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#pragma warning disable 0420
// ==++==
//
// Copyright (c) Microsoft Corporation. All rights reserved.
//
// ==--==
//
// <OWNER>Microsoft</OWNER>
////////////////////////////////////////////////////////////////////////////////
using System;
using System.Security;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using System.Security.Permissions;
using System.Diagnostics.Contracts;
using System.Runtime;
namespace System.Threading
{
/// <summary>
/// Signals to a <see cref="System.Threading.CancellationToken"/> that it should be canceled.
/// </summary>
/// <remarks>
/// <para>
/// <see cref="T:System.Threading.CancellationTokenSource"/> is used to instantiate a <see
/// cref="T:System.Threading.CancellationToken"/>
/// (via the source's <see cref="System.Threading.CancellationTokenSource.Token">Token</see> property)
/// that can be handed to operations that wish to be notified of cancellation or that can be used to
/// register asynchronous operations for cancellation. That token may have cancellation requested by
/// calling to the source's <see cref="System.Threading.CancellationTokenSource.Cancel()">Cancel</see>
/// method.
/// </para>
/// <para>
/// All members of this class, except <see cref="Dispose">Dispose</see>, are thread-safe and may be used
/// concurrently from multiple threads.
/// </para>
/// </remarks>
[ComVisible(false)]
[HostProtection(Synchronization = true, ExternalThreading = true)]
public class CancellationTokenSource : IDisposable
{
//static sources that can be used as the backing source for 'fixed' CancellationTokens that never change state.
private static readonly CancellationTokenSource _staticSource_Set = new CancellationTokenSource(true);
private static readonly CancellationTokenSource _staticSource_NotCancelable = new CancellationTokenSource(false);
//Note: the callback lists array is only created on first registration.
// the actual callback lists are only created on demand.
// Storing a registered callback costs around >60bytes, hence some overhead for the lists array is OK
// At most 24 lists seems reasonable, and caps the cost of the listsArray to 96bytes(32-bit,24-way) or 192bytes(64-bit,24-way).
private static readonly int s_nLists = (PlatformHelper.ProcessorCount > 24) ? 24 : PlatformHelper.ProcessorCount;
private volatile ManualResetEvent m_kernelEvent; //lazily initialized if required.
private volatile SparselyPopulatedArray<CancellationCallbackInfo>[] m_registeredCallbacksLists;
// legal values for m_state
private const int CANNOT_BE_CANCELED = 0;
private const int NOT_CANCELED = 1;
private const int NOTIFYING = 2;
private const int NOTIFYINGCOMPLETE = 3;
//m_state uses the pattern "volatile int32 reads, with cmpxch writes" which is safe for updates and cannot suffer torn reads.
private volatile int m_state;
/// The ID of the thread currently executing the main body of CTS.Cancel()
/// this helps us to know if a call to ctr.Dispose() is running 'within' a cancellation callback.
/// This is updated as we move between the main thread calling cts.Cancel() and any syncContexts that are used to
/// actually run the callbacks.
private volatile int m_threadIDExecutingCallbacks = -1;
private bool m_disposed;
private CancellationTokenRegistration [] m_linkingRegistrations; //lazily initialized if required.
private static readonly Action<object> s_LinkedTokenCancelDelegate = new Action<object>(LinkedTokenCancelDelegate);
// we track the running callback to assist ctr.Dispose() to wait for the target callback to complete.
private volatile CancellationCallbackInfo m_executingCallback;
// provided for CancelAfter and timer-related constructors
private volatile Timer m_timer;
private static void LinkedTokenCancelDelegate(object source)
{
CancellationTokenSource cts = source as CancellationTokenSource;
Contract.Assert(source != null);
cts.Cancel();
}
// ----------------------
// ** public properties
/// <summary>
/// Gets whether cancellation has been requested for this <see
/// cref="System.Threading.CancellationTokenSource">CancellationTokenSource</see>.
/// </summary>
/// <value>Whether cancellation has been requested for this <see
/// cref="System.Threading.CancellationTokenSource">CancellationTokenSource</see>.</value>
/// <remarks>
/// <para>
/// This property indicates whether cancellation has been requested for this token source, such as
/// due to a call to its
/// <see cref="System.Threading.CancellationTokenSource.Cancel()">Cancel</see> method.
/// </para>
/// <para>
/// If this property returns true, it only guarantees that cancellation has been requested. It does not
/// guarantee that every handler registered with the corresponding token has finished executing, nor
/// that cancellation requests have finished propagating to all registered handlers. Additional
/// synchronization may be required, particularly in situations where related objects are being
/// canceled concurrently.
/// </para>
/// </remarks>
public bool IsCancellationRequested
{
get { return m_state >= NOTIFYING; }
}
/// <summary>
/// A simple helper to determine whether cancellation has finished.
/// </summary>
internal bool IsCancellationCompleted
{
get { return m_state == NOTIFYINGCOMPLETE; }
}
/// <summary>
/// A simple helper to determine whether disposal has occured.
/// </summary>
internal bool IsDisposed
{
get { return m_disposed; }
}
/// <summary>
/// The ID of the thread that is running callbacks.
/// </summary>
internal int ThreadIDExecutingCallbacks
{
set { m_threadIDExecutingCallbacks = value; }
get { return m_threadIDExecutingCallbacks; }
}
/// <summary>
/// Gets the <see cref="System.Threading.CancellationToken">CancellationToken</see>
/// associated with this <see cref="CancellationTokenSource"/>.
