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// ==++==
//
// Copyright (c) Microsoft Corporation. All rights reserved.
//
// ==--==
// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
// YieldAwaitable.cs
//
// <OWNER>Microsoft</OWNER>
//
// Compiler-targeted type for switching back into the current execution context, e.g.
//
// await Task.Yield();
// =====================
// var $awaiter = Task.Yield().GetAwaiter();
// if (!$awaiter.IsCompleted)
// {
// $builder.AwaitUnsafeOnCompleted(ref $awaiter, ref this);
// return;
// Label:
// }
// $awaiter.GetResult();
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
using System;
using System.Security;
using System.Diagnostics;
using System.Diagnostics.Contracts;
using System.Diagnostics.Tracing;
using System.Threading;
using System.Threading.Tasks;
using System.Security.Permissions;
namespace System.Runtime.CompilerServices
{
// NOTE: YieldAwaitable currently has no state; while developers are encouraged to use Task.Yield() to produce one,
// no validation is performed to ensure that the developer isn't doing "await new YieldAwaitable()". Such validation
// would require additional, useless state to be stored, and as this is a type in the CompilerServices namespace, and
// as the above example isn't harmful, we take the cheaper approach of not validating anything.
/// <summary>Provides an awaitable context for switching into a target environment.</summary>
/// <remarks>This type is intended for compiler use only.</remarks>
public readonly struct YieldAwaitable
{
/// <summary>Gets an awaiter for this <see cref="YieldAwaitable"/>.</summary>
/// <returns>An awaiter for this awaitable.</returns>
/// <remarks>This method is intended for compiler user rather than use directly in code.</remarks>
public YieldAwaiter GetAwaiter() { return new YieldAwaiter(); }
/// <summary>Provides an awaiter that switches into a target environment.</summary>
/// <remarks>This type is intended for compiler use only.</remarks>
[HostProtection(Synchronization = true, ExternalThreading = true)]
public readonly struct YieldAwaiter : ICriticalNotifyCompletion
{
/// <summary>Gets whether a yield is not required.</summary>
/// <remarks>This property is intended for compiler user rather than use directly in code.</remarks>
public bool IsCompleted { get { return false; } } // yielding is always required for YieldAwaiter, hence false
/// <summary>Posts the <paramref name="continuation"/> back to the current context.</summary>
/// <param name="continuation">The action to invoke asynchronously.</param>
/// <exception cref="System.ArgumentNullException">The <paramref name="continuation"/> argument is null (Nothing in Visual Basic).</exception>
[SecuritySafeCritical]
public void OnCompleted(Action continuation)
{
QueueContinuation(continuation, flowContext: true);
}
/// <summary>Posts the <paramref name="continuation"/> back to the current context.</summary>
/// <param name="continuation">The action to invoke asynchronously.</param>
/// <exception cref="System.ArgumentNullException">The <paramref name="continuation"/> argument is null (Nothing in Visual Basic).</exception>
[SecurityCritical]
public void UnsafeOnCompleted(Action continuation)
{
QueueContinuation(continuation, flowContext: false);
}
/// <summary>Posts the <paramref name="continuation"/> back to the current context.</summary>
/// <param name="continuation">The action to invoke asynchronously.</param>
/// <param name="flowContext">true to flow ExecutionContext; false if flowing is not required.</param>
/// <exception cref="System.ArgumentNullException">The <paramref name="continuation"/> argument is null (Nothing in Visual Basic).</exception>
[SecurityCritical]
private static void QueueContinuation(Action continuation, bool flowContext)
{
// Validate arguments
if (continuation == null) throw new ArgumentNullException("continuation");
Contract.EndContractBlock();
#if !MONO
if (TplEtwProvider.Log.IsEnabled())
{
continuation = OutputCorrelationEtwEvent(continuation);
}
#endif
// Get the current SynchronizationContext, and if there is one,
// post the continuation to it. However, treat the base type
// as if there wasn't a SynchronizationContext, since that's what it
// logically represents.
var syncCtx = SynchronizationContext.CurrentNoFlow;
if (syncCtx != null && syncCtx.GetType() != typeof(SynchronizationContext))
{
syncCtx.Post(s_sendOrPostCallbackRunAction, continuation);
}
else
{
// If we're targeting the default scheduler, queue to the thread pool, so that we go into the global
// queue. As we're going into the global queue, we might as well use QUWI, which for the global queue is
// just a tad faster than task, due to a smaller object getting allocated and less work on the execution path.
TaskScheduler scheduler = TaskScheduler.Current;
if (scheduler == TaskScheduler.Default)
{
if (flowContext)
{
ThreadPool.QueueUserWorkItem(s_waitCallbackRunAction, continuation);
}
else
{
ThreadPool.UnsafeQueueUserWorkItem(s_waitCallbackRunAction, continuation);
}
}
// We're targeting a custom scheduler, so queue a task.
else
{
Task.Factory.StartNew(continuation, default(CancellationToken), TaskCreationOptions.PreferFairness, scheduler);
}
}
}
#if !MONO
private static Action OutputCorrelationEtwEvent(Action continuation)
{
int continuationId = Task.NewId();
Task currentTask = Task.InternalCurrent;
// fire the correlation ETW event
TplEtwProvider.Log.AwaitTaskContinuationScheduled(TaskScheduler.Current.Id, (currentTask != null) ? currentTask.Id : 0, continuationId);
return AsyncMethodBuilderCore.CreateContinuationWrapper(continuation, () =>
{
var etwLog = TplEtwProvider.Log;
etwLog.TaskWaitContinuationStarted(continuationId);
// ETW event for Task Wait End.
Guid prevActivityId = new Guid();
// Ensure the continuation runs under the correlated activity ID generated above
if (etwLog.TasksSetActivityIds)
EventSource.SetCurrentThreadActivityId(TplEtwProvider.CreateGuidForTaskID(continuationId), out prevActivityId);
// Invoke the original continuation provided to OnCompleted.
continuation();
// Restore activity ID
if (etwLog.TasksSetActivityIds)
EventSource.SetCurrentThreadActivityId(prevActivityId);
etwLog.TaskWaitContinuationComplete(continuationId);
});
}
#endif
/// <summary>WaitCallback that invokes the Action supplied as object state.</summary>
private static readonly WaitCallback s_waitCallbackRunAction = RunAction;
/// <summary>SendOrPostCallback that invokes the Action supplied as object state.</summary>
private static readonly SendOrPostCallback s_sendOrPostCallbackRunAction = RunAction;
/// <summary>Runs an Action delegate provided as state.</summary>
/// <param name="state">The Action delegate to invoke.</param>
private static void RunAction(object state) { ((Action)state)(); }
/// <summary>Ends the await operation.</summary>
public void GetResult() {} // Nop. It exists purely because the compiler pattern demands it.
}
}
}
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