SocketAsyncEventArgs的释放问题

起因是发现一个同事编写的程序运行两个月左右，占用了服务器20G左右的内存。用WinDbg查看发现存在大量的Async Pinned
Handles，而它们的gcroot都来自于SocketAsyncEventArgs。下面是场景的简易模拟代码（为了说明问题添加了手动GC）：

for (var i = 0; i < 1000; ++i)

{

    var endPoint = new IPEndPoint(IPAddress.Parse(host), port);

    var socket = new Socket(endPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp);

    socket.ReceiveBufferSize = bufferSize;

    socket.SendBufferSize = bufferSize;

var iocp = new SocketAsyncEventArgs();

    iocp.Completed += new EventHandler<SocketAsyncEventArgs>(OnIoSocketCompleted);

    iocp.SetBuffer(new Byte[bufferSize], 0, bufferSize);

    iocp.AcceptSocket = socket;
try

    {

        socket.Connect(endPoint);

        Console.WriteLine(i);

    }

    catch (SocketException ex)

    {

        Console.WriteLine(ex.Message);

    }
socket.Close();

    //iocp.Dispose();

}
GC.Collect();

GC.WaitForPendingFinalizers();

GC.Collect();

SocketAsyncEventArgs的SetBuffer函数内部会pin住buffer数据（查阅SetBufferInternal实现），它的析构函数会调用FreeOverlapped函数释放资源。而问题就是在于FreeOverlapped函数的实现和传递参数。来看一下Dispose、~SocketAsyncEventArgs的实现：

public void Dispose()

{

    this.m_DisposeCalled = true;

    if (Interlocked.CompareExchange(ref this.m_Operating, 2, 0) != 0)

    {

        return;

    }

    this.FreeOverlapped(false);

    GC.SuppressFinalize(this);

}

~SocketAsyncEventArgs()

{

    this.FreeOverlapped(true);

}

两者都会调用FreeOverlapped函数释放，但是一个传递了false、一个传递了true参数。再来看FreeOverlapped实现：

private void FreeOverlapped(bool checkForShutdown)

{

    if (!checkForShutdown || !NclUtilities.HasShutdownStarted)

    {

        if (this.m_PtrNativeOverlapped != null && !this.m_PtrNativeOverlapped.IsInvalid)

        {

            this.m_PtrNativeOverlapped.Dispose();

            this.m_PtrNativeOverlapped = null;

            this.m_Overlapped = null;

            this.m_PinState = SocketAsyncEventArgs.PinState.None;

            this.m_PinnedAcceptBuffer = null;

            this.m_PinnedSingleBuffer = null;

            this.m_PinnedSingleBufferOffset = 0;

            this.m_PinnedSingleBufferCount = 0;

        }

        if (this.m_SocketAddressGCHandle.IsAllocated)

        {

            this.m_SocketAddressGCHandle.Free();

        }

        if (this.m_WSAMessageBufferGCHandle.IsAllocated)

        {

            this.m_WSAMessageBufferGCHandle.Free();

        }

        if (this.m_WSARecvMsgWSABufferArrayGCHandle.IsAllocated)

        {

            this.m_WSARecvMsgWSABufferArrayGCHandle.Free();

        }

        if (this.m_ControlBufferGCHandle.IsAllocated)

        {

            this.m_ControlBufferGCHandle.Free();

        }

    }

}

用WinDbg查看gchandles统计：

Statistics:

              MT    Count    TotalSize Class Name

000007fb1bdb6ae8        1           24 System.Object

000007fb1bdb6b80        1           48 System.SharedStatics

000007fb1bdb7f58        1           64 System.Security.PermissionSet

000007fb1bdb6a10        1          160 System.ExecutionEngineException

000007fb1bdb6998        1          160 System.StackOverflowException

000007fb1bdb6920        1          160 System.OutOfMemoryException

000007fb1bdb6738        1          160 System.Exception

000007fb1bdb7b90        2          192 System.Threading.Thread

000007fb1bdb6c40        1          216 System.AppDomain

000007fb1bdb6a88        2          320 System.Threading.ThreadAbortException

000007fb1ae19770        6          336 System.Net.Logging+NclTraceSource

000007fb1bdbf958        3          480 System.RuntimeType+RuntimeTypeCache

000007fb1ae19900        6          480 System.Diagnostics.SourceSwitch

000007fb1bd64458        6        34520 System.Object[]

000007fb1b6f5e40     1000       112000 System.Threading.OverlappedData

Total 1033 objects

Handles:

    Strong Handles:       12

    Pinned Handles:       5

    Async Pinned Handles: 1000

    Weak Long Handles:    3

    Weak Short Handles:   13

确实存在1000个Async Pinned
Handles，也就是说无法通过SocketAsyncEventArgs的析构函数释放SafeNativeOverlapped相关的资源。将示例代码的"iocp.Dispose();"注释去除，并重新执行再次查看gchandles：

Statistics:

              MT    Count    TotalSize Class Name

000007fb1bdb6ae8        1           24 System.Object

000007fb1bdb6b80        1           48 System.SharedStatics

000007fb1bdb7f58        1           64 System.Security.PermissionSet

000007fb1bdb6a10        1          160 System.ExecutionEngineException

000007fb1bdb6998        1          160 System.StackOverflowException

000007fb1bdb6920        1          160 System.OutOfMemoryException

000007fb1bdb6738        1          160 System.Exception

000007fb1bdb7b90        2          192 System.Threading.Thread

000007fb1bdb6c40        1          216 System.AppDomain

000007fb1bdb6a88        2          320 System.Threading.ThreadAbortException

000007fb1ae19770        6          336 System.Net.Logging+NclTraceSource

000007fb1bdbf958        3          480 System.RuntimeType+RuntimeTypeCache

000007fb1ae19900        6          480 System.Diagnostics.SourceSwitch

000007fb1bd64458        6        34520 System.Object[]

Total 33 objects

Handles:

    Strong Handles:       12

    Pinned Handles:       5

    Weak Long Handles:    3

    Weak Short Handles:   13

1000个Async Pinned
Handles已不存在，但SocketAsyncEventArgs的析构函数从实现来看应该也可以完成释放，为什么失败了？

用!bpmd命令添加NclUtilities.HasShutdownStarted、 m_PtrNativeOverlapped.Dispose()的断点。

!bpmd System.dll System.Net.NclUtilities.get_HasShutdownStarted

!bpmd mscorlib.dll System.Runtime.InteropServices.SafeHandle.Dispose

SocketAsyncEventArgs的释放问题