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

[Copilot]

Description

When performing synchronous I/O on async file handles (FILE_FLAG_OVERLAPPED), RandomAccess.Windows.cs previously used ThreadPoolBoundHandle.UnsafeAllocateNativeOverlapped with an IOCompletionCallback and a custom CallbackResetEvent class that tracked a reference count to handle the race between the IOCP callback and the caller's GetOverlappedResult.

This callback path is unnecessary. Setting the low-order bit of hEvent in the OVERLAPPED structure prevents the I/O completion from being queued to the completion port, eliminating the callback.

Changes:

• Allocate NativeOverlapped via NativeMemory.AllocZeroed instead of ThreadPoolBoundHandle
• Set EventHandle = handle | 1 to suppress IOCP notification
• Cache a ManualResetEvent in SafeFileHandle using an Interlocked rent/return pattern (following the existing _reusableOverlapped caching pattern in SafeFileHandle.OverlappedValueTaskSource.Windows.cs) to avoid creating/disposing an event on every sync I/O call
• Wait for I/O completion via ManualResetEvent.WaitOne() only for ERROR_IO_PENDING (not ERROR_SUCCESS) to respect SynchronizationContext and COM message pumping while avoiding unnecessary waits
• Fix use-after-free when WaitOne() throws arbitrary exceptions (e.g., via SynchronizationContext): catch any exception, cancel the pending I/O with CancelIoEx, then wait for completion via GetOverlappedResult(bWait: true) before freeing the overlapped
• Free overlapped with NativeMemory.Free and return cached event in finally
• Remove s_callback, AllocateCallback(), GetNativeOverlappedForAsyncHandle(), and CallbackResetEvent
• Remove EnsureThreadPoolBindingInitialized() from sync-over-async paths (no longer uses ThreadPoolBoundHandle)
• Remove unnecessary overlapped->InternalLow = IntPtr.Zero in EOF path (only needed for ThreadPoolBoundHandle.FreeNativeOverlapped assertion, not NativeMemory.Free)
• Fix sync-over-async read/write error handling to only call GetLastWin32ErrorAndDisposeHandleIfInvalid when ReadFile/WriteFile returns failure (0), and treat synchronous completion as ERROR_SUCCESS (avoids using undefined/stale GetLastError values)
• Add a ThrowingSynchronizationContext test to demonstrate exceptions from WaitOne() are handled safely without memory corruption
• Update that test to use SafeFileHandle.CreateAnonymousPipe(..., asyncRead: true, asyncWrite: false) so pending I/O is exercised deterministically, and verify the handle remains usable for subsequent reads with content validation after the failure path

This reduces allocations and complexity, preserves managed wait behavior, fixes a potential use-after-free when WaitOne throws via SynchronizationContext, and fixes incorrect error handling when overlapped operations complete synchronously.

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[adamsitnik]

@copilot please address my feedback (nits)

[adamsitnik]

@copilot address my feedback

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[adamsitnik]

@copilot address my feedback

[Copilot]

@copilot address my feedback

Done. Addressed in a8e69ef and c4d02a6: sync-over-async read/write now only consume last-error on Win32 failure, and the SynchronizationContext regression test now uses async anonymous pipes to reliably hit the pending wait path and verify post-failure usability.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[adamsitnik]

The changes LGTM, the perf is much better despite still using ManualResetEvent:

(managed_cached is the PR)

BenchmarkDotNet v0.16.0-nightly.20260302.459, Windows 11 (10.0.26200.8390/25H2/2025Update/HudsonValley2)
AMD Ryzen Threadripper PRO 3945WX 12-Cores 3.99GHz, 1 CPU, 24 logical and 12 physical cores
Memory: 63.86 GB Total, 32.62 GB Available
.NET SDK 11.0.100-preview.5.26255.101
  [Host]     : .NET 11.0.0 (11.0.0-preview.5.26255.101, 11.0.26.25601), X64 RyuJIT x86-64-v3

Method	Toolchain	fileSize	userBufferSize	options	Mean	Ratio	Allocated
Read	\main\corerun.exe	1024	1024	None	145.7 us	1.00	4360 B
Read	\main\corerun.exe	1024	1024	Asynchronous	173.9 us	1.00	4576 B
Read	\managed_cached\corerun.exe	1024	1024	Asynchronous	156.9 us	0.90	4464 B
Read	\native_cached\corerun.exe	1024	1024	Asynchronous	154.6 us	0.89	4440 B
Write	\main\corerun.exe	1024	1024	None	609.2 us	1.00	4360 B
Write	\main\corerun.exe	1024	1024	Asynchronous	639.3 us	1.00	4577 B
Write	\managed_cached\corerun.exe	1024	1024	Asynchronous	627.9 us	0.98	4465 B
Write	\native_cached\corerun.exe	1024	1024	Asynchronous	623.3 us	0.98	4441 B
Read	\main\corerun.exe	1048576	512	None	779.2 us	1.00	4361 B
Read	\main\corerun.exe	1048576	512	Asynchronous	2,756.3 us	1.00	49460 B
Read	\managed_cached\corerun.exe	1048576	512	Asynchronous	1,262.3 us	0.46	4466 B
Read	\native_cached\corerun.exe	1048576	512	Asynchronous	1,113.6 us	0.40	4442 B
Write	\main\corerun.exe	1048576	512	None	7,438.8 us	1.00	4365 B
Write	\main\corerun.exe	1048576	512	Asynchronous	7,788.8 us	1.00	49470 B
Write	\managed_cached\corerun.exe	1048576	512	Asynchronous	7,507.1 us	0.96	4473 B
Write	\native_cached\corerun.exe	1048576	512	Asynchronous	7,387.7 us	0.95	4449 B
Read	\main\corerun.exe	1048576	4096	None	709.8 us	1.00	241 B
Read	\main\corerun.exe	1048576	4096	Asynchronous	2,929.2 us	1.00	45340 B
Read	\managed_cached\corerun.exe	1048576	4096	Asynchronous	1,235.6 us	0.42	346 B
Read	\native_cached\corerun.exe	1048576	4096	Asynchronous	1,049.8 us	0.36	322 B
Write	\main\corerun.exe	1048576	4096	None	7,197.4 us	1.00	245 B
Write	\main\corerun.exe	1048576	4096	Asynchronous	7,838.3 us	1.00	45350 B
Write	\managed_cached\corerun.exe	1048576	4096	Asynchronous	7,351.9 us	0.94	353 B
Write	\native_cached\corerun.exe	1048576	4096	Asynchronous	7,477.0 us	0.96	329 B
Read	\main\corerun.exe	104857600	4096	None	82,969.7 us	1.00	352 B
Read	\main\corerun.exe	104857600	4096	Asynchronous	329,198.7 us	1.00	4506328 B
Read	\managed_cached\corerun.exe	104857600	4096	Asynchronous	154,884.8 us	0.47	568 B
Read	\native_cached\corerun.exe	104857600	4096	Asynchronous	118,937.8 us	0.36	544 B
Write	\main\corerun.exe	104857600	4096	None	172,746.9 us	1.00	464 B
Write	\main\corerun.exe	104857600	4096	Asynchronous	458,918.6 us	1.00	4506328 B
Write	\managed_cached\corerun.exe	104857600	4096	Asynchronous	208,779.5 us	0.46	792 B
Write	\native_cached\corerun.exe	104857600	4096	Asynchronous	170,257.2 us	0.37	768 B