/// </summary>
/// <value>The <see cref="System.Threading.CancellationToken">CancellationToken</see>
/// associated with this <see cref="CancellationTokenSource"/>.</value>
/// <exception cref="T:System.ObjectDisposedException">The token source has been
/// disposed.</exception>
public CancellationToken Token
{
get
{
ThrowIfDisposed();
return new CancellationToken(this);
}
}
// ----------------------
// ** internal and private properties.
/// <summary>
///
/// </summary>
internal bool CanBeCanceled
{
get { return m_state != CANNOT_BE_CANCELED; }
}
/// <summary>
///
/// </summary>
internal WaitHandle WaitHandle
{
get
{
ThrowIfDisposed();
// fast path if already allocated.
if (m_kernelEvent != null)
return m_kernelEvent;
// lazy-init the mre.
ManualResetEvent mre = new ManualResetEvent(false);
if (Interlocked.CompareExchange(ref m_kernelEvent, mre, null) != null)
{
((IDisposable)mre).Dispose();
}
// There is a ---- between checking IsCancellationRequested and setting the event.
// However, at this point, the kernel object definitely exists and the cases are:
// 1. if IsCancellationRequested = true, then we will call Set()
// 2. if IsCancellationRequested = false, then NotifyCancellation will see that the event exists, and will call Set().
if (IsCancellationRequested)
m_kernelEvent.Set();
return m_kernelEvent;
}
}
/// <summary>
/// The currently executing callback
/// </summary>
internal CancellationCallbackInfo ExecutingCallback
{
get { return m_executingCallback; }
}
#if DEBUG
/// <summary>
/// Used by the dev unit tests to check the number of outstanding registrations.
/// They use private reflection to gain access. Because this would be dead retail
/// code, however, it is ifdef'd out to work only in debug builds.
/// </summary>
private int CallbackCount
{
get
{
SparselyPopulatedArray<CancellationCallbackInfo>[] callbackLists = m_registeredCallbacksLists;
if (callbackLists == null)
return 0;
int count = 0;
foreach(SparselyPopulatedArray<CancellationCallbackInfo> sparseArray in callbackLists)
{
if(sparseArray != null)
{
SparselyPopulatedArrayFragment<CancellationCallbackInfo> currCallbacks = sparseArray.Head;
while (currCallbacks != null)
{
for (int i = 0; i < currCallbacks.Length; i++)
if (currCallbacks[i] != null)
count++;
currCallbacks = currCallbacks.Next;
}
}
}
return count;
}
}
#endif
// ** Public Constructors
/// <summary>
/// Initializes the <see cref="T:System.Threading.CancellationTokenSource"/>.
/// </summary>
public CancellationTokenSource()
{
m_state = NOT_CANCELED;
}
// ** Private constructors for static sources.
// set=false ==> cannot be canceled.
// set=true ==> is canceled.
private CancellationTokenSource(bool set)
{
m_state = set ? NOTIFYINGCOMPLETE : CANNOT_BE_CANCELED;
}
/// <summary>
/// Constructs a <see cref="T:System.Threading.CancellationTokenSource"/> that will be canceled after a specified time span.
/// </summary>
/// <param name="delay">The time span to wait before canceling this <see cref="T:System.Threading.CancellationTokenSource"/></param>
/// <exception cref="T:System.ArgumentOutOfRangeException">
/// The exception that is thrown when <paramref name="delay"/> is less than -1 or greater than Int32.MaxValue.
/// </exception>
/// <remarks>
/// <para>
/// The countdown for the delay starts during the call to the constructor. When the delay expires,
/// the constructed <see cref="T:System.Threading.CancellationTokenSource"/> is canceled, if it has
/// not been canceled already.
/// </para>
/// <para>
/// Subsequent calls to CancelAfter will reset the delay for the constructed
/// <see cref="T:System.Threading.CancellationTokenSource"/>, if it has not been
/// canceled already.
/// </para>
/// </remarks>
public CancellationTokenSource(TimeSpan delay)
{
long totalMilliseconds = (long)delay.TotalMilliseconds;
if (totalMilliseconds < -1 || totalMilliseconds > Int32.MaxValue)
{
throw new ArgumentOutOfRangeException("delay");
}
InitializeWithTimer((int)totalMilliseconds);
}
/// <summary>
/// Constructs a <see cref="T:System.Threading.CancellationTokenSource"/> that will be canceled after a specified time span.
/// </summary>
/// <param name="millisecondsDelay">The time span to wait before canceling this <see cref="T:System.Threading.CancellationTokenSource"/></param>
/// <exception cref="T:System.ArgumentOutOfRangeException">
/// The exception that is thrown when <paramref name="millisecondsDelay"/> is less than -1.
/// </exception>
/// <remarks>
/// <para>
/// The countdown for the millisecondsDelay starts during the call to the constructor. When the millisecondsDelay expires,
/// the constructed <see cref="T:System.Threading.CancellationTokenSource"/> is canceled (if it has
/// not been canceled already).
/// </para>
/// <para>
/// Subsequent calls to CancelAfter will reset the millisecondsDelay for the constructed
/// <see cref="T:System.Threading.CancellationTokenSource"/>, if it has not been
/// canceled already.
/// </para>
/// </remarks>
public CancellationTokenSource(Int32 millisecondsDelay)
{
if (millisecondsDelay < -1)
{
throw new ArgumentOutOfRangeException("millisecondsDelay");
}
InitializeWithTimer(millisecondsDelay);
}
// Common initialization logic when constructing a CTS with a delay parameter
private void InitializeWithTimer(Int32 millisecondsDelay)
{
m_state = NOT_CANCELED;
m_timer = new Timer(s_timerCallback, this, millisecondsDelay, -1);
}
// ** Public Methods
/// <summary>
/// Communicates a request for cancellation.
