xenadmin/XenModel/Actions/CancellingAction.cs

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using System;
using System.Threading;
using XenAdmin.Network;
using XenAPI;

namespace XenAdmin.Actions
{
    public class CancellingAction : ActionBase
    {
        private static readonly log4net.ILog log = log4net.LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);

        private Session _cancel_session = null;
        private Session _session;

        /// <summary>
        /// Whether, the last time we checked on the server, this task could be cancelled.  This is a cached
        /// value, so may be stale.
        /// </summary>
        private volatile bool can_cancel = false;

        /// <summary>
        /// Whether this operation is being cancelled.  This takes precedence over can_cancel.
        /// </summary>
        private volatile bool cancelling = false;

        /// <summary>
        /// Whether this operation was cancelled or not.
        /// </summary>
        private volatile bool cancelled = false;

        protected CancellingAction(string title, string description, bool suppressHistory) 
            : base(title, description, suppressHistory)
        {
        }

        protected CancellingAction(string title, string description, bool suppressHistory, bool completeImmediately)
            : base(title, description, suppressHistory, completeImmediately)
        {
        }

        private XenRef<Task> _relatedTask;

        private readonly object connectionLock = new object();

        public sealed override IXenConnection Connection
        {
            protected set
            {
                // lock any changes to the connection
                // This only affect actions which run across multiple connections
                lock (connectionLock)
                {
                    base.Connection = value;
                }
            }
        }

        public Session Session
        {
            get { return _session; }
            set { _session = value; }
        }

        /// <summary>
        /// The XenAPI.Task object (if any) that corresponds to this action.
        /// </summary>
        public XenRef<Task> RelatedTask
        {
            get { return _relatedTask; }
            set
            {
                //Program.AssertOffEventThread();
                _relatedTask = value;
                if (_relatedTask != null && _session != null)
                {
                    Task.SetXenCenterUUID(_session, _relatedTask.opaque_ref, XenAdminConfigManager.Provider.XenCenterUUID);
                    Task.SetAppliesTo(_session, _relatedTask.opaque_ref, AppliesTo);
                    Task.SetMeddlingActionTitle(_session, _relatedTask.opaque_ref, Title);
                    RecomputeCanCancel();
                }
            }
        }

        public override sealed bool CanCancel
        {
            get { return !cancelling && can_cancel; }
            protected set
            {
                if (can_cancel != value)
                {
                    can_cancel = value;
                    OnChanged();
                }
            }
        }

        public bool Cancelling
        {
            get { return cancelling; }
            protected set { cancelling = value; }
        }

        // You can't refer to property getters in order to treat them as a delegate, so this is
        // a substitute.
        public bool GetCancelling()
        {
            return cancelling;
        }

        public bool Cancelled
        {
            get { return cancelled; }
            protected set { cancelled = value; }
        }

        /// <summary>
        /// Check again whether this task may be cancelled.  Must be called from off the event thread,
        /// whereas CanCancel is called on it.
        /// </summary>
        public virtual void RecomputeCanCancel()
        {
            //Program.AssertOffEventThread();

            try
            {
                XenRef<Task> task = _relatedTask;
                if (task == null)
                {
                    can_cancel = false;
                    return;
                }

                Session local_session = GetCancelSession();
                if (local_session == null || string.IsNullOrEmpty(local_session.uuid))
                {
                    can_cancel = false;
                    return;
                }
                CanCancel = Task.get_allowed_operations(local_session, task.opaque_ref).Contains(task_allowed_operations.cancel);
            }
            catch (Exception exn)
            {
                log.Error(exn, exn);
                LogoutCancelSession();
                can_cancel = false;
            }
        }

        /// <summary>
        /// Will return null if Connection is null.
        /// </summary>
        /// <returns></returns>
        protected Session GetCancelSession()
        {
            lock (connectionLock)
            {
                if (Connection == null || !Connection.IsConnected)
                    return null;

                if (_cancel_session == null)
                {
                    if (_session == null)
                    {
                        _cancel_session = Connection.DuplicateSession(XenAdminConfigManager.Provider.ConnectionTimeout);
                    }
                    else if (_session.Url == null)  // DbProxy
                    {
                        return null;
                    }
                    else
                    {
                        _cancel_session = XenAdminConfigManager.Provider.CreateActionSession(_session, Connection);
                    }
                }

                return _cancel_session;
            }
        }

        protected void LogoutCancelSession()
        {
            lock (connectionLock)
            {
                _cancel_session = null;
            }
        }

