENTSTATE structure.
 *
 * @note Saving the session ID not necessary, as they're not persistent across
 *       state save/restore.
 */
static SSMFIELD const s_aShClSSMClientState[] =
{
    SSMFIELD_ENTRY(SHCLCLIENTSTATE, fGuestFeatures0),
    SSMFIELD_ENTRY(SHCLCLIENTSTATE, fGuestFeatures1),
    SSMFIELD_ENTRY(SHCLCLIENTSTATE, cbChunkSize),
    SSMFIELD_ENTRY(SHCLCLIENTSTATE, enmSource),
    SSMFIELD_ENTRY(SHCLCLIENTSTATE, fFlags),
    SSMFIELD_ENTRY_TERM()
};

/**
 * VBox 6.1 Beta 1 version of s_aShClSSMClientState (no flags).
 */
static SSMFIELD const s_aShClSSMClientState61B1[] =
{
    SSMFIELD_ENTRY(SHCLCLIENTSTATE, fGuestFeatures0),
    SSMFIELD_ENTRY(SHCLCLIENTSTATE, fGuestFeatures1),
    SSMFIELD_ENTRY(SHCLCLIENTSTATE, cbChunkSize),
    SSMFIELD_ENTRY(SHCLCLIENTSTATE, enmSource),
    SSMFIELD_ENTRY_TERM()
};

/**
 * SSM descriptor table for the SHCLCLIENTPODSTATE structure.
 */
static SSMFIELD const s_aShClSSMClientPODState[] =
{
    SSMFIELD_ENTRY(SHCLCLIENTPODSTATE, enmDir),
    SSMFIELD_ENTRY(SHCLCLIENTPODSTATE, uFormat),
    SSMFIELD_ENTRY(SHCLCLIENTPODSTATE, cbToReadWriteTotal),
    SSMFIELD_ENTRY(SHCLCLIENTPODSTATE, cbReadWritten),
    SSMFIELD_ENTRY(SHCLCLIENTPODSTATE, tsLastReadWrittenMs),
    SSMFIELD_ENTRY_TERM()
};

/**
 * SSM descriptor table for the SHCLCLIENTURISTATE structure.
 */
static SSMFIELD const s_aShClSSMClientTransferState[] =
{
    SSMFIELD_ENTRY(SHCLCLIENTTRANSFERSTATE, enmTransferDir),
    SSMFIELD_ENTRY_TERM()
};

/**
 * SSM descriptor table for the header of the SHCLCLIENTMSG structure.
 * The actual message parameters will be serialized separately.
 */
static SSMFIELD const s_aShClSSMClientMsgHdr[] =
{
    SSMFIELD_ENTRY(SHCLCLIENTMSG, idMsg),
    SSMFIELD_ENTRY(SHCLCLIENTMSG, cParms),
    SSMFIELD_ENTRY_TERM()
};

/**
 * SSM descriptor table for what used to be the VBOXSHCLMSGCTX structure but is
 * now part of SHCLCLIENTMSG.
 */
static SSMFIELD const s_aShClSSMClientMsgCtx[] =
{
    SSMFIELD_ENTRY(SHCLCLIENTMSG, idCtx),
    SSMFIELD_ENTRY_TERM()
};
#endif /* !UNIT_TEST */

static DECLCALLBACK(int) svcSaveState(void *, uint32_t u32ClientID, void *pvClient, PSSMHANDLE pSSM, PCVMMR3VTABLE pVMM)
{
    LogFlowFuncEnter();

#ifndef UNIT_TEST
    /*
     * When the state will be restored, pending requests will be reissued
     * by VMMDev. The service therefore must save state as if there were no
     * pending request.
     * Pending requests, if any, will be completed in svcDisconnect.
     */
    RT_NOREF(u32ClientID);
    LogFunc(("u32ClientID=%RU32\n", u32ClientID));

    PSHCLCLIENT pClient = (PSHCLCLIENT)pvClient;
    AssertPtr(pClient);

