/// <summary>
/// Disposes of the registration and unregisters the target callback from the associated
/// <see cref="System.Threading.CancellationToken">CancellationToken</see>.
/// The returned <see cref="ValueTask"/> will complete once the associated callback
/// is unregistered without having executed or once it's finished executing, except
/// in the degenerate case where the callback itself is unregistering itself.
/// </summary>
public ValueTask DisposeAsync()
{
CancellationTokenSource.CallbackNode node = _node;
return node != null && !node.Partition.Unregister(_id, node) ?
WaitForCallbackIfNecessaryAsync() :
default;
}
/// <summary>
/// Disposes of the registration and unregisters the target callback from the associated
/// <see cref="T:System.Threading.CancellationToken">CancellationToken</see>.
/// If the target callback is currently executing this method will wait until it completes, except
/// in the degenerate cases where a callback method deregisters itself.
/// </summary>
public void Dispose()
{
CancellationTokenSource.CallbackNode node = _node;
if (node != null && !node.Partition.Unregister(_id, node))
{
WaitForCallbackIfNecessary();
}
}
static void WaitForCallbackIfNecessary(long id, CancellationTokenSource.CallbackNode node)
{
// We're a valid registration but we were unable to unregister, which means the callback wasn't in the list,
// which means either it already executed or it's currently executing. We guarantee that we will not return
// if the callback is being executed (assuming we are not currently called by the callback itself)
// We achieve this by the following rules:
// 1. If we are called in the context of an executing callback, no need to wait (determined by tracking callback-executor threadID)
// - if the currently executing callback is this CTR, then waiting would deadlock. (We choose to return rather than deadlock)
// - if not, then this CTR cannot be the one executing, hence no need to wait
// 2. If unregistration failed, and we are on a different thread, then the callback may be running under control of cts.Cancel()
// => poll until cts.ExecutingCallback is not the one we are trying to unregister.
CancellationTokenSource source = node.Registrations.Source;
if (source.IsCancellationRequested && // Running callbacks has commenced.
!source.IsCancellationCompleted && // Running callbacks hasn't finished.
node.Registrations.ThreadIDExecutingCallbacks != Environment.CurrentManagedThreadId) // The executing thread ID is not this thread's ID.
{
// Callback execution is in progress, the executing thread is different from this thread and has taken the callback for execution
// so observe and wait until this target callback is no longer the executing callback.
node.Registrations.WaitForCallbackToComplete(id);
}
}
/// <summary>
/// Disposes of the registration and unregisters the target callback from the associated
/// <see cref="T:System.Threading.CancellationToken">CancellationToken</see>.
/// </summary>
internal bool TryDeregister() // corefx currently has an InternalsVisibleTo dependency on this
{
CancellationTokenSource.CallbackNode node = _node;
return(node != null && node.Partition.Unregister(_id, node));
}
internal CancellationTokenRegistration(long id, CancellationTokenSource.CallbackNode node)
{
_id = id;
_node = node;
}
/// <summary>
/// Disposes of the registration and unregisters the target callback from the associated
/// <see cref="T:System.Threading.CancellationToken">CancellationToken</see>.
/// </summary>
public bool Unregister()
{
CancellationTokenSource.CallbackNode node = _node;
return(node != null && node.Partition.Unregister(_id, node));
}
/// <summary>
/// Disposes of the registration and unregisters the target callback from the associated
/// <see cref="T:System.Threading.CancellationToken">CancellationToken</see>.
/// </summary>
public bool Unregister() // TODO dotnet/corefx#14903: This is "public" for corefx usage, but it's not yet in the refs or approved public API.
{
CancellationTokenSource.CallbackNode node = _node;
return(node != null && node.Partition.Unregister(_id, node));
}
