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Fix locking regression #9438

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Merged
michaelstaib merged 2 commits into from
Mar 24, 2026
Merged

Fix locking regression #9438

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6f11b81
Fix locking regression
michaelstaib Mar 24, 2026
cf56dbf
Merge branch 'main' into mst/revert-optimization
michaelstaib Mar 24, 2026
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119 changes: 53 additions & 66 deletions src/HotChocolate/Core/src/Types/Fetching/AsyncAutoResetEvent.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,90 +5,69 @@ namespace HotChocolate.Fetching;

internal sealed class AsyncAutoResetEvent : INotifyCompletion
{
private const int Idle = 0;
private const int Signaled = 1;
private const int Waiting = 2;

private int _state;
#if NET9_0_OR_GREATER
private readonly Lock _sync = new();
#else
private readonly object _sync = new();
#endif
private Action? _continuation;
private bool _isSignaled;

public bool IsSignaled => Volatile.Read(ref _state) == Signaled;
public bool IsSignaled => Volatile.Read(ref _isSignaled);

public bool IsCompleted => Volatile.Read(ref _state) == Signaled;
public bool IsCompleted => false;

public void GetResult()
{
// Consume the signal when completing synchronously (via IsCompleted == true).
// CAS failure is benign (e.g. TryResetToIdle cleared it first).
Interlocked.CompareExchange(ref _state, Idle, Signaled);
}
public void GetResult() { }

public void OnCompleted(Action continuation)
{
Debug.Assert(_continuation is null, "There should only be one awaiter.");
_continuation = continuation;
bool wasSignaled;

while (true)
lock (_sync)
{
switch (Volatile.Read(ref _state))
{
case Idle:
// Register waiter: IDLE -> WAITING
if (Interlocked.CompareExchange(ref _state, Waiting, Idle) == Idle)
{
return;
}
break; // CAS failed, retry

case Signaled:
// Consume signal immediately: SIGNALED -> IDLE
if (Interlocked.CompareExchange(ref _state, Idle, Signaled) == Signaled)
{
_continuation = null;
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
return;
}
break; // CAS failed, retry
wasSignaled = _isSignaled;

default:
Debug.Fail("OnCompleted called while already waiting.");
return;
if (wasSignaled)
{
// consume the signal
_isSignaled = false;
}
else
{
Debug.Assert(_continuation is null, "There should only be one awaiter.");
_continuation = continuation;
}
}

if (wasSignaled)
{
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
}
}

public void Set()
{
while (true)
Action? continuation = null;

lock (_sync)
{
switch (Volatile.Read(ref _state))
if (_continuation is not null)
{
case Idle:
// Store signal: IDLE -> SIGNALED
if (Interlocked.CompareExchange(ref _state, Signaled, Idle) == Idle)
{
return;
}
break; // CAS failed, retry

case Waiting:
// Wake waiter: WAITING -> IDLE
if (Interlocked.CompareExchange(ref _state, Idle, Waiting) == Waiting)
{
var c = _continuation!;
_continuation = null;
ThreadPool.QueueUserWorkItem(static c => c(), c, preferLocal: true);
return;
}
break; // CAS failed, retry

case Signaled:
// Already signaled, nothing to do
return;

default:
return;
// someone is waiting - release them immediately
// we don't set _isSignaled since we're consuming it immediately
continuation = _continuation;
_continuation = null;
}
else
{
// since no one waiting we are storing the signal for the next awaiter
_isSignaled = true;
}
}

if (continuation is not null)
{
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
}
}

Expand All @@ -98,7 +77,15 @@ public void Set()
/// </summary>
public bool TryResetToIdle()
{
return Interlocked.CompareExchange(ref _state, Idle, Signaled) == Signaled;
lock (_sync)
{
if (_continuation is null && _isSignaled)
{
_isSignaled = false;
return true;
}
return false;
}
}

public AsyncAutoResetEvent GetAwaiter() => this;
Expand Down
119 changes: 53 additions & 66 deletions src/HotChocolate/Fusion/src/Fusion.Execution/Execution/AsyncAutoResetEvent.cs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -5,90 +5,69 @@ namespace HotChocolate.Fusion.Execution;

internal sealed class AsyncAutoResetEvent : INotifyCompletion
{
private const int Idle = 0;
private const int Signaled = 1;
private const int Waiting = 2;

private int _state;
#if NET9_0_OR_GREATER
private readonly Lock _sync = new();
#else
private readonly object _sync = new();
#endif
private Action? _continuation;
private bool _isSignaled;

public bool IsSignaled => Volatile.Read(ref _state) == Signaled;
public bool IsSignaled => Volatile.Read(ref _isSignaled);

public bool IsCompleted => Volatile.Read(ref _state) == Signaled;
public bool IsCompleted => false;

public void GetResult()
{
// Consume the signal when completing synchronously (via IsCompleted == true).
// CAS failure is benign (e.g. TryResetToIdle cleared it first).
Interlocked.CompareExchange(ref _state, Idle, Signaled);
}
public void GetResult() { }

public void OnCompleted(Action continuation)
{
Debug.Assert(_continuation is null, "There should only be one awaiter.");
_continuation = continuation;
bool wasSignaled;

while (true)
lock (_sync)
{
switch (Volatile.Read(ref _state))
{
case Idle:
// Register waiter: IDLE -> WAITING
if (Interlocked.CompareExchange(ref _state, Waiting, Idle) == Idle)
{
return;
}
break; // CAS failed, retry

case Signaled:
// Consume signal immediately: SIGNALED -> IDLE
if (Interlocked.CompareExchange(ref _state, Idle, Signaled) == Signaled)
{
_continuation = null;
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
return;
}
break; // CAS failed, retry
wasSignaled = _isSignaled;

default:
Debug.Fail("OnCompleted called while already waiting.");
return;
if (wasSignaled)
{
// consume the signal
_isSignaled = false;
}
else
{
Debug.Assert(_continuation is null, "There should only be one awaiter.");
_continuation = continuation;
}
}

if (wasSignaled)
{
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
}
}

public void Set()
{
while (true)
Action? continuation = null;

lock (_sync)
{
switch (Volatile.Read(ref _state))
if (_continuation is not null)
{
case Idle:
// Store signal: IDLE -> SIGNALED
if (Interlocked.CompareExchange(ref _state, Signaled, Idle) == Idle)
{
return;
}
break; // CAS failed, retry

case Waiting:
// Wake waiter: WAITING -> IDLE
if (Interlocked.CompareExchange(ref _state, Idle, Waiting) == Waiting)
{
var c = _continuation!;
_continuation = null;
ThreadPool.QueueUserWorkItem(static c => c(), c, preferLocal: true);
return;
}
break; // CAS failed, retry

case Signaled:
// Already signaled, nothing to do
return;

default:
return;
// someone is waiting - release them immediately
// we don't set _isSignaled since we're consuming it immediately
continuation = _continuation;
_continuation = null;
}
else
{
// since no one waiting we are storing the signal for the next awaiter
_isSignaled = true;
}
}

if (continuation is not null)
{
ThreadPool.QueueUserWorkItem(static c => c(), continuation, preferLocal: true);
}
}

Expand All @@ -98,7 +77,15 @@ public void Set()
/// </summary>
public bool TryResetToIdle()
{
return Interlocked.CompareExchange(ref _state, Idle, Signaled) == Signaled;
lock (_sync)
{
if (_continuation is null && _isSignaled)
{
_isSignaled = false;
return true;
}
return false;
}
}

public AsyncAutoResetEvent GetAwaiter() => this;
Expand Down
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