/// </summary>
/// <remarks>
/// <para>
/// The associated <see cref="T:System.Threading.CancellationToken" /> will be
/// notified of the cancellation and will transition to a state where
/// <see cref="System.Threading.CancellationToken.IsCancellationRequested">IsCancellationRequested</see> returns true.
/// Any callbacks or cancelable operations
/// registered with the <see cref="T:System.Threading.CancellationToken"/> will be executed.
/// </para>
/// <para>
/// Cancelable operations and callbacks registered with the token should not throw exceptions.
/// However, this overload of Cancel will aggregate any exceptions thrown into a <see cref="System.AggregateException"/>,
/// such that one callback throwing an exception will not prevent other registered callbacks from being executed.
/// </para>
/// <para>
/// The <see cref="T:System.Threading.ExecutionContext"/> that was captured when each callback was registered
/// will be reestablished when the callback is invoked.
/// </para>
/// </remarks>
/// <exception cref="T:System.AggregateException">An aggregate exception containing all the exceptions thrown
/// by the registered callbacks on the associated <see cref="T:System.Threading.CancellationToken"/>.</exception>
/// <exception cref="T:System.ObjectDisposedException">This <see
/// cref="T:System.Threading.CancellationTokenSource"/> has been disposed.</exception>
public void Cancel()
{
Cancel(false);
}
/// <summary>
/// Communicates a request for cancellation.
/// </summary>
/// <remarks>
/// <para>
/// The associated <see cref="T:System.Threading.CancellationToken" /> will be
/// notified of the cancellation and will transition to a state where
/// <see cref="System.Threading.CancellationToken.IsCancellationRequested">IsCancellationRequested</see> returns true.
/// Any callbacks or cancelable operations
/// registered with the <see cref="T:System.Threading.CancellationToken"/> will be executed.
/// </para>
/// <para>
/// Cancelable operations and callbacks registered with the token should not throw exceptions.
/// If <paramref name="throwOnFirstException"/> is true, an exception will immediately propagate out of the
/// call to Cancel, preventing the remaining callbacks and cancelable operations from being processed.
/// If <paramref name="throwOnFirstException"/> is false, this overload will aggregate any
/// exceptions thrown into a <see cref="System.AggregateException"/>,
/// such that one callback throwing an exception will not prevent other registered callbacks from being executed.
/// </para>
/// <para>
/// The <see cref="T:System.Threading.ExecutionContext"/> that was captured when each callback was registered
/// will be reestablished when the callback is invoked.
/// </para>
/// </remarks>
/// <param name="throwOnFirstException">Specifies whether exceptions should immediately propagate.</param>
/// <exception cref="T:System.AggregateException">An aggregate exception containing all the exceptions thrown
/// by the registered callbacks on the associated <see cref="T:System.Threading.CancellationToken"/>.</exception>
/// <exception cref="T:System.ObjectDisposedException">This <see
/// cref="T:System.Threading.CancellationTokenSource"/> has been disposed.</exception>
public void Cancel(bool throwOnFirstException)
{
ThrowIfDisposed();
NotifyCancellation(throwOnFirstException);
}
/// <summary>
/// Schedules a Cancel operation on this <see cref="T:System.Threading.CancellationTokenSource"/>.
/// </summary>
/// <param name="delay">The time span to wait before canceling this <see
/// cref="T:System.Threading.CancellationTokenSource"/>.
/// </param>
/// <exception cref="T:System.ObjectDisposedException">The exception thrown when this <see
/// cref="T:System.Threading.CancellationTokenSource"/> has been disposed.
/// </exception>
/// <exception cref="T:System.ArgumentOutOfRangeException">
/// The exception thrown when <paramref name="delay"/> is less than -1 or
/// greater than Int32.MaxValue.
/// </exception>
/// <remarks>
/// <para>
/// The countdown for the delay starts during this call. When the delay expires,
/// this <see cref="T:System.Threading.CancellationTokenSource"/> is canceled, if it has
/// not been canceled already.
/// </para>
/// <para>
/// Subsequent calls to CancelAfter will reset the delay for this
/// <see cref="T:System.Threading.CancellationTokenSource"/>, if it has not been
/// canceled already.
/// </para>
/// </remarks>
public void CancelAfter(TimeSpan delay)
{
long totalMilliseconds = (long)delay.TotalMilliseconds;
if (totalMilliseconds < -1 || totalMilliseconds > Int32.MaxValue)
{
throw new ArgumentOutOfRangeException("delay");
}
CancelAfter((int)totalMilliseconds);
}
/// <summary>
/// Schedules a Cancel operation on this <see cref="T:System.Threading.CancellationTokenSource"/>.
/// </summary>
/// <param name="millisecondsDelay">The time span to wait before canceling this <see
/// cref="T:System.Threading.CancellationTokenSource"/>.
/// </param>
/// <exception cref="T:System.ObjectDisposedException">The exception thrown when this <see
/// cref="T:System.Threading.CancellationTokenSource"/> has been disposed.
/// </exception>
/// <exception cref="T:System.ArgumentOutOfRangeException">
/// The exception thrown when <paramref name="millisecondsDelay"/> is less than -1.
/// </exception>
/// <remarks>
/// <para>
/// The countdown for the millisecondsDelay starts during this call. When the millisecondsDelay expires,
/// this <see cref="T:System.Threading.CancellationTokenSource"/> is canceled, if it has
/// not been canceled already.
/// </para>
/// <para>
/// Subsequent calls to CancelAfter will reset the millisecondsDelay for this
/// <see cref="T:System.Threading.CancellationTokenSource"/>, if it has not been
/// canceled already.
