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//------------------------------------------------------------------------------
// <copyright file="_LazyAsyncResult.cs" company="Microsoft">
// Copyright (c) Microsoft Corporation. All rights reserved.
// </copyright>
//------------------------------------------------------------------------------
namespace System.Net
{
using System.Threading;
using System.Diagnostics;
using System.Collections;
// LazyAsyncResult - Base class for all IAsyncResult classes
// that want to take advantage of lazy allocated event handles
internal class LazyAsyncResult : IAsyncResult
{
private const int c_HighBit = unchecked((int) 0x80000000);
private const int c_ForceAsyncCount = 50;
#if !NET_PERF
// This is to avoid user mistakes when they queue another async op from a callback the completes sync.
[ThreadStatic]
private static ThreadContext t_ThreadContext;
private static ThreadContext CurrentThreadContext
{
get
{
ThreadContext threadContext = t_ThreadContext;
if (threadContext == null)
{
threadContext = new ThreadContext();
t_ThreadContext = threadContext;
}
return threadContext;
}
}
private class ThreadContext
{
internal int m_NestedIOCount;
}
#endif
#if DEBUG
internal object _DebugAsyncChain; // Optionally used to track chains of async calls.
private bool _ProtectState; // Used by ContextAwareResult to prevent some calls.
#endif
#if TRACK_LAR
internal static Hashtable _PendingResults = Hashtable.Synchronized(new Hashtable());
internal static int _PendingIndex = 0;
internal int _MyIndex;
#endif
//
// class members
//
private object m_AsyncObject; // Caller's async object.
private object m_AsyncState; // Caller's state object.
private AsyncCallback m_AsyncCallback; // Caller's callback method.
private object m_Result; // Final IO result to be returned byt the End*() method.
private int m_ErrorCode; // Win32 error code for Win32 IO async calls (that want to throw).
private int m_IntCompleted; // Sign bit indicates synchronous completion if set.
// Remaining bits count the number of InvokeCallbak() calls.
private bool m_EndCalled; // true if the user called the End*() method.
private bool m_UserEvent; // true if the event has been (or is about to be) handed to the user
private object m_Event; // lazy allocated event to be returned in the IAsyncResult for the client to wait on
internal LazyAsyncResult(object myObject, object myState, AsyncCallback myCallBack) {
m_AsyncObject = myObject;
m_AsyncState = myState;
m_AsyncCallback = myCallBack;
m_Result = DBNull.Value;
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::.ctor()");
#if TRACK_LAR
_MyIndex = Interlocked.Increment(ref _PendingIndex);
_PendingResults.Add(_MyIndex, this);
#endif
}
// Allows creating a pre-completed result with less interlockeds. Beware! Constructor calls the callback.
// if a derived class ever uses this and overloads Cleanup, this may need to change
internal LazyAsyncResult(object myObject, object myState, AsyncCallback myCallBack, object result)
{
GlobalLog.Assert(result != DBNull.Value, "LazyAsyncResult#{0}::.ctor()|Result can't be set to DBNull - it's a special internal value.", ValidationHelper.HashString(this));
m_AsyncObject = myObject;
m_AsyncState = myState;
m_AsyncCallback = myCallBack;
m_Result = result;
m_IntCompleted = 1;
if (m_AsyncCallback != null) {
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::Complete() invoking callback");
m_AsyncCallback(this);
}
else {
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::Complete() no callback to invoke");
}
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::.ctor() (pre-completed)");
}
// Interface method to return the original async object:
internal object AsyncObject {
get {
return m_AsyncObject;
}
}
// Interface method to return the caller's state object.
public object AsyncState {
get {
return m_AsyncState;
}
}
protected AsyncCallback AsyncCallback
{
get
{
return m_AsyncCallback;
}
set
{
m_AsyncCallback = value;
}
}
// Interface property to return a WaitHandle that can be waited on for I/O completion.
// This property implements lazy event creation.
// the event object is only created when this property is accessed,
// since we're internally only using callbacks, as long as the user is using
// callbacks as well we will not create an event at all.
// If this is used, the event cannot be disposed because it is under the control of the
// application. Internal should use InternalWaitForCompletion instead - never AsyncWaitHandle.
public WaitHandle AsyncWaitHandle
{
get
{
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::get_AsyncWaitHandle()");
#if DEBUG
// Can't be called when state is protected.
if (_ProtectState)
{
throw new InvalidOperationException("get_AsyncWaitHandle called in protected state");
}
#endif
ManualResetEvent asyncEvent;
// Indicates that the user has seen the event; it can't be disposed.
m_UserEvent = true;
// The user has access to this object. Lock-in CompletedSynchronously.
if (m_IntCompleted == 0)
{
Interlocked.CompareExchange(ref m_IntCompleted, c_HighBit, 0);
}
// Because InternalWaitForCompletion() tries to dispose this event, it's
// possible for m_Event to become null immediately after being set, but only if
// IsCompleted has become true. Therefore it's possible for this property
// to give different (set) events to different callers when IsCompleted is true.
asyncEvent = (ManualResetEvent) m_Event;
while (asyncEvent == null)
{
LazilyCreateEvent(out asyncEvent);
}
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::get_AsyncWaitHandle() m_Event:" + ValidationHelper.HashString(m_Event));
return asyncEvent;
}
}
// Returns true if this call created the event.
