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

[Copilot]

Description

When performing synchronous IO 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 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 entirely.

Changes:

• Allocate NativeOverlapped via NativeMemory.AllocZeroed instead of ThreadPoolBoundHandle
• Set EventHandle = handle | 1 to suppress IOCP notification
• Create a native event handle via CreateEventExW with minimal access rights (SYNCHRONIZE | EVENT_MODIFY_STATE) instead of managed ManualResetEvent
• Wait for I/O completion via GetOverlappedResult(bWait: true) instead of managed WaitOne() — this avoids use-after-free if WaitOne throws (e.g., via SynchronizationContext callback) since GetOverlappedResult is a native blocking call that cannot throw managed exceptions
• Free overlapped with NativeMemory.Free and close event handle with CloseHandle in finally — no ref counting needed
• Remove s_callback, AllocateCallback(), GetNativeOverlappedForAsyncHandle(), CallbackResetEvent class
• 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)
• Add new CreateEventExW P/Invoke overload returning raw IntPtr for use without SafeWaitHandle

Reduces allocations and complexity with no behavioral change, and fixes a potential use-after-free when WaitOne throws arbitrary exceptions via SynchronizationContext.

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[adamsitnik]

@copilot please address my feedback (nits)

[Copilot]

@copilot please invoke the code-review skill and post your analysis/results as a comment on this PR

🤖 Copilot Code Review — PR #126845

Note

This review was AI/Copilot-generated via the code-review skill, using multi-model analysis (Gemini 2.5 Pro + GPT-5.3-Codex + my own assessment).

Holistic Assessment

Motivation: Justified. The old code used ThreadPoolBoundHandle.UnsafeAllocateNativeOverlapped with a CallbackResetEvent ref-counting scheme to handle the race between the IOCP callback and GetOverlappedResult. This was necessary because the event handle alone did not suppress IOCP notifications. The Windows API provides a documented mechanism (setting the low-order bit of hEvent) to prevent completion packets from being queued to the IOCP, making the entire callback infrastructure unnecessary.

Approach: Sound. The hEvent | 1 technique is well-documented in the GetQueuedCompletionStatus documentation. This eliminates the callback, the ref counting, and the thread pool binding requirement for these sync paths — a clean simplification with −71/+13 lines.

Summary: ⚠️ Needs Human Review. The core approach is correct and well-documented. One concern about resource lifetime during exceptional WaitOne() interruption was flagged by one model but is pre-existing in the old code. Human reviewers should confirm they are comfortable with this pre-existing behavior.

---

Detailed Findings

✅ Core Approach — hEvent low-order bit to suppress IOCP notifications

Setting result->EventHandle = waitHandle.SafeWaitHandle.DangerousGetHandle() | 1 is the canonical Windows pattern for performing overlapped I/O on an IOCP-bound handle without queuing a completion packet. This is documented in the GetQueuedCompletionStatus and OVERLAPPED documentation. Both sub-agent models and my own analysis confirm this is correct.

✅ Removal of EnsureThreadPoolBindingInitialized() from sync paths

Since the overlapped is now allocated via NativeMemory.AllocZeroed rather than ThreadPoolBoundHandle.UnsafeAllocateNativeOverlapped, there is no need to initialize the thread pool binding for these sync-over-async paths. The async paths (QueueAsyncReadFile, etc.) still call EnsureThreadPoolBindingInitialized() correctly. Verified via grep that all remaining async callers still invoke it.

✅ Removal of CallbackResetEvent ref-counting and else error branch

The old else branch (for errors other than ERROR_IO_PENDING/ERROR_SUCCESS) only called resetEvent.ReleaseRefCount(overlapped) to balance the ref count since the callback wouldn't fire. With no callback in the new code, this branch is unnecessary — the finally block handles cleanup uniformly. The entire CallbackResetEvent class, s_callback field, and AllocateCallback() method are correctly removed.

✅ DangerousGetHandle() lifetime safety

The ManualResetEvent is created at method entry and not disposed until the finally block, which runs after the overlapped I/O has completed (via WaitOne + GetOverlappedResult). The native event handle remains valid for the full duration of the overlapped operation in all normal execution paths.