/// </para>
/// </remarks>
public void CancelAfter(Int32 millisecondsDelay)
{
ThrowIfDisposed();
if (millisecondsDelay < -1)
{
throw new ArgumentOutOfRangeException("millisecondsDelay");
}
if (IsCancellationRequested) return;
// There is a race condition here as a Cancel could occur between the check of
// IsCancellationRequested and the creation of the timer. This is benign; in the
// worst case, a timer will be created that has no effect when it expires.
// Also, if Dispose() is called right here (after ThrowIfDisposed(), before timer
// creation), it would result in a leaked Timer object (at least until the timer
// expired and Disposed itself). But this would be considered bad behavior, as
// Dispose() is not thread-safe and should not be called concurrently with CancelAfter().
if (m_timer == null)
{
// Lazily initialize the timer in a thread-safe fashion.
// Initially set to "never go off" because we don't want to take a
// chance on a timer "losing" the initialization ---- and then
// cancelling the token before it (the timer) can be disposed.
Timer newTimer = new Timer(s_timerCallback, this, -1, -1);
if (Interlocked.CompareExchange(ref m_timer, newTimer, null) != null)
{
// We lost the ---- to initialize the timer. Dispose the new timer.
newTimer.Dispose();
}
}
// It is possible that m_timer has already been disposed, so we must do
// the following in a try/catch block.
try
{
m_timer.Change(millisecondsDelay, -1);
}
catch (ObjectDisposedException)
{
// Just eat the exception. There is no other way to tell that
// the timer has been disposed, and even if there were, there
// would not be a good way to deal with the observe/dispose
// race condition.
}
}
private static readonly TimerCallback s_timerCallback = new TimerCallback(TimerCallbackLogic);
// Common logic for a timer delegate
private static void TimerCallbackLogic(object obj)
{
CancellationTokenSource cts = (CancellationTokenSource)obj;
// Cancel the source; handle a race condition with cts.Dispose()
if (!cts.IsDisposed)
{
// There is a small window for a race condition where a cts.Dispose can sneak
// in right here. I'll wrap the cts.Cancel() in a try/catch to proof us
// against this ----.
try
{
cts.Cancel(); // will take care of disposing of m_timer
}
catch (ObjectDisposedException)
{
// If the ODE was not due to the target cts being disposed, then propagate the ODE.
if (!cts.IsDisposed) throw;
}
}
}
/// <summary>
/// Releases the resources used by this <see cref="T:System.Threading.CancellationTokenSource" />.
/// </summary>
/// <remarks>
/// This method is not thread-safe for any other concurrent calls.
/// </remarks>
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
/// <summary>
/// Releases the unmanaged resources used by the <see cref="T:System.Threading.CancellationTokenSource" /> class and optionally releases the managed resources.
/// </summary>
/// <param name="disposing">true to release both managed and unmanaged resources; false to release only unmanaged resources.</param>
protected virtual void Dispose(bool disposing)
{
// There is nothing to do if disposing=false because the CancellationTokenSource holds no unmanaged resources.
if (disposing)
{
//NOTE: We specifically tolerate that a callback can be deregistered
// after the CTS has been disposed and/or concurrently with cts.Dispose().
// This is safe without locks because the reg.Dispose() only
// mutates a sparseArrayFragment and then reads from properties of the CTS that are not
// invalidated by cts.Dispose().
//
// We also tolerate that a callback can be registered after the CTS has been
// disposed. This is safe without locks because InternalRegister is tolerant
// of m_registeredCallbacksLists becoming null during its execution. However,
// we run the acceptable risk of m_registeredCallbacksLists getting reinitialized
// to non-null if there is a ---- between Dispose and Register, in which case this
// instance may unnecessarily hold onto a registered callback. But that's no worse
// than if Dispose wasn't safe to use concurrently, as Dispose would never be called,
// and thus no handlers would be dropped.
if (m_disposed)
return;
if (m_timer != null) m_timer.Dispose();
var linkingRegistrations = m_linkingRegistrations;
if (linkingRegistrations != null)
{
m_linkingRegistrations = null; // free for GC once we're done enumerating
for (int i = 0; i < linkingRegistrations.Length; i++)
{
linkingRegistrations[i].Dispose();
}
}
// registered callbacks are now either complete or will never run, due to guarantees made by ctr.Dispose()
// so we can now perform main disposal work without risk of linking callbacks trying to use this CTS.
m_registeredCallbacksLists = null; // free for GC.
if (m_kernelEvent != null)
{
m_kernelEvent.Close(); // the critical cleanup to release an OS handle
m_kernelEvent = null; // free for GC.
}
m_disposed = true;
}
}
// -- Internal methods.
/// <summary>
/// Throws an exception if the source has been disposed.
/// </summary>
internal void ThrowIfDisposed()
{
if (m_disposed)
ThrowObjectDisposedException();
}
// separation enables inlining of ThrowIfDisposed
private static void ThrowObjectDisposedException()
{
throw new ObjectDisposedException(null, Environment.GetResourceString("CancellationTokenSource_Disposed"));
}
/// <summary>
/// InternalGetStaticSource()
/// </summary>
/// <param name="set">Whether the source should be set.</param>
/// <returns>A static source to be shared among multiple tokens.</returns>
internal static CancellationTokenSource InternalGetStaticSource(bool set)
{
return set ? _staticSource_Set : _staticSource_NotCancelable;
}
/// <summary>
/// Registers a callback object. If cancellation has already occurred, the
/// callback will have been run by the time this method returns.