    /* Write Shared Clipboard saved state version. */
    pVMM->pfnSSMR3PutU32(pSSM, VBOX_SHCL_SAVED_STATE_VER_CURRENT);

    int rc = pVMM->pfnSSMR3PutStructEx(pSSM, &pClient->State, sizeof(pClient->State), 0 /*fFlags*/, &s_aShClSSMClientState[0], NULL);
    AssertRCReturn(rc, rc);

    rc = pVMM->pfnSSMR3PutStructEx(pSSM, &pClient->State.POD, sizeof(pClient->State.POD), 0 /*fFlags*/, &s_aShClSSMClientPODState[0], NULL);
    AssertRCReturn(rc, rc);

    rc = pVMM->pfnSSMR3PutStructEx(pSSM, &pClient->State.Transfers, sizeof(pClient->State.Transfers), 0 /*fFlags*/, &s_aShClSSMClientTransferState[0], NULL);
    AssertRCReturn(rc, rc);

    /* Serialize the client's internal message queue. */
    rc = pVMM->pfnSSMR3PutU64(pSSM, pClient->cMsgAllocated);
    AssertRCReturn(rc, rc);

    PSHCLCLIENTMSG pMsg;
    RTListForEach(&pClient->MsgQueue, pMsg, SHCLCLIENTMSG, ListEntry)
    {
        pVMM->pfnSSMR3PutStructEx(pSSM, pMsg, sizeof(SHCLCLIENTMSG), 0 /*fFlags*/, &s_aShClSSMClientMsgHdr[0], NULL);
        pVMM->pfnSSMR3PutStructEx(pSSM, pMsg, sizeof(SHCLCLIENTMSG), 0 /*fFlags*/, &s_aShClSSMClientMsgCtx[0], NULL);

        for (uint32_t iParm = 0; iParm < pMsg->cParms; iParm++)
            HGCMSvcSSMR3Put(&pMsg->aParms[iParm], pSSM, pVMM);
    }

    rc = pVMM->pfnSSMR3PutU64(pSSM, pClient->Legacy.cCID);
    AssertRCReturn(rc, rc);

    PSHCLCLIENTLEGACYCID pCID;
    RTListForEach(&pClient->Legacy.lstCID, pCID, SHCLCLIENTLEGACYCID, Node)
    {
        rc = pVMM->pfnSSMR3PutStructEx(pSSM, pCID, sizeof(SHCLCLIENTLEGACYCID), 0 /*fFlags*/, &s_aShClSSMClientLegacyCID[0], NULL);
        AssertRCReturn(rc, rc);
    }
#else  /* UNIT_TEST */
    RT_NOREF(u32ClientID, pvClient, pSSM, pVMM);
#endif /* UNIT_TEST */
    return VINF_SUCCESS;
}

#ifndef UNIT_TEST
static int svcLoadStateV0(uint32_t u32ClientID, void *pvClient, PSSMHANDLE pSSM, PCVMMR3VTABLE pVMM, uint32_t uVersion)
{
    RT_NOREF(u32ClientID, pvClient, pSSM, uVersion);

    uint32_t uMarker;
    int rc = pVMM->pfnSSMR3GetU32(pSSM, &uMarker);   /* Begin marker. */
    AssertRC(rc);
    Assert(uMarker == UINT32_C(0x19200102)  /* SSMR3STRUCT_BEGIN */);

    rc = pVMM->pfnSSMR3Skip(pSSM, sizeof(uint32_t)); /* Client ID */
    AssertRCReturn(rc, rc);

    bool fValue;
    rc = pVMM->pfnSSMR3GetBool(pSSM, &fValue);       /* fHostMsgQuit */
    AssertRCReturn(rc, rc);

    rc = pVMM->pfnSSMR3GetBool(pSSM, &fValue);       /* fHostMsgReadData */
    AssertRCReturn(rc, rc);

    rc = pVMM->pfnSSMR3GetBool(pSSM, &fValue);       /* fHostMsgFormats */
    AssertRCReturn(rc, rc);

    uint32_t fFormats;
    rc = pVMM->pfnSSMR3GetU32(pSSM, &fFormats);      /* u32RequestedFormat */
    AssertRCReturn(rc, rc);

    rc = pVMM->pfnSSMR3GetU32(pSSM, &uMarker);       /* End marker. */
    AssertRCReturn(rc, rc);
    Assert(uMarker == UINT32_C(0x19920406) /* SSMR3STRUCT_END */);

    return VINF_SUCCESS;
}
#endif /* UNIT_TEST */

static DECLCALLBACK(int) svcLoadState(void *, uint32_t u32ClientID, void *pvClient,
                                      PSSMHANDLE pSSM, PCVMMR3VTABLE pVMM, uint32_t uVersion)
{
    LogFlowFuncEnter();