⚠️ Resource lifetime if WaitOne() throws (pre-existing) — Needs Human Review

One model flagged that if resetEvent.WaitOne() throws (e.g., ThreadInterruptedException), the finally block will free the NativeOverlapped* and dispose the event while the kernel I/O may still be pending. However, this concern is pre-existing:

• In the old code, the finally block called resetEvent.ReleaseRefCount(overlapped) (decrementing ref count to 1, deferring the free) but still called resetEvent.Dispose(), closing the event handle the kernel is trying to signal.
• In both old and new code, the fixed block exits when the exception propagates, unpinning the buffer the kernel is writing to.

So the old code was equally vulnerable to buffer and event-handle use-after-free in this scenario — it only deferred the overlapped free itself. ThreadInterruptedException is legacy, and @adamsitnik already noted he defers to other reviewers on this concern. The new code is not materially worse than the old code for this edge case.

💡 overlapped->InternalLow = IntPtr.Zero in EOF path

The line overlapped->InternalLow = IntPtr.Zero (line 104) with the comment "failure to do so looks like we are freeing a pending overlapped later" was present in the old code. In the old code, this was needed because ThreadPoolBoundHandle.FreeNativeOverlapped asserts the overlapped is not pending. With NativeMemory.Free, no such assertion exists. This line is harmless but may be removable as a follow-up cleanup. Low priority.

---

Test Coverage

The existing test file RandomAccess/Mixed.Windows.cs specifically tests all combinations of sync/async read/write on both File.OpenHandle handles (with ThreadPoolBinding initialized) and raw CreateFileW handles (without binding). This covers the exact paths modified by this PR.

No New Public API Surface

No ref assembly changes detected. No new public members introduced. API approval check is not applicable.

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[adamsitnik]

💡 overlapped->InternalLow = IntPtr.Zero in EOF path

The line overlapped->InternalLow = IntPtr.Zero (line 104) with the comment "failure to do so looks like we are freeing a pending overlapped later" was present in the old code. In the old code, this was needed because ThreadPoolBoundHandle.FreeNativeOverlapped asserts the overlapped is not pending. With NativeMemory.Free, no such assertion exists. This line is harmless but may be removable as a follow-up cleanup. Low priority.

@copilot please address this

[Copilot]

💡 overlapped->InternalLow = IntPtr.Zero in EOF path

The line overlapped->InternalLow = IntPtr.Zero (line 104) with the comment "failure to do so looks like we are freeing a pending ov...

Removed the overlapped->InternalLow = IntPtr.Zero line and its comment in the EOF path of ReadSyncUsingAsyncHandle. This was only needed because ThreadPoolBoundHandle.FreeNativeOverlapped asserts that the overlapped is not pending — NativeMemory.Free has no such assertion. The two remaining occurrences in async paths (lines 273, 579) still use ThreadPoolBoundHandle.FreeNativeOverlapped and correctly remain. Done in 99c34d4.

[adamsitnik]

@copilot please invoke the code-review skill and post your analysis/results as a comment on this PR

[Copilot]

@copilot please invoke the code-review skill and post your analysis/results as a comment on this PR

🤖 Copilot Code Review — PR #126845

Note

This review was AI/Copilot-generated via the code-review skill, using multi-model analysis (Gemini 2.5 Pro + GPT-5.3-Codex + my own assessment).

Holistic Assessment

Motivation: Justified. The old code used ThreadPoolBoundHandle.UnsafeAllocateNativeOverlapped with a CallbackResetEvent ref-counting scheme to handle the race between the IOCP callback and GetOverlappedResult. The Windows API provides a documented, simpler mechanism (setting the low-order bit of hEvent) to suppress IOCP completion queuing entirely.

Approach: Sound. The hEvent | 1 technique is well-documented in the GetQueuedCompletionStatus documentation. This eliminates the callback, the ref counting, the CallbackResetEvent class, and the thread pool binding requirement for sync paths — a clean simplification.

Summary: ✅ LGTM. All three review models (Gemini 2.5 Pro, GPT-5.3-Codex, and my own analysis) agree the change is correct. No blocking or advisory issues remain after the latest commit (99c34d4) removed the unnecessary InternalLow zeroing.