/// </summary>
internal CancellationTokenRegistration InternalRegister(
Action<object> callback, object stateForCallback, SynchronizationContext targetSyncContext, ExecutionContext executionContext)
{
if (AppContextSwitches.ThrowExceptionIfDisposedCancellationTokenSource)
{
ThrowIfDisposed();
}
// the CancellationToken has already checked that the token is cancelable before calling this method.
Contract.Assert(CanBeCanceled, "Cannot register for uncancelable token src");
// if not canceled, register the event handlers
// if canceled already, run the callback synchronously
// Apart from the semantics of late-enlistment, this also ensures that during ExecuteCallbackHandlers() there
// will be no mutation of the _registeredCallbacks list
if (!IsCancellationRequested)
{
// In order to enable code to not leak too many handlers, we allow Dispose to be called concurrently
// with Register. While this is not a recommended practice, consumers can and do use it this way.
// We don't make any guarantees about whether the CTS will hold onto the supplied callback
// if the CTS has already been disposed when the callback is registered, but we try not to
// while at the same time not paying any non-negligible overhead. The simple compromise
// is to check whether we're disposed (not volatile), and if we see we are, to return an empty
// registration, just as if CanBeCanceled was false for the check made in CancellationToken.Register.
// If there's a ---- and m_disposed is false even though it's been disposed, or if the disposal request
// comes in after this line, we simply run the minor risk of having m_registeredCallbacksLists reinitialized
// (after it was cleared to null during Dispose).
if (m_disposed && !AppContextSwitches.ThrowExceptionIfDisposedCancellationTokenSource)
return new CancellationTokenRegistration();
int myIndex = Thread.CurrentThread.ManagedThreadId % s_nLists;
CancellationCallbackInfo callbackInfo = new CancellationCallbackInfo(callback, stateForCallback, targetSyncContext, executionContext, this);
//allocate the callback list array
var registeredCallbacksLists = m_registeredCallbacksLists;
if (registeredCallbacksLists == null)
{
SparselyPopulatedArray<CancellationCallbackInfo>[] list = new SparselyPopulatedArray<CancellationCallbackInfo>[s_nLists];
registeredCallbacksLists = Interlocked.CompareExchange(ref m_registeredCallbacksLists, list, null);
if (registeredCallbacksLists == null) registeredCallbacksLists = list;
}
//allocate the actual lists on-demand to save mem in low-use situations, and to avoid false-sharing.
var callbacks = Volatile.Read<SparselyPopulatedArray<CancellationCallbackInfo>>(ref registeredCallbacksLists[myIndex]);
if (callbacks == null)
{
SparselyPopulatedArray<CancellationCallbackInfo> callBackArray = new SparselyPopulatedArray<CancellationCallbackInfo>(4);
Interlocked.CompareExchange(ref (registeredCallbacksLists[myIndex]), callBackArray, null);
callbacks = registeredCallbacksLists[myIndex];
}
// Now add the registration to the list.
SparselyPopulatedArrayAddInfo<CancellationCallbackInfo> addInfo = callbacks.Add(callbackInfo);
CancellationTokenRegistration registration = new CancellationTokenRegistration(callbackInfo, addInfo);
if (!IsCancellationRequested)
return registration;
// If a cancellation has since come in, we will try to undo the registration and run the callback ourselves.
// (this avoids leaving the callback orphaned)
bool deregisterOccurred = registration.TryDeregister();
if (!deregisterOccurred)
{
// The thread that is running Cancel() snagged our callback for execution.
// So we don't need to run it, but we do return the registration so that
// ctr.Dispose() will wait for callback completion.
return registration;
}
}
// If cancellation already occurred, we run the callback on this thread and return an empty registration.
callback(stateForCallback);
return new CancellationTokenRegistration();
}
/// <summary>
///
/// </summary>
private void NotifyCancellation(bool throwOnFirstException)
{
// fast-path test to check if Notify has been called previously
if (IsCancellationRequested)
return;
// If we're the first to signal cancellation, do the main extra work.
if (Interlocked.CompareExchange(ref m_state, NOTIFYING, NOT_CANCELED) == NOT_CANCELED)
{
// Dispose of the timer, if any
Timer timer = m_timer;
if(timer != null) timer.Dispose();
//record the threadID being used for running the callbacks.
ThreadIDExecutingCallbacks = Thread.CurrentThread.ManagedThreadId;
//If the kernel event is null at this point, it will be set during lazy construction.
if (m_kernelEvent != null)
m_kernelEvent.Set(); // update the MRE value.
// - late enlisters to the Canceled event will have their callbacks called immediately in the Register() methods.
// - Callbacks are not called inside a lock.
// - After transition, no more delegates will be added to the
// - list of handlers, and hence it can be consumed and cleared at leisure by ExecuteCallbackHandlers.
ExecuteCallbackHandlers(throwOnFirstException);
Contract.Assert(IsCancellationCompleted, "Expected cancellation to have finished");
}
}
/// <summary>
/// Invoke the Canceled event.
/// </summary>
/// <remarks>
/// The handlers are invoked synchronously in LIFO order.
/// </remarks>
private void ExecuteCallbackHandlers(bool throwOnFirstException)
{
Contract.Assert(IsCancellationRequested, "ExecuteCallbackHandlers should only be called after setting IsCancellationRequested->true");
Contract.Assert(ThreadIDExecutingCallbacks != -1, "ThreadIDExecutingCallbacks should have been set.");
// Design decision: call the delegates in LIFO order so that callbacks fire 'deepest first'.
// This is intended to help with nesting scenarios so that child enlisters cancel before their parents.