// May return with a null handle. That means it thought it got one, but it was disposed in the mean time.
private bool LazilyCreateEvent(out ManualResetEvent waitHandle)
{
// lazy allocation of the event:
// if this property is never accessed this object is never created
waitHandle = new ManualResetEvent(false);
try
{
if (Interlocked.CompareExchange(ref m_Event, waitHandle, null) == null)
{
if (InternalPeekCompleted)
{
waitHandle.Set();
}
return true;
}
else
{
waitHandle.Close();
waitHandle = (ManualResetEvent) m_Event;
// There's a chance here that m_Event became null. But the only way is if another thread completed
// in InternalWaitForCompletion and disposed it. If we're in InternalWaitForCompletion, we now know
// IsCompleted is set, so we can avoid the wait when waitHandle comes back null. AsyncWaitHandle
// will try again in this case.
return false;
}
}
catch
{
// This should be very rare, but doing this will reduce the chance of deadlock.
m_Event = null;
if (waitHandle != null)
waitHandle.Close();
throw;
}
}
// This allows ContextAwareResult to not let anyone trigger the CompletedSynchronously tripwire while the context is being captured.
[Conditional("DEBUG")]
protected void DebugProtectState(bool protect)
{
#if DEBUG
_ProtectState = protect;
#endif
}
// Interface property, returning synchronous completion status.
public bool CompletedSynchronously {
get {
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::get_CompletedSynchronously()");
#if DEBUG
// Can't be called when state is protected.
if (_ProtectState)
{
throw new InvalidOperationException("get_CompletedSynchronously called in protected state");
}
#endif
// If this returns greater than zero, it means it was incremented by InvokeCallback before anyone ever saw it.
int result = m_IntCompleted;
if (result == 0)
{
result = Interlocked.CompareExchange(ref m_IntCompleted, c_HighBit, 0);
}
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::get_CompletedSynchronously() returns: "+((result>0)?"true":"false"));
return result > 0;
}
}
// Interface property, returning completion status.
public bool IsCompleted {
get {
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::get_IsCompleted()");
#if DEBUG
// Can't be called when state is protected.
if (_ProtectState)
{
throw new InvalidOperationException("get_IsCompleted called in protected state");
}
#endif
// Look at just the low bits to see if it's been incremented. If it hasn't, set the high bit
// to show that it's been looked at.
int result = m_IntCompleted;
if (result == 0)
{
result = Interlocked.CompareExchange(ref m_IntCompleted, c_HighBit, 0);
}
return (result & ~c_HighBit) != 0;
}
}
// Use to see if something's completed without fixing CompletedSynchronously
internal bool InternalPeekCompleted
{
get
{
return (m_IntCompleted & ~c_HighBit) != 0;
}
}
// Internal property for setting the IO result.
internal object Result {
get {
return m_Result == DBNull.Value ? null : m_Result;
}
set {
// Ideally this should never be called, since setting
// the result object really makes sense when the IO completes.
//
// But if the result was set here (as a preemptive error or for some other reason),
// then the "result" parameter passed to InvokeCallback() will be ignored.
//
// It's an error to call after the result has been completed or with DBNull.
GlobalLog.Assert(value != DBNull.Value, "LazyAsyncResult#{0}::set_Result()|Result can't be set to DBNull - it's a special internal value.", ValidationHelper.HashString(this));
GlobalLog.Assert(!InternalPeekCompleted, "LazyAsyncResult#{0}::set_Result()|Called on completed result.", ValidationHelper.HashString(this));
m_Result = value;
}
}
internal bool EndCalled {
get {
return m_EndCalled;
}
set {
m_EndCalled = value;
}
}
// Internal property for setting the Win32 IO async error code.
internal int ErrorCode {
get {
return m_ErrorCode;
}
set {
m_ErrorCode = value;
}
}
// A method for completing the IO with a result
// and invoking the user's callback.
// Used by derived classes to pass context into an overridden Complete(). Useful
// for determining the 'winning' thread in case several may simultaneously call
// the equivalent of InvokeCallback().
protected void ProtectedInvokeCallback(object result, IntPtr userToken)
{
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::ProtectedInvokeCallback() result = " +
(result is Exception? ((Exception)result).Message: result == null? "<null>": result.ToString()) +
", userToken:" + userToken.ToString());
// Critical to disallow DBNull here - it could result in a stuck spinlock in WaitForCompletion.
if (result == DBNull.Value)
{
throw new ArgumentNullException("result");
}
#if DEBUG
// Always safe to ask for the state now.
