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diff --git a/src/VBox/Main/include/ClientWatcher.h b/src/VBox/Main/include/ClientWatcher.h
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+++ b/src/VBox/Main/include/ClientWatcher.h
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+/* $Id: ClientWatcher.h $ */
+/** @file
+ * VirtualBox API client session watcher
+ */
+
+/*
+ * Copyright (C) 2013-2022 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
+ */
+
+#ifndef MAIN_INCLUDED_ClientWatcher_h
+#define MAIN_INCLUDED_ClientWatcher_h
+#ifndef RT_WITHOUT_PRAGMA_ONCE
+# pragma once
+#endif
+
+
+#include <list>
+#include <VBox/com/ptr.h>
+#include <VBox/com/AutoLock.h>
+
+#include "VirtualBoxImpl.h"
+
+#if defined(RT_OS_WINDOWS)
+# define CWUPDATEREQARG NULL
+# define CWUPDATEREQTYPE HANDLE
+# define CW_MAX_CLIENTS  _16K            /**< Max number of clients we can watch (windows). */
+# ifndef DEBUG /* The debug version triggers worker thread code much much earlier. */
+#  define CW_MAX_CLIENTS_PER_THREAD 63   /**< Max clients per watcher thread (windows). */
+# else
+#  define CW_MAX_CLIENTS_PER_THREAD 3    /**< Max clients per watcher thread (windows). */
+# endif
+# define CW_MAX_HANDLES_PER_THREAD (CW_MAX_CLIENTS_PER_THREAD + 1) /**< Max handles per thread. */
+
+#elif defined(RT_OS_OS2)
+# define CWUPDATEREQARG NIL_RTSEMEVENT
+# define CWUPDATEREQTYPE RTSEMEVENT
+
+#elif defined(VBOX_WITH_SYS_V_IPC_SESSION_WATCHER) || defined(VBOX_WITH_GENERIC_SESSION_WATCHER)
+# define CWUPDATEREQARG NIL_RTSEMEVENT
+# define CWUPDATEREQTYPE RTSEMEVENT
+
+#else
+# error "Port me!"
+#endif
+
+/**
+ * Class which checks for API clients which have crashed/exited, and takes
+ * the necessary cleanup actions. Singleton.
+ */
+class VirtualBox::ClientWatcher
+{
+public:
+    /**
+     * Constructor which creates a usable instance
+     *
+     * @param pVirtualBox   Reference to VirtualBox object
+     */
+    ClientWatcher(const ComObjPtr<VirtualBox> &pVirtualBox);
+
+    /**
+     * Default destructor. Cleans everything up.
+     */
+    ~ClientWatcher();
+
+    bool isReady();
+
+    void update();
+    void addProcess(RTPROCESS pid);
+
+private:
+    /**
+     * Default constructor. Don't use, will not create a sensible instance.
+     */
+    ClientWatcher();
+
+    static DECLCALLBACK(int) worker(RTTHREAD hThreadSelf, void *pvUser);
+    uint32_t reapProcesses(void);
+
+    VirtualBox *mVirtualBox;
+    RTTHREAD mThread;
+    CWUPDATEREQTYPE mUpdateReq;
+    util::RWLockHandle mLock;
+
+    typedef std::list<RTPROCESS> ProcessList;
+    ProcessList mProcesses;
+
+#if defined(VBOX_WITH_SYS_V_IPC_SESSION_WATCHER) || defined(VBOX_WITH_GENERIC_SESSION_WATCHER)
+    uint8_t mUpdateAdaptCtr;
+#endif
+#ifdef RT_OS_WINDOWS
+    /** Indicate a real update request is pending.
+     * To avoid race conditions this must be set before mUpdateReq is signalled and
+     * read after resetting mUpdateReq. */
+    volatile bool mfUpdateReq;
+    /** Set when the worker threads are supposed to shut down. */
+    volatile bool mfTerminate;
+    /** Number of active subworkers.
+     * When decremented to 0, subworker zero is signalled. */
+    uint32_t volatile mcActiveSubworkers;
+    /** Number of valid handles in mahWaitHandles. */
+    uint32_t    mcWaitHandles;
+    /** The wait interval (usually INFINITE). */
+    uint32_t    mcMsWait;
+    /** Per subworker data. Subworker 0 is the main worker and does not have a
+     *  pReq pointer since. */
+    struct PerSubworker
+    {
+        /** The wait result. */
+        DWORD                       dwWait;
+        /** The subworker index. */
+        uint32_t                    iSubworker;
+        /** The subworker thread handle. */
+        RTTHREAD                    hThread;
+        /** Self pointer (for worker thread). */
+        VirtualBox::ClientWatcher  *pSelf;
+    } maSubworkers[(CW_MAX_CLIENTS + CW_MAX_CLIENTS_PER_THREAD - 1) / CW_MAX_CLIENTS_PER_THREAD];
+    /** Wait handle array. The mUpdateReq manual reset event handle is inserted
+     * every 64 entries, first entry being 0. */
+    HANDLE      mahWaitHandles[CW_MAX_CLIENTS + (CW_MAX_CLIENTS + CW_MAX_CLIENTS_PER_THREAD - 1) / CW_MAX_CLIENTS_PER_THREAD];
+
+    void subworkerWait(VirtualBox::ClientWatcher::PerSubworker *pSubworker, uint32_t cMsWait);
+    static DECLCALLBACK(int) subworkerThread(RTTHREAD hThreadSelf, void *pvUser);
+    void winResetHandleArray(uint32_t cProcHandles);
+#endif
+};
+
+#endif /* !MAIN_INCLUDED_ClientWatcher_h */
+/* vi: set tabstop=4 shiftwidth=4 expandtab: */