List<Exception> exceptionList = null;
SparselyPopulatedArray<CancellationCallbackInfo>[] callbackLists = m_registeredCallbacksLists;
// If there are no callbacks to run, we can safely exit. Any ----s to lazy initialize it
// will see IsCancellationRequested and will then run the callback themselves.
if (callbackLists == null)
{
Interlocked.Exchange(ref m_state, NOTIFYINGCOMPLETE);
return;
}
try
{
for (int index = 0; index < callbackLists.Length; index++)
{
SparselyPopulatedArray<CancellationCallbackInfo> list = Volatile.Read<SparselyPopulatedArray<CancellationCallbackInfo>>(ref callbackLists[index]);
if (list != null)
{
SparselyPopulatedArrayFragment<CancellationCallbackInfo> currArrayFragment = list.Tail;
while (currArrayFragment != null)
{
for (int i = currArrayFragment.Length - 1; i >= 0; i--)
{
// 1a. publish the indended callback, to ensure ctr.Dipose can tell if a wait is necessary.
// 1b. transition to the target syncContext and continue there..
// On the target SyncContext.
// 2. actually remove the callback
// 3. execute the callback
// re:#2 we do the remove on the syncCtx so that we can be sure we have control of the syncCtx before
// grabbing the callback. This prevents a deadlock if ctr.Dispose() might run on the syncCtx too.
m_executingCallback = currArrayFragment[i];
if (m_executingCallback != null)
{
//Transition to the target sync context (if necessary), and continue our work there.
CancellationCallbackCoreWorkArguments args = new CancellationCallbackCoreWorkArguments(currArrayFragment, i);
// marshal exceptions: either aggregate or perform an immediate rethrow
// We assume that syncCtx.Send() has forwarded on user exceptions when appropriate.
try
{
if (m_executingCallback.TargetSyncContext != null)
{
m_executingCallback.TargetSyncContext.Send(CancellationCallbackCoreWork_OnSyncContext, args);
// CancellationCallbackCoreWork_OnSyncContext may have altered ThreadIDExecutingCallbacks, so reset it.
ThreadIDExecutingCallbacks = Thread.CurrentThread.ManagedThreadId;
}
else
{
CancellationCallbackCoreWork(args);
}
}
catch(Exception ex)
{
if (throwOnFirstException)
throw;
// Otherwise, log it and proceed.
if(exceptionList == null)
exceptionList = new List<Exception>();
exceptionList.Add(ex);
}
}
}
currArrayFragment = currArrayFragment.Prev;
}
}
}
}
finally
{
m_state = NOTIFYINGCOMPLETE;
m_executingCallback = null;
Thread.MemoryBarrier(); // for safety, prevent reorderings crossing this point and seeing inconsistent state.
}
if (exceptionList != null)
{
Contract.Assert(exceptionList.Count > 0, "Expected exception count > 0");
throw new AggregateException(exceptionList);
}
}
// The main callback work that executes on the target synchronization context
private void CancellationCallbackCoreWork_OnSyncContext(object obj)
{
CancellationCallbackCoreWork((CancellationCallbackCoreWorkArguments)obj);
}
private void CancellationCallbackCoreWork(CancellationCallbackCoreWorkArguments args)
{
// remove the intended callback..and ensure that it worked.
// otherwise the callback has disappeared in the interim and we can immediately return.
CancellationCallbackInfo callback = args.m_currArrayFragment.SafeAtomicRemove(args.m_currArrayIndex, m_executingCallback);
if (callback == m_executingCallback)
{
if (callback.TargetExecutionContext != null)
{
// we are running via a custom sync context, so update the executing threadID
callback.CancellationTokenSource.ThreadIDExecutingCallbacks = Thread.CurrentThread.ManagedThreadId;
}
callback.ExecuteCallback();
}
}
/// <summary>
/// Creates a <see cref="T:System.Threading.CancellationTokenSource">CancellationTokenSource</see> that will be in the canceled state
/// when any of the source tokens are in the canceled state.
/// </summary>
/// <param name="token1">The first <see cref="T:System.Threading.CancellationToken">CancellationToken</see> to observe.</param>
/// <param name="token2">The second <see cref="T:System.Threading.CancellationToken">CancellationToken</see> to observe.</param>
/// <returns>A <see cref="T:System.Threading.CancellationTokenSource">CancellationTokenSource</see> that is linked
/// to the source tokens.</returns>
public static CancellationTokenSource CreateLinkedTokenSource(CancellationToken token1, CancellationToken token2)
{
CancellationTokenSource linkedTokenSource = new CancellationTokenSource();
bool token2CanBeCanceled = token2.CanBeCanceled;
if( token1.CanBeCanceled )
{
linkedTokenSource.m_linkingRegistrations = new CancellationTokenRegistration[token2CanBeCanceled ? 2 : 1]; // there will be at least 1 and at most 2 linkings
linkedTokenSource.m_linkingRegistrations[0] = token1.InternalRegisterWithoutEC(s_LinkedTokenCancelDelegate, linkedTokenSource);
}
if( token2CanBeCanceled )
{
int index = 1;
if( linkedTokenSource.m_linkingRegistrations == null )
{
linkedTokenSource.m_linkingRegistrations = new CancellationTokenRegistration[1]; // this will be the only linking
index = 0;
}
linkedTokenSource.m_linkingRegistrations[index] = token2.InternalRegisterWithoutEC(s_LinkedTokenCancelDelegate, linkedTokenSource);
}
return linkedTokenSource;
}
/// <summary>
/// Creates a <see cref="T:System.Threading.CancellationTokenSource">CancellationTokenSource</see> that will be in the canceled state
/// when any of the source tokens are in the canceled state.