#ifndef UNIT_TEST

    RT_NOREF(u32ClientID, uVersion);

    PSHCLCLIENT pClient = (PSHCLCLIENT)pvClient;
    AssertPtr(pClient);

    /* Restore the client data. */
    uint32_t lenOrVer;
    int rc = pVMM->pfnSSMR3GetU32(pSSM, &lenOrVer);
    AssertRCReturn(rc, rc);

    LogFunc(("u32ClientID=%RU32, lenOrVer=%#RX64\n", u32ClientID, lenOrVer));

    if (lenOrVer == VBOX_SHCL_SAVED_STATE_VER_3_1)
        return svcLoadStateV0(u32ClientID, pvClient, pSSM, pVMM, uVersion);

    if (   lenOrVer >= VBOX_SHCL_SAVED_STATE_VER_6_1B2
        && lenOrVer <= VBOX_SHCL_SAVED_STATE_VER_CURRENT)
    {
        if (lenOrVer >= VBOX_SHCL_SAVED_STATE_VER_6_1RC1)
        {
            pVMM->pfnSSMR3GetStructEx(pSSM, &pClient->State, sizeof(pClient->State), 0 /* fFlags */,
                                      &s_aShClSSMClientState[0], NULL);
            pVMM->pfnSSMR3GetStructEx(pSSM, &pClient->State.POD, sizeof(pClient->State.POD), 0 /* fFlags */,
                                      &s_aShClSSMClientPODState[0], NULL);
        }
        else
            pVMM->pfnSSMR3GetStructEx(pSSM, &pClient->State, sizeof(pClient->State), 0 /* fFlags */,
                                      &s_aShClSSMClientState61B1[0], NULL);
        rc = pVMM->pfnSSMR3GetStructEx(pSSM, &pClient->State.Transfers, sizeof(pClient->State.Transfers), 0 /* fFlags */,
                                       &s_aShClSSMClientTransferState[0], NULL);
        AssertRCReturn(rc, rc);

        /* Load the client's internal message queue. */
        uint64_t cMsgs;
        rc = pVMM->pfnSSMR3GetU64(pSSM, &cMsgs);
        AssertRCReturn(rc, rc);
        AssertLogRelMsgReturn(cMsgs < _16K, ("Too many messages: %u (%x)\n", cMsgs, cMsgs), VERR_SSM_DATA_UNIT_FORMAT_CHANGED);

        for (uint64_t i = 0; i < cMsgs; i++)
        {
            union
            {
                SHCLCLIENTMSG Msg;
                uint8_t abPadding[RT_UOFFSETOF(SHCLCLIENTMSG, aParms) + sizeof(VBOXHGCMSVCPARM) * 2];
            } u;

            pVMM->pfnSSMR3GetStructEx(pSSM, &u.Msg, RT_UOFFSETOF(SHCLCLIENTMSG, aParms), 0 /*fFlags*/,
                                      &s_aShClSSMClientMsgHdr[0], NULL);
            rc = pVMM->pfnSSMR3GetStructEx(pSSM, &u.Msg, RT_UOFFSETOF(SHCLCLIENTMSG, aParms), 0 /*fFlags*/,
                                           &s_aShClSSMClientMsgCtx[0], NULL);
            AssertRCReturn(rc, rc);

            AssertLogRelMsgReturn(u.Msg.cParms <= VMMDEV_MAX_HGCM_PARMS,
                                  ("Too many HGCM message parameters: %u (%#x)\n", u.Msg.cParms, u.Msg.cParms),
                                  VERR_SSM_DATA_UNIT_FORMAT_CHANGED);