        /// <summary>
        /// Cancels this action.
        /// 
        /// 1. Must be called on the event thread.
        /// 2. Will return if Cancelling = true
        /// 3. Runs RecomputeCanCancel() on a bg thread, then if CanCancel == true, sets Cancelling to true and runs Cancel_()
        /// </summary>
        public override sealed void Cancel()
        {
            //Program.AssertOnEventThread();
            log.Debug("Cancel() was called. Attempting to cancel action");

            // We can always cancel before the action starts running
            lock (_startedRunningLock)
            {
                if (!_startedRunning)
                {
                    cancelled = true;
                    new Thread(delegate()
                    {
                        AuditLogCancelled();
                        MarkCompleted(new CancelledException());
                        Clean();
                        CleanOnError();
                    }).Start();
                    return;
                }
            }

            lock (_cancellinglock)
            {
                if (Cancelling)
                    return;

                Cancelling = true;
            }

            Thread t = new Thread(DoCancel);
            t.Name = string.Format("Cancelling task {0}", Title);
            t.IsBackground = true;
            t.Priority = ThreadPriority.Lowest;
            t.Start();
        }

        private void DoCancel()
        {
            RecomputeCanCancel();
            if (can_cancel)
            {
                try
                {
                    CancelRelatedTask();
                }
                catch (Exception e)
                {
                    log.DebugFormat("Exception when cancelling action {0}", this.Description);
                    log.Debug(e, e);
                    LogoutCancelSession();
                    Cancelling = false;
                }
            }
        }

        /// <summary>
        /// Called by Cancel on a background thread, to do any heavy lifting.
        /// Creates a new Session and cancels RelatedTask.
        /// 
        ///   * Only called if RecomputeCanCancel sets CanCancel to true;
        ///   * Cancelling set to true before this call, but after call to RecomputeCanCancel and CanCancel.
        ///   * DO NOT CHECK CANCANCEL HERE, you will only be called with it as true, but it may change after.
        ///
        /// </summary>
        protected virtual void CancelRelatedTask()
        {
            PerformSilentTaskOp(delegate()
            {
                // Create a new session since this.Session may be in-use by the thread pool thread, and, in particular, TaskPoller
                Session local_session = GetCancelSession();

                XenRef<Task> r = RelatedTask;
                if (r != null)
                {
                    XenAPI.Task.cancel(local_session, r);
                }
            });
        }

        protected void PerformSilentTaskOp(Action f)
        {
            if (_relatedTask == null)
                return;
            try
            {
                f();
            }
            catch (XenAPI.Failure exn)
            {
                if (exn.ErrorDescription.Count > 1 &&
                    exn.ErrorDescription[0] == XenAPI.Failure.HANDLE_INVALID &&
                    exn.ErrorDescription[1] == "task")
                {
                    log.Debug(exn, exn);
                    // The task has disappeared.
                    _relatedTask = null;
                }
                else
                {
                    log.Error(exn, exn);
                    // Ignore, and hope that this isn't a problem.
                }
            }
            catch (Exception exn)
            {
                log.Error(exn, exn);
                // Ignore, and hope that this isn't a problem.
            }
        }

        private readonly object _cancellinglock = new object();

        // _startedRunningLock controls the case that two threads try and Cancel and Run an action
        // at the same time: one of them has to win and prevent the other.
        private object _startedRunningLock = new object();
        
        private bool _startedRunning = false;
        public bool StartedRunning
        {
            get { return _startedRunning; }
            protected set
            {
                lock(_startedRunningLock)
                {
                    _startedRunning = value;
                }
            }
        }
        

        /// <summary>
        /// If there has been an exception this code will always execute after the action has finished, use for tidyup
        /// </summary>
        protected virtual void CleanOnError() { }

        /// <summary>
        /// This code will always execute after the action has finished, use for tidyup
        /// </summary>
        protected virtual void Clean() { }

    }
}