---

Detailed Findings

✅ Core Approach — hEvent low-order bit to suppress IOCP notifications (all 3 models agree)

Setting result->EventHandle = waitHandle.SafeWaitHandle.DangerousGetHandle() | 1 is the canonical Windows pattern for overlapped I/O on an IOCP-bound handle without queuing a completion packet. Well-documented and correct.

✅ NativeMemory.AllocZeroed / NativeMemory.Free lifecycle (all 3 models agree)

The overlapped is allocated per-call in the try block and unconditionally freed in finally. Since the method waits for completion (WaitOne + GetOverlappedResult) before reaching finally in all normal paths, the lifetime is correctly scoped.

✅ Removal of EnsureThreadPoolBindingInitialized() from sync paths (all 3 models agree)

Since the overlapped is now allocated via NativeMemory.AllocZeroed rather than ThreadPoolBoundHandle.UnsafeAllocateNativeOverlapped, the thread pool binding is unnecessary for these paths. All remaining async callers (QueueAsyncReadFile, QueueAsyncWriteFile, etc.) still call EnsureThreadPoolBindingInitialized().

✅ Removal of CallbackResetEvent and the else error branch (all 3 models agree)

The old else branch only balanced the ref count since the callback wouldn't fire on immediate errors. With no callback, this is unnecessary — the finally block handles cleanup uniformly. The entire CallbackResetEvent class, s_callback field, and AllocateCallback() method are correctly removed.

✅ Removal of overlapped->InternalLow = IntPtr.Zero in EOF path (all 3 models agree)

This was needed because ThreadPoolBoundHandle.FreeNativeOverlapped asserts the overlapped is not pending. NativeMemory.Free has no such assertion. The two remaining occurrences (lines 273, 579 in async read paths) correctly remain since they still use ThreadPoolBoundHandle.FreeNativeOverlapped.

✅ Thread safety (all 3 models agree)

Each call creates a dedicated ManualResetEvent and NativeOverlapped, ensuring operations on the same file handle from different threads don't interfere. The hEvent | 1 pattern isolates each operation from the IOCP.

✅ DangerousGetHandle() lifetime safety

The ManualResetEvent is created at method entry and disposed only in finally after the I/O completes. The native event handle remains valid throughout the overlapped operation.

✅ Test coverage

The existing RandomAccess/Mixed.Windows.cs tests all combinations of sync/async read/write on both File.OpenHandle handles (with ThreadPoolBinding) and raw CreateFileW handles (without binding), covering the exact paths modified.

---

No New Public API Surface

No ref assembly changes detected. No new public members introduced.

[adamsitnik]

LGTM, to be honest I have not expected such nice performance improvements (EgorBot/Benchmarks#122).

This is definitely going to improve synchronous std out and err reads as well.

[adamsitnik]

@EgorBot -windows_x64 -windows_arm64 --filter "*Perf_FileStream.Read(Asynchronous" "*Perf_FileStream.Write(Asynchronous" "Perf_Process.ReadOutputToEnd"

[adamsitnik]

@copilot please give my suggestion a try

[adamsitnik]

@copilot let's try to implement following approach:

• don't allocate ManualResetEvent, instead perform the windows sys-call with minimal access right required to perform ReadFile

CreateEventExW(
    NULL,
    NULL,
    CREATE_EVENT_MANUAL_RESET,
    SYNCHRONIZE | EVENT_MODIFY_STATE
);

• pass the event to AllocNativeOverlappedWithEventHandle instead of ManualResetEvent
• instead of waiting on the event handle, pass bWait = true to GetOverlappedResult
• close the event handle your own by calling CloseHandle

[jkotas]

instead of waiting on the event handle, pass bWait = true to GetOverlappedResult

This will be an observable behavior change if the user code uses SynchronizationContext and overrides waiting....

[Copilot]

@copilot please invoke the code-review skill and post your analysis/results as a comment on this PR

🤖 Copilot Code Review — PR #126845

Note

This review was AI/Copilot-generated via the code-review skill, using multi-model analysis (Gemini 2.5 Pro + GPT-5.3-Codex + my own assessment).