/// </summary>
/// <param name="tokens">The <see cref="T:System.Threading.CancellationToken">CancellationToken</see> instances to observe.</param>
/// <returns>A <see cref="T:System.Threading.CancellationTokenSource">CancellationTokenSource</see> that is linked
/// to the source tokens.</returns>
/// <exception cref="T:System.ArgumentNullException"><paramref name="tokens"/> is null.</exception>
public static CancellationTokenSource CreateLinkedTokenSource(params CancellationToken[] tokens)
{
if (tokens == null)
throw new ArgumentNullException("tokens");
if (tokens.Length == 0)
throw new ArgumentException(Environment.GetResourceString("CancellationToken_CreateLinkedToken_TokensIsEmpty"));
// a defensive copy is not required as the array has value-items that have only a single IntPtr field,
// hence each item cannot be null itself, and reads of the payloads cannot be torn.
Contract.EndContractBlock();
CancellationTokenSource linkedTokenSource = new CancellationTokenSource();
linkedTokenSource.m_linkingRegistrations = new CancellationTokenRegistration[tokens.Length];
for (int i = 0; i < tokens.Length; i++)
{
if (tokens[i].CanBeCanceled)
{
linkedTokenSource.m_linkingRegistrations[i] = tokens[i].InternalRegisterWithoutEC(s_LinkedTokenCancelDelegate, linkedTokenSource);
}
// Empty slots in the array will be default(CancellationTokenRegistration), which are nops to Dispose.
// Based on usage patterns, such occurrences should also be rare, such that it's not worth resizing
// the array and incurring the related costs.
}
return linkedTokenSource;
}
// Wait for a single callback to complete (or, more specifically, to not be running).
// It is ok to call this method if the callback has already finished.
// Calling this method before the target callback has been selected for execution would be an error.
internal void WaitForCallbackToComplete(CancellationCallbackInfo callbackInfo)
{
SpinWait sw = new SpinWait();
while (ExecutingCallback == callbackInfo)
{
sw.SpinOnce(); //spin as we assume callback execution is fast and that this situation is rare.
}
}
}
// ----------------------------------------------------------
// -- CancellationCallbackCoreWorkArguments --
// ----------------------------------------------------------
// Helper struct for passing data to the target sync context
internal struct CancellationCallbackCoreWorkArguments
{
internal SparselyPopulatedArrayFragment<CancellationCallbackInfo> m_currArrayFragment;
internal int m_currArrayIndex;
public CancellationCallbackCoreWorkArguments(SparselyPopulatedArrayFragment<CancellationCallbackInfo> currArrayFragment, int currArrayIndex)
{
m_currArrayFragment = currArrayFragment;
m_currArrayIndex = currArrayIndex;
}
}
// ----------------------------------------------------------
// -- CancellationCallbackInfo --
// ----------------------------------------------------------
/// <summary>
/// A helper class for collating the various bits of information required to execute
/// cancellation callbacks.
/// </summary>
internal class CancellationCallbackInfo
{
internal readonly Action<object> Callback;
internal readonly object StateForCallback;
internal readonly SynchronizationContext TargetSyncContext;
internal readonly ExecutionContext TargetExecutionContext;
internal readonly CancellationTokenSource CancellationTokenSource;
internal CancellationCallbackInfo(
Action<object> callback, object stateForCallback, SynchronizationContext targetSyncContext, ExecutionContext targetExecutionContext,
CancellationTokenSource cancellationTokenSource)
{
Callback = callback;
StateForCallback = stateForCallback;
TargetSyncContext = targetSyncContext;
TargetExecutionContext = targetExecutionContext;
CancellationTokenSource = cancellationTokenSource;
}
// Cached callback delegate that's lazily initialized due to ContextCallback being SecurityCritical
[SecurityCritical]
private static ContextCallback s_executionContextCallback;
/// <summary>
/// InternalExecuteCallbackSynchronously_GeneralPath
/// This will be called on the target synchronization context, however, we still need to restore the required execution context
/// </summary>
[SecuritySafeCritical]
internal void ExecuteCallback()
{
if (TargetExecutionContext != null)
{
// Lazily initialize the callback delegate; benign ----
var callback = s_executionContextCallback;
if (callback == null) s_executionContextCallback = callback = new ContextCallback(ExecutionContextCallback);
ExecutionContext.Run(
TargetExecutionContext,
callback,
this);
}
else
{
//otherwise run directly
ExecutionContextCallback(this);
}
}
// the worker method to actually run the callback
// The signature is such that it can be used as a 'ContextCallback'
[SecurityCritical]
private static void ExecutionContextCallback(object obj)
{
CancellationCallbackInfo callbackInfo = obj as CancellationCallbackInfo;
Contract.Assert(callbackInfo != null);
callbackInfo.Callback(callbackInfo.StateForCallback);
}
}
// ----------------------------------------------------------
// -- SparselyPopulatedArray --
// ----------------------------------------------------------
/// <summary>
/// A sparsely populated array. Elements can be sparse and some null, but this allows for
/// lock-free additions and growth, and also for constant time removal (by nulling out).
/// </summary>
/// <typeparam name="T">The kind of elements contained within.</typeparam>
internal class SparselyPopulatedArray<T> where T : class
{
private readonly SparselyPopulatedArrayFragment<T> m_head;
private volatile SparselyPopulatedArrayFragment<T> m_tail;
/// <summary>
/// Allocates a new array with the given initial size.
/// </summary>
/// <param name="initialSize">How many array slots to pre-allocate.</param>
internal SparselyPopulatedArray(int initialSize)
{
m_head = m_tail = new SparselyPopulatedArrayFragment<T>(initialSize);
}
#if DEBUG
// Used in DEBUG mode by CancellationTokenSource.CallbackCount
/// <summary>
/// The head of the doubly linked list.
