|
//------------------------------------------------------------------------------
// <copyright file="SynchronizationHelper.cs" company="Microsoft">
// Copyright (c) Microsoft Corporation. All rights reserved.
// </copyright>
//------------------------------------------------------------------------------
namespace System.Web.Util {
using System;
using System.Runtime.ExceptionServices;
using System.Threading;
using System.Threading.Tasks;
// This class is used by the AspNetSynchronizationContext to assist with scheduling tasks in a non-blocking fashion.
// Asynchronous work will be queued and will execute sequentially, never consuming more than a single thread at a time.
// Synchronous work will block and will execute on the current thread.
internal sealed class SynchronizationHelper {
private Task _completionTask; // the Task that will run when all in-flight operations have completed
private Thread _currentThread; // the Thread that's running the current Task; all threads must see the same value for this field
private Task _lastScheduledTask = CreateInitialTask(); // the last Task that was queued to this helper, used to hook future Tasks (not volatile since always accessed under lock)
private Task _lastScheduledTaskAsync = CreateInitialTask(); // the last async Task that was queued to this helper
private readonly object _lockObj = new object(); // synchronizes access to _lastScheduledTask
private int _operationsInFlight; // operation counter
private readonly ISyncContext _syncContext; // a context that wraps an operation with pre- and post-execution phases
private readonly Action<bool> _appVerifierCallback; // for making sure that developers don't try calling us after the request has completed
public SynchronizationHelper(ISyncContext syncContext) {
_syncContext = syncContext;
_appVerifierCallback = AppVerifier.GetSyncContextCheckDelegate(syncContext);
}
// If an operation results in an exception, this property will provide access to it.
public ExceptionDispatchInfo Error { get; set; }
// Helper to access the _currentThread field in a thread-safe fashion.
// It is not enough to mark the _currentThread field volatile, since that only guarantees
// read / write ordering and doesn't ensure that each thread sees the same value.
private Thread CurrentThread {
get { return Interlocked.CompareExchange(ref _currentThread, null, null); }
set { Interlocked.Exchange(ref _currentThread, value); }
}
// Returns the number of pending operations
public int PendingCount { get { return ChangeOperationCount(0); } }
public int ChangeOperationCount(int addend) {
int newOperationCount = Interlocked.Add(ref _operationsInFlight, addend);
if (newOperationCount == 0) {
// if an asynchronous completion operation is queued, run it
Task completionTask = Interlocked.Exchange(ref _completionTask, null);
if (completionTask != null) {
completionTask.Start();
}
}
return newOperationCount;
}
private void CheckForRequestStateIfRequired(bool checkForReEntry) {
if (_appVerifierCallback != null) {
_appVerifierCallback(checkForReEntry);
}
}
// Creates the initial hook that future operations can ride off of
private static Task CreateInitialTask() {
return Task.FromResult<object>(null);
}
// Takes control of this SynchronizationHelper instance synchronously. Asynchronous operations
// will be queued but will not be dispatched until control is released (by disposing of the
// returned object). This operation might block if a different thread is currently in
// control of the context.
public IDisposable EnterSynchronousControl() {
if (CurrentThread == Thread.CurrentThread) {
// If the current thread already has control of this context, there's nothing extra to do.
return DisposableAction.Empty;
}
// used to mark the end of the synchronous task
TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
Task lastTask;
lock (_lockObj) {
lastTask = _lastScheduledTask;
_lastScheduledTask = tcs.Task; // future work can be scheduled off this Task
}
// The original task may end up Faulted, which would make its Wait() method throw an exception.
// To avoid this, we instead wait on a continuation which is always guaranteed to complete successfully.
if (!lastTask.IsCompleted) { lastTask.ContinueWith(_ => { }, TaskContinuationOptions.ExecuteSynchronously).Wait(); }
CurrentThread = Thread.CurrentThread;
// synchronous control is released by marking the Task as complete
return new DisposableAction(() => {
CurrentThread = null;
tcs.TrySetResult(null);
});
}
public void QueueAsynchronous(Action action) {
CheckForRequestStateIfRequired(checkForReEntry: true);
ChangeOperationCount(+1);
// This method only schedules work; it doesn't itself do any work. The lock is held for a very
// short period of time.
lock (_lockObj) {
Task newTask = _lastScheduledTask.ContinueWith(_ => SafeWrapCallback(action), TaskScheduler.Default);
_lastScheduledTask = newTask; // the newly-created task is now the last one
}
}
// QueueAsynchronousAsync and SafeWrapCallbackAsync guarantee:
// 1. For funcs posted here, it's would first come, first complete.
