Fix reentrant Monitor wait

[eduardo-vp]

Fixes #123173.

There's currently a problem with Monitor using a single thread-static Waiter (and AutoResetEvent) per thread. When using synchronization contexts that allow message pumping the following scenario can happen:

• The main thread calls Monitor.Wait on lock A.
• SynchronizationContext.Wait pumps a message and the main thread calls Monitor.Wait on lock B.
• Since both operations were run on the same thread, both share the same waiter.
• A background thread calls Monitor.Pulse on lock A but lock B steals the signal since they share the same waiter.

This change nulls out the thread-static Waiter while a wait is in progress, so any reentrant Monitor.Wait gets its own Waiter with a distinct AutoResetEvent. The Waiter is restored after the wait completes.

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[Copilot]

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.Waiter thread-static reuse to clear the cached waiter while a wait is in progress, ensuring overlapping waits use distinct AutoResetEvent instances.
• 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.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[VSadov]

LGTM!