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Fix NullReferenceException race condition in TraceLog.AllocLookup/FreeLookup #2387

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Merged
brianrob merged 2 commits into from
Mar 20, 2026
Merged

Fix NullReferenceException race condition in TraceLog.AllocLookup/FreeLookup #2387

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4a14ad3
Initial plan
Copilot Mar 19, 2026
be0b76a
Fix thread-safety race in AllocLookup/FreeLookup using Interlocked op…
Copilot Mar 19, 2026
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15 changes: 7 additions & 8 deletions src/TraceEvent/TraceLog.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3662,21 +3662,20 @@ private void AddTemplatesForParser(TraceEventParser parser, TraceLogEventSource
/// <returns></returns>
internal unsafe TraceEventDispatcher AllocLookup()
{
if (freeLookup == null)
// Use Interlocked.Exchange to atomically take the cached dispatcher (setting the field to null).
// This prevents a race where two threads both read the non-null value before either sets it to null.
TraceEventDispatcher ret = Interlocked.Exchange(ref freeLookup, null);
if (ret == null)
{
freeLookup = AddAllTemplatesToDispatcher(new TraceLogEventSource(events));
ret = AddAllTemplatesToDispatcher(new TraceLogEventSource(events));
}

TraceEventDispatcher ret = freeLookup;
freeLookup = null;
return ret;
}
internal unsafe void FreeLookup(TraceEventDispatcher lookup)
{
if (freeLookup == null)
{
freeLookup = lookup;
}
// Use Interlocked.CompareExchange to atomically store the dispatcher back only if the slot is empty.
Interlocked.CompareExchange(ref freeLookup, lookup, null);
}

private void InitializeFromFile(string etlxFilePath)
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