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March 24, 2026 17:51
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Pull request overview
Fixes a Monitor.Wait reentrancy issue where nested waits on the same thread could share a single thread-static waiter/event, allowing one wait to consume (“steal”) another wait’s pulse, particularly when SynchronizationContext.Wait pumps messages.
Changes:
- Adjust
Condition.Waiterthread-static reuse to clear the cached waiter while a wait is in progress, ensuring overlapping waits use distinctAutoResetEventinstances. - Add a regression test that reproduces the message-pumping reentrant wait scenario via a custom
SynchronizationContext.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| src/libraries/System.Private.CoreLib/src/System/Threading/Condition.cs | Updates waiter caching logic so overlapping waits don’t share the same thread-static waiter/event. |
| src/libraries/System.Threading/tests/MonitorTests.cs | Adds a regression test validating reentrant waits don’t steal each other’s pulse signals. |
src/libraries/System.Private.CoreLib/src/System/Threading/Condition.cs
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…ition.cs Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
jkotas
approved these changes
Mar 25, 2026
jkoritzinsky
approved these changes
Mar 25, 2026
VSadov
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Mar 25, 2026
src/libraries/System.Private.CoreLib/src/System/Threading/Condition.cs
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Fixes #123173.
There's currently a problem with Monitor using a single thread-static
Waiter(andAutoResetEvent) per thread. When using synchronization contexts that allow message pumping the following scenario can happen:Monitor.Waiton lock A.SynchronizationContext.Waitpumps a message and the main thread callsMonitor.Waiton lock B.Monitor.Pulseon lock A but lock B steals the signal since they share the same waiter.This change nulls out the thread-static
Waiterwhile a wait is in progress, so any reentrantMonitor.Waitgets its ownWaiterwith a distinctAutoResetEvent. TheWaiteris restored after the wait completes.