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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
using System.Diagnostics;
using System.Runtime.InteropServices;
namespace System.Threading
{
/// <summary>
/// A LIFO semaphore implemented using Win32 IO Completion Ports.
/// </summary>
/// <remarks>
/// IO Completion ports release waiting threads in LIFO order, so we can use them to create a LIFO semaphore.
/// See https://msdn.microsoft.com/en-us/library/windows/desktop/aa365198(v=vs.85).aspx under How I/O Completion Ports Work.
/// From the docs "Threads that block their execution on an I/O completion port are released in last-in-first-out (LIFO) order."
/// </remarks>
internal sealed class LowLevelLifoSemaphore : IDisposable
{
private IntPtr _completionPort;
public LowLevelLifoSemaphore(int initialSignalCount, int maximumSignalCount)
{
Debug.Assert(initialSignalCount >= 0, "Windows LowLevelLifoSemaphore does not support a negative signal count"); // TODO: Track actual signal count to enable this
_completionPort = Interop.Kernel32.CreateIoCompletionPort(new IntPtr(-1), IntPtr.Zero, UIntPtr.Zero, 1);
if (_completionPort == IntPtr.Zero)
{
var error = Marshal.GetLastWin32Error();
var exception = new OutOfMemoryException();
exception.HResult = error;
throw exception;
}
Release(initialSignalCount);
}
public bool Wait(int timeoutMs)
{
bool success = Interop.Kernel32.GetQueuedCompletionStatus(_completionPort, out var numberOfBytes, out var completionKey, out var pointerToOverlapped, timeoutMs);
Debug.Assert(success || (Marshal.GetLastWin32Error() == WaitHandle.WaitTimeout));
return success;
}
public int Release(int count)
{
for (int i = 0; i < count; i++)
{
if(!Interop.Kernel32.PostQueuedCompletionStatus(_completionPort, 1, UIntPtr.Zero, IntPtr.Zero))
{
var lastError = Marshal.GetLastWin32Error();
var exception = new OutOfMemoryException();
exception.HResult = lastError;
throw exception;
}
}
return 0; // TODO: Track actual signal count to calculate this
}
public void Dispose()
{
Interop.Kernel32.CloseHandle(_completionPort);
}
}
}
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