Don't use callbacks when performing sync IO with async handles

[adamsitnik]

In RandomAccess.Windows.cs, we create a dedicated callback:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs

Line 19 in ad023e0

private static readonly IOCompletionCallback s_callback = AllocateCallback();

Then provide it in these places:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs

Line 731 in ad023e0

NativeOverlapped* result = handle.ThreadPoolBinding!.UnsafeAllocateNativeOverlapped(s_callback, resetEvent, null);

And handle the callback in a dedicated class:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs

Lines 801 to 825 in ad023e0

	// We need to store the reference count (see the comment in ReleaseRefCount) and an EventHandle to signal the completion.
	// We could keep these two things separate, but since ManualResetEvent is sealed and we want to avoid any extra allocations, this type has been created.
	// It's basically ManualResetEvent with reference count.
	private sealed class CallbackResetEvent : EventWaitHandle
	{
	private readonly ThreadPoolBoundHandle _threadPoolBoundHandle;
	private int _freeWhenZero = 2; // one for the callback and another for the method that calls GetOverlappedResult
	internal CallbackResetEvent(ThreadPoolBoundHandle threadPoolBoundHandle) : base(initialState: false, EventResetMode.ManualReset)
	{
	_threadPoolBoundHandle = threadPoolBoundHandle;
	}
	internal unsafe void ReleaseRefCount(NativeOverlapped* pOverlapped)
	{
	// Each SafeFileHandle opened for async IO is bound to ThreadPool.
	// It requires us to provide a callback even if we want to use EventHandle and use GetOverlappedResult to obtain the result.
	// There can be a race condition between the call to GetOverlappedResult and the callback invocation,
	// so we need to track the number of references, and when it drops to zero, then free the native overlapped.
	if (Interlocked.Decrement(ref _freeWhenZero) == 0)
	{
	_threadPoolBoundHandle.FreeNativeOverlapped(pOverlapped);
	}
	}
	}

Based on my finding from #126807, we don't need to provide a callback when configuring WaitHandle for overlapped provided by ReadFile/WriteFile. What we need to do is setting the low-order bit of hEvent in the OVERLAPPED structure:

runtime/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Multiplexing.Windows.cs

Lines 146 to 152 in ce6b594

	private static unsafe NativeOverlapped* AllocateOverlapped(EventWaitHandle waitHandle)
	{
	NativeOverlapped* overlapped = (NativeOverlapped*)NativeMemory.AllocZeroed((nuint)sizeof(NativeOverlapped));
	overlapped->EventHandle = SetLowOrderBit(waitHandle);
	return overlapped;
	}

runtime/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Multiplexing.Windows.cs

Lines 218 to 226 in ce6b594

	/// <summary>
	/// Returns the event handle with the low-order bit set.
	/// Per https://learn.microsoft.com/windows/win32/api/ioapiset/nf-ioapiset-getqueuedcompletionstatus,
	/// setting the low-order bit of hEvent in the OVERLAPPED structure prevents the I/O completion
	/// from being queued to a completion port bound to the same file object. The kernel masks off
	/// the bit when signaling, so the event still works normally.
	/// </summary>
	private static nint SetLowOrderBit(EventWaitHandle waitHandle)
	=> waitHandle.SafeWaitHandle.DangerousGetHandle() | 1;

This should allow us to get rid of CallbackResetEvent, slightly reduce allocations and complexity. The perf might also get better.

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