Holistic Assessment

Motivation: Justified. The old code used ThreadPoolBoundHandle.UnsafeAllocateNativeOverlapped with a CallbackResetEvent ref-counting scheme. The ManualResetEvent.WaitOne() replacement was a step forward, but @jkotas correctly identified that WaitOne can throw arbitrary exceptions (via SynchronizationContext), leading to potential use-after-free of the NativeOverlapped*.

Approach: Sound. Using native CreateEventExW + GetOverlappedResult(bWait: true) eliminates all managed wait paths, ensuring the I/O is guaranteed to complete before the finally block frees resources. The hEvent | 1 technique remains correct for IOCP suppression.

Summary: ✅ LGTM. All three review models (Gemini 2.5 Pro, GPT-5.3-Codex, and my own analysis) agree the change is correct. The latest commit (d0fc732f) addresses the use-after-free concern by removing all managed wait paths.

---

Detailed Findings

✅ Core Approach — Native event + GetOverlappedResult(bWait: true) (all 3 models agree)

Using GetOverlappedResult with bWait: true performs the wait in the kernel via the event handle in the OVERLAPPED. This is a native blocking call that cannot throw managed exceptions, eliminating the use-after-free risk identified by @jkotas. The I/O is guaranteed to complete before the finally block runs.

✅ CreateEventExW with minimal access rights (all 3 models agree)

CreateSyncOverlappedEvent() creates a native manual-reset event with SYNCHRONIZE | EVENT_MODIFY_STATE — the minimal rights needed for GetOverlappedResult to wait on it. The IntPtr-returning CreateEventExNoName P/Invoke avoids SafeWaitHandle allocation overhead. Error checking for IntPtr.Zero is correct.

✅ Resource lifecycle safety (all 3 models agree)

Both NativeMemory.Free(overlapped) and Interop.Kernel32.CloseHandle(eventHandle) in finally are now safe because GetOverlappedResult(bWait: true) guarantees I/O completion before any normal return or exception from the try block. If ReadFile/WriteFile fails immediately (not ERROR_IO_PENDING or ERROR_SUCCESS), no I/O is pending, so NativeMemory.Free is also safe.

✅ Merged ERROR_IO_PENDING/ERROR_SUCCESS handling (all 3 models agree)

The if (errorCode is Interop.Errors.ERROR_IO_PENDING or Interop.Errors.ERROR_SUCCESS) pattern is correct. GetOverlappedResult(bWait: true) handles both cases: if pending, it waits; if already complete, it returns immediately. This simplifies the control flow.

✅ EnsureThreadPoolBindingInitialized removal from sync paths (all 3 models agree)

Verified via grep: all four remaining EnsureThreadPoolBindingInitialized calls (lines 235, 328, 543, 640) are in the async paths (QueueAsyncReadFile, QueueAsyncWriteFile, and scatter/gather variants).

✅ No new public API surface

No ref assembly changes. No new public members. The CreateEventExNoName P/Invoke is internal.

✅ Test coverage

Existing RandomAccess/Mixed.Windows.cs tests all combinations of sync/async read/write on both File.OpenHandle and raw CreateFileW handles, covering the exact paths modified.

@copilot please give my suggestion a try

Implemented in d0fc732f. Both ReadSyncUsingAsyncHandle and WriteSyncUsingAsyncHandle now:

• Create a native event via CreateEventExW(NULL, NULL, CREATE_EVENT_MANUAL_RESET, SYNCHRONIZE | EVENT_MODIFY_STATE)
• Pass the raw IntPtr event handle to AllocNativeOverlappedWithEventHandle
• Use GetOverlappedResult(bWait: true) instead of managed WaitOne()
• Close the event via CloseHandle in finally

[adamsitnik]

@EgorBot -windows_x64 -windows_arm64 -commits d0fc732,d0fc732~1 --filter "*Perf_FileStream.Read(Asynchronous" "*Perf_FileStream.Write(Asynchronous" "Perf_Process.ReadOutputToEnd"

[adamsitnik]

@EgorBot -windows_x64 -windows_arm64 --filter "*Perf_FileStream.Read(Asynchronous" "*Perf_FileStream.Write(Asynchronous" "Perf_Process.ReadOutputToEnd"