// 2. There is no overlapping execution.
public void QueueAsynchronousAsync(Func<object, Task> func, object state) {
CheckForRequestStateIfRequired(checkForReEntry: true);
ChangeOperationCount(+1);
// This method only schedules work; it doesn't itself do any work. The lock is held for a very
// short period of time.
lock (_lockObj) {
// 1. Note that we are chaining newTask with _lastScheduledTaskAsync, not _lastScheduledTask.
// Chaining newTask with _lastScheduledTask would cause deadlock.
// 2. Unwrap() is necessary to be called here. When chaining multiple tasks using the ContinueWith
// method, your return type will be Task<T> whereas T is the return type of the delegate/method
// passed to ContinueWith. As the return type of an async delegate is a Task, you will end up with
// a Task<Task> and end up waiting for the async delegate to return you the Task which is done after
// the first await.
Task newTask = _lastScheduledTaskAsync.ContinueWith(
async _ => { await SafeWrapCallbackAsync(func, state); }).Unwrap();
_lastScheduledTaskAsync = newTask; // the newly-created task is now the last one
}
}
public void QueueSynchronous(Action action) {
CheckForRequestStateIfRequired(checkForReEntry: false);
if (CurrentThread == Thread.CurrentThread) {
// current thread already owns the context, so just execute inline to prevent deadlocks
action();
return;
}
ChangeOperationCount(+1);
using (EnterSynchronousControl()) {
SafeWrapCallback(action);
}
}
private void SafeWrapCallback(Action action) {
// This method will try to catch exceptions so that they don't bubble up to our
// callers. However, ThreadAbortExceptions will continue to bubble up.
try {
CurrentThread = Thread.CurrentThread;
ISyncContextLock syncContextLock = null;
try {
syncContextLock = (_syncContext != null) ? _syncContext.Enter() : null;
try {
action();
}
catch (Exception ex) {
Error = ExceptionDispatchInfo.Capture(ex);
}
}
finally {
if (syncContextLock != null) {
syncContextLock.Leave();
}
}
}
finally {
CurrentThread = null;
ChangeOperationCount(-1);
}
}
// This method does not run the func by itself. It simply queues the func into the existing
// syncContext queue.
private async Task SafeWrapCallbackAsync(Func<object, Task> func, object state) {
try {
TaskCompletionSource<Task> tcs = new TaskCompletionSource<Task>();
QueueAsynchronous(() => {
var t = func(state);
t.ContinueWith((_) => {
if (t.IsFaulted) {
tcs.TrySetException(t.Exception.InnerExceptions);
}
else if (t.IsCanceled) {
tcs.TrySetCanceled();
}
else {
tcs.TrySetResult(t);
}
}, TaskContinuationOptions.ExecuteSynchronously);
});
await tcs.Task;
}
catch (Exception ex) {
Error = ExceptionDispatchInfo.Capture(ex);
}
finally {
ChangeOperationCount(-1);
}
}
// Sets the continuation that will asynchronously execute when the pending operation counter
// hits zero. Returns true if asynchronous execution is expected, false if the operation
// counter is already at zero and the caller should run the continuation inline.
public bool TrySetCompletionContinuation(Action continuation) {
int newOperationCount = ChangeOperationCount(+1); // prevent the operation counter from hitting zero while we're setting the field
bool scheduledAsynchronously = (newOperationCount > 1);
if (scheduledAsynchronously) {
Interlocked.Exchange(ref _completionTask, new Task(continuation));
}
ChangeOperationCount(-1);
return scheduledAsynchronously;
}
}
}
|