_ProtectState = false;
#endif
if ((m_IntCompleted & ~c_HighBit) == 0 && (Interlocked.Increment(ref m_IntCompleted) & ~c_HighBit) == 1)
{
// DBNull.Value is used to guarantee that the first caller wins,
// even if the result was set to null.
if (m_Result == DBNull.Value)
m_Result = result;
// Does this need a memory barrier to be sure this thread gets the m_Event if it's set? I don't think so
// because the Interlockeds on m_IntCompleted/m_Event should serve as the barrier.
ManualResetEvent asyncEvent = (ManualResetEvent) m_Event;
if (asyncEvent != null)
{
try {
asyncEvent.Set();
}
catch (ObjectDisposedException) {
// Simply ignore this exception - There is apparently a rare race condition
// where the event is disposed before the completion method is called.
}
}
Complete(userToken);
}
}
// A method for completing the IO with a result
// and invoking the user's callback.
internal void InvokeCallback(object result)
{
ProtectedInvokeCallback(result, IntPtr.Zero);
}
// A method for completing the IO without a result
// and invoking the user's callback.
internal void InvokeCallback()
{
ProtectedInvokeCallback(null, IntPtr.Zero);
}
//
// MUST NOT BE CALLED DIRECTLY
// A protected method that does callback job and it is guaranteed to be called exactly once.
// A derived overriding method must call the base class somewhere or the completion is lost.
//
protected virtual void Complete(IntPtr userToken)
{
#if !NET_PERF
bool offloaded = false;
ThreadContext threadContext = CurrentThreadContext;
try {
++threadContext.m_NestedIOCount;
#else
try
{
#endif
if (m_AsyncCallback != null) {
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::Complete() invoking callback");
#if !NET_PERF
if (threadContext.m_NestedIOCount >= c_ForceAsyncCount)
{
GlobalLog.Print("LazyAsyncResult::Complete *** OFFLOADED the user callback ***");
ThreadPool.QueueUserWorkItem(new WaitCallback(WorkerThreadComplete));
offloaded = true;
}
else
#endif
{
m_AsyncCallback(this);
}
}
else {
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::Complete() no callback to invoke");
}
}
finally {
#if !NET_PERF
--threadContext.m_NestedIOCount;
// Never call this method unless interlocked m_IntCompleted check has succeeded (like in this case)
if (!offloaded)
#endif
{
Cleanup();
}
}
}
#if !NET_PERF
// Only called in the above method
void WorkerThreadComplete(object state)
{
try
{
m_AsyncCallback(this);
}
finally
{
Cleanup();
}
}
#endif
// Custom instance cleanup method.
// Derived types override this method to release unmanaged resources associated with an IO request.
protected virtual void Cleanup()
{
#if TRACK_LAR
_PendingResults.Remove(_MyIndex);
#endif
}
internal object InternalWaitForCompletion()
{
return WaitForCompletion(true);
}
/*
internal object InternalWaitForCompletionNoSideEffects()
{
return WaitForCompletion(false);
}
*/
private object WaitForCompletion(bool snap) {
ManualResetEvent waitHandle = null;
bool createdByMe = false;
bool complete = snap ? IsCompleted : InternalPeekCompleted;
if (!complete)
{
// Not done yet, so wait:
waitHandle = (ManualResetEvent) m_Event;
if (waitHandle == null)
{
createdByMe = LazilyCreateEvent(out waitHandle);
}
}
if (waitHandle != null)
{
try
{
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::InternalWaitForCompletion() Waiting for completion m_Event#" + ValidationHelper.HashString(waitHandle));
waitHandle.WaitOne(Timeout.Infinite, false);
}
catch (ObjectDisposedException)
{
// This can occur if this method is called from two different threads.
// This possibility is the trade-off for not locking.
}
finally {
// We also want to dispose the event although we can't unless we did wait on it here.
if (createdByMe && !m_UserEvent)
{
// Does m_UserEvent need to be volatile (or m_Event set via Interlocked) in order
// to avoid giving a user a disposed event?
ManualResetEvent oldEvent = (ManualResetEvent) m_Event;
m_Event = null;
if (!m_UserEvent)
{
oldEvent.Close();
}
}
}
}
// A race condition exists because InvokeCallback sets m_IntCompleted before m_Result (so that m_Result
// can benefit from the synchronization of m_IntCompleted). That means you can get here before m_Result got
// set (although rarely - once every eight hours of stress). Handle that case with a spin-lock.
while (m_Result == DBNull.Value)
Thread.SpinWait(1);
GlobalLog.Print("LazyAsyncResult#" + ValidationHelper.HashString(this) + "::InternalWaitForCompletion() done: " +
(m_Result is Exception? ((Exception)m_Result).Message: m_Result == null? "<null>": m_Result.ToString()));
return m_Result;
}
// A general interface that is called to release unmanaged resources associated with the class.
// It completes the result but doesn't do any of the notifications.
internal void InternalCleanup()
{
if ((m_IntCompleted & ~c_HighBit) == 0 && (Interlocked.Increment(ref m_IntCompleted) & ~c_HighBit) == 1)
{
// Set no result so that just in case there are waiters, they don't hang in the spin lock.
m_Result = null;
Cleanup();
}
}
}
}
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