            PSHCLCLIENTMSG pMsg = shClSvcMsgAlloc(pClient, u.Msg.idMsg, u.Msg.cParms);
            AssertReturn(pMsg, VERR_NO_MEMORY);
            pMsg->idCtx = u.Msg.idCtx;

            for (uint32_t p = 0; p < pMsg->cParms; p++)
            {
                rc = HGCMSvcSSMR3Get(&pMsg->aParms[p], pSSM, pVMM);
                AssertRCReturnStmt(rc, shClSvcMsgFree(pClient, pMsg), rc);
            }

            RTCritSectEnter(&pClient->CritSect);
            shClSvcMsgAdd(pClient, pMsg, true /* fAppend */);
            RTCritSectLeave(&pClient->CritSect);
        }

        if (lenOrVer >= VBOX_SHCL_SAVED_STATE_LEGACY_CID)
        {
            uint64_t cCID;
            rc = pVMM->pfnSSMR3GetU64(pSSM, &cCID);
            AssertRCReturn(rc, rc);
            AssertLogRelMsgReturn(cCID < _16K, ("Too many context IDs: %u (%x)\n", cCID, cCID), VERR_SSM_DATA_UNIT_FORMAT_CHANGED);

            for (uint64_t i = 0; i < cCID; i++)
            {
                PSHCLCLIENTLEGACYCID pCID = (PSHCLCLIENTLEGACYCID)RTMemAlloc(sizeof(SHCLCLIENTLEGACYCID));
                AssertPtrReturn(pCID, VERR_NO_MEMORY);

                pVMM->pfnSSMR3GetStructEx(pSSM, pCID, sizeof(SHCLCLIENTLEGACYCID), 0 /* fFlags */,
                                          &s_aShClSSMClientLegacyCID[0], NULL);
                RTListAppend(&pClient->Legacy.lstCID, &pCID->Node);
            }
        }
    }
    else
    {
        LogRel(("Shared Clipboard: Unsupported saved state version (%#x)\n", lenOrVer));
        return VERR_SSM_DATA_UNIT_FORMAT_CHANGED;
    }

    /* Actual host data are to be reported to guest (SYNC). */
    ShClBackendSync(&g_ShClBackend, pClient);

#else  /* UNIT_TEST */
    RT_NOREF(u32ClientID, pvClient, pSSM, pVMM, uVersion);
#endif /* UNIT_TEST */
    return VINF_SUCCESS;
}

static DECLCALLBACK(int) extCallback(uint32_t u32Function, uint32_t u32Format, void *pvData, uint32_t cbData)
{
    RT_NOREF(pvData, cbData);

    LogFlowFunc(("u32Function=%RU32\n", u32Function));

    int rc = VINF_SUCCESS;

    /* Figure out if the client in charge for the service extension still is connected. */
    ClipboardClientMap::const_iterator itClient = g_mapClients.find(g_ExtState.uClientID);
    if (itClient != g_mapClients.end())
    {
        PSHCLCLIENT pClient = itClient->second;
        AssertPtr(pClient);

        switch (u32Function)
        {
            /* The service extension announces formats to the guest. */
            case VBOX_CLIPBOARD_EXT_FN_FORMAT_ANNOUNCE:
            {
                LogFlowFunc(("VBOX_CLIPBOARD_EXT_FN_FORMAT_ANNOUNCE: g_ExtState.fReadingData=%RTbool\n", g_ExtState.fReadingData));
                if (!g_ExtState.fReadingData)
                    rc = ShClSvcHostReportFormats(pClient, u32Format);
                else
                {
                    g_ExtState.fDelayedAnnouncement = true;
                    g_ExtState.fDelayedFormats = u32Format;
                    rc = VINF_SUCCESS;
                }
                break;
            }

            /* The service extension wants read data from the guest. */
            case VBOX_CLIPBOARD_EXT_FN_DATA_READ:
                rc = ShClSvcGuestDataRequest(pClient, u32Format, NULL /* pidEvent */);
                break;

            default:
                /* Just skip other messages. */
                break;
        }
    }
    else
        rc = VERR_NOT_FOUND;

