[release/10.0] Backport #127638: Fix !m_RedirectContextInUse assert in RestoreContextSimulated on win-x86

[daigs]

Summary

This is a request to backport PR #127638 to the release/10.0 branch so it ships in a future .NET 10 servicing update.

The original bug is tracked in #127637 (now closed). The fix was merged to main on 2026-05-01 (merge commit bd9e85e4b37557e649fccc083f33a50acdda2dde) but has not been backported to release/10.0. I verified that v10.0.8, v10.0.9, and v10.0.10 all still contain the buggy code:

• Thread::RestoreContextSimulated still calls HandleThreadAbort() while m_RedirectContextInUse is true
• The x86 RestoreContextSimulated call still happens before COMPlusCheckForAbort()

Why this matters

This is not just a Windows 7 compatibility issue. The root cause is a correctness bug in the x86 RestoreContextSimulated path that can affect any x86 .NET 10 process running in an environment where RtlRestoreContext is unavailable in ntdll.dll (x86 Win7, WOW64 on Win7, and potentially other setups).

The double-redirect race silently corrupts the saved thread context and causes random, hard-to-diagnose crashes — exactly the kind of issue that LTS users will hit in production.

Why it's safe to backport

• The change is minimal: +10 / -12 lines in a single file (src/coreclr/vm/threadsuspend.cpp)
• It only reorders the abort check before the context-restore call and removes the inlined HandleThreadAbort() from RestoreContextSimulated
• It was authored/reviewed by the runtime team (jkotas) and is already live on main

Request

Could this please be backported to release/10.0 for an upcoming servicing release? .NET 10 is an LTS release supported through November 2028, so a large number of users will remain on it for years.

Related:

• Original issue: !m_RedirectContextInUse Assert on win-x86 #127637
• Fix PR (merged to main): Fix !m_RedirectContextInUse assert in RestoreContextSimulated on win-x86 #127638

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