/// </summary>
internal SparselyPopulatedArrayFragment<T> Head
{
get { return m_head; }
}
#endif
/// <summary>
/// The tail of the doubly linked list.
/// </summary>
internal SparselyPopulatedArrayFragment<T> Tail
{
get { return m_tail; }
}
/// <summary>
/// Adds an element in the first available slot, beginning the search from the tail-to-head.
/// If no slots are available, the array is grown. The method doesn't return until successful.
/// </summary>
/// <param name="element">The element to add.</param>
/// <returns>Information about where the add happened, to enable O(1) deregistration.</returns>
internal SparselyPopulatedArrayAddInfo<T> Add(T element)
{
while (true)
{
// Get the tail, and ensure it's up to date.
SparselyPopulatedArrayFragment<T> tail = m_tail;
while (tail.m_next != null)
m_tail = (tail = tail.m_next);
// Search for a free index, starting from the tail.
SparselyPopulatedArrayFragment<T> curr = tail;
while (curr != null)
{
const int RE_SEARCH_THRESHOLD = -10; // Every 10 skips, force a search.
if (curr.m_freeCount < 1)
--curr.m_freeCount;
if (curr.m_freeCount > 0 || curr.m_freeCount < RE_SEARCH_THRESHOLD)
{
int c = curr.Length;
// We'll compute a start offset based on how many free slots we think there
// are. This optimizes for ordinary the LIFO deregistration pattern, and is
// far from perfect due to the non-threadsafe ++ and -- of the free counter.
int start = ((c - curr.m_freeCount) % c);
if (start < 0)
{
start = 0;
curr.m_freeCount--; // Too many free elements; fix up.
}
Contract.Assert(start >= 0 && start < c, "start is outside of bounds");
// Now walk the array until we find a free slot (or reach the end).
for (int i = 0; i < c; i++)
{
// If the slot is null, try to CAS our element into it.
int tryIndex = (start + i) % c;
Contract.Assert(tryIndex >= 0 && tryIndex < curr.m_elements.Length, "tryIndex is outside of bounds");
if (curr.m_elements[tryIndex] == null && Interlocked.CompareExchange(ref curr.m_elements[tryIndex], element, null) == null)
{
// We adjust the free count by --. Note: if this drops to 0, we will skip
// the fragment on the next search iteration. Searching threads will -- the
// count and force a search every so often, just in case fragmentation occurs.
int newFreeCount = curr.m_freeCount - 1;
curr.m_freeCount = newFreeCount > 0 ? newFreeCount : 0;
return new SparselyPopulatedArrayAddInfo<T>(curr, tryIndex);
}
}
}
curr = curr.m_prev;
}
// If we got here, we need to add a new chunk to the tail and try again.
SparselyPopulatedArrayFragment<T> newTail = new SparselyPopulatedArrayFragment<T>(
tail.m_elements.Length == 4096 ? 4096 : tail.m_elements.Length * 2, tail);
if (Interlocked.CompareExchange(ref tail.m_next, newTail, null) == null)
{
m_tail = newTail;
}
}
}
}
/// <summary>
/// A struct to hold a link to the exact spot in an array an element was inserted, enabling
/// constant time removal later on.
/// </summary>
internal struct SparselyPopulatedArrayAddInfo<T> where T : class
{
private SparselyPopulatedArrayFragment<T> m_source;
private int m_index;
internal SparselyPopulatedArrayAddInfo(SparselyPopulatedArrayFragment<T> source, int index)
{
Contract.Assert(source != null);
Contract.Assert(index >= 0 && index < source.Length);
m_source = source;
m_index = index;
}
internal SparselyPopulatedArrayFragment<T> Source
{
get { return m_source; }
}
internal int Index
{
get { return m_index; }
}
}
/// <summary>
/// A fragment of a sparsely populated array, doubly linked.
/// </summary>
/// <typeparam name="T">The kind of elements contained within.</typeparam>
internal class SparselyPopulatedArrayFragment<T> where T : class
{
internal readonly T[] m_elements; // The contents, sparsely populated (with nulls).
internal volatile int m_freeCount; // A hint of the number of free elements.
internal volatile SparselyPopulatedArrayFragment<T> m_next; // The next fragment in the chain.
internal volatile SparselyPopulatedArrayFragment<T> m_prev; // The previous fragment in the chain.
internal SparselyPopulatedArrayFragment(int size) : this(size, null)
{
}
internal SparselyPopulatedArrayFragment(int size, SparselyPopulatedArrayFragment<T> prev)
{
m_elements = new T[size];
m_freeCount = size;
m_prev = prev;
}
internal T this[int index]
{
get { return Volatile.Read<T>(ref m_elements[index]); }
}
internal int Length
{
get { return m_elements.Length; }
}
#if DEBUG
// Used in DEBUG mode by CancellationTokenSource.CallbackCount
internal SparselyPopulatedArrayFragment<T> Next
{
get { return m_next; }
}
#endif
internal SparselyPopulatedArrayFragment<T> Prev
{
get { return m_prev; }
}
// only removes the item at the specified index if it is still the expected one.
// Returns the prevailing value.
// The remove occured successfully if the return value == expected element
// otherwise the remove did not occur.
internal T SafeAtomicRemove(int index, T expectedElement)
{
T prevailingValue = Interlocked.CompareExchange(ref m_elements[index], null, expectedElement);
if (prevailingValue != null)
++m_freeCount;
return prevailingValue;
}
}
}
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