    LogFlowFuncLeaveRC(rc);
    return rc;
}

static DECLCALLBACK(int) svcRegisterExtension(void *, PFNHGCMSVCEXT pfnExtension, void *pvExtension)
{
    LogFlowFunc(("pfnExtension=%p\n", pfnExtension));

    SHCLEXTPARMS parms;
    RT_ZERO(parms);

    /*
     * Reference counting for service extension registration is done a few
     * layers up (in ConsoleVRDPServer::ClipboardCreate()).
     */

    int rc = RTCritSectEnter(&g_CritSect);
    AssertLogRelRCReturn(rc, rc);

    if (pfnExtension)
    {
        /* Install extension. */
        g_ExtState.pfnExtension = pfnExtension;
        g_ExtState.pvExtension  = pvExtension;

        parms.u.pfnCallback = extCallback;
        g_ExtState.pfnExtension(g_ExtState.pvExtension, VBOX_CLIPBOARD_EXT_FN_SET_CALLBACK, &parms, sizeof(parms));

        LogRel2(("Shared Clipboard: registered service extension\n"));
    }
    else
    {
        if (g_ExtState.pfnExtension)
            g_ExtState.pfnExtension(g_ExtState.pvExtension, VBOX_CLIPBOARD_EXT_FN_SET_CALLBACK, &parms, sizeof(parms));

        /* Uninstall extension. */
        g_ExtState.pvExtension  = NULL;
        g_ExtState.pfnExtension = NULL;

        LogRel2(("Shared Clipboard: de-registered service extension\n"));
    }

    RTCritSectLeave(&g_CritSect);

    return VINF_SUCCESS;
}

extern "C" DECLCALLBACK(DECLEXPORT(int)) VBoxHGCMSvcLoad(VBOXHGCMSVCFNTABLE *pTable)
{
    int rc = VINF_SUCCESS;

    LogFlowFunc(("pTable=%p\n", pTable));

    if (!RT_VALID_PTR(pTable))
    {
        rc = VERR_INVALID_PARAMETER;
    }
    else
    {
        LogFunc(("pTable->cbSize = %d, ptable->u32Version = 0x%08X\n", pTable->cbSize, pTable->u32Version));

        if (   pTable->cbSize     != sizeof (VBOXHGCMSVCFNTABLE)
            || pTable->u32Version != VBOX_HGCM_SVC_VERSION)
        {
            rc = VERR_VERSION_MISMATCH;
        }
        else
        {
            g_pHelpers = pTable->pHelpers;

            pTable->cbClient = sizeof(SHCLCLIENT);

            /* Map legacy clients to root. */
            pTable->idxLegacyClientCategory = HGCM_CLIENT_CATEGORY_ROOT;

            /* Limit the number of clients to 128 in each category (should be enough),
               but set kernel clients to 1. */
            for (uintptr_t i = 0; i < RT_ELEMENTS(pTable->acMaxClients); i++)
                pTable->acMaxClients[i] = 128;
            pTable->acMaxClients[HGCM_CLIENT_CATEGORY_KERNEL] = 1;

            /* Only 16 pending calls per client (1 should be enough). */
            for (uintptr_t i = 0; i < RT_ELEMENTS(pTable->acMaxClients); i++)
                pTable->acMaxCallsPerClient[i] = 16;

            pTable->pfnUnload            = svcUnload;
            pTable->pfnConnect           = svcConnect;
            pTable->pfnDisconnect        = svcDisconnect;
            pTable->pfnCall              = svcCall;
            pTable->pfnHostCall          = svcHostCall;
            pTable->pfnSaveState         = svcSaveState;
            pTable->pfnLoadState         = svcLoadState;
            pTable->pfnRegisterExtension = svcRegisterExtension;
            pTable->pfnNotify            = NULL;
            pTable->pvService            = NULL;

            /* Service specific initialization. */
            rc = svcInit(pTable);
        }
    }

    LogFlowFunc(("Returning %Rrc\n", rc));
    return rc;
}