Bug: ci-mstest-testhost-crash-in-folder-tree-service-tests #1004

• Work Mode: full-bug

Summary

The CI mstest-coverage job intermittently aborts with "Test host process crashed" while the UtilitiesCS.Test OutlookFolderTreeService concurrency, disposal and invalidation tests are running. No assertion fails; the test host process dies, so the run reports an unknown total and fails the job.

Environment

• OS/version: GitHub Actions windows runner, Visual Studio 18 test platform (vstest.console.exe)
• Python version: not applicable (C# .NET Framework 4.8 MSTest)
• Command/flags used: scripts/vscode/Invoke-MSTestWithCoverage.ps1 via the CI mstest-coverage job
• Data source or fixture: UtilitiesCS.Test OutlookFolderTreeServiceConcurrencyTests, OutlookFolderTreeServiceDisposalTests, OutlookFolderTreeServiceInvalidationTests

Steps to Reproduce

1. Run the CI workflow on a pull request (observed on PR fix(quickfiler): EFC recursive subfolder selection, trash selection, and QFC nested indentation (#1002) #1003, run 39794073798, attempt 1, head e94c261).
2. Observe the mstest-coverage job: 4622 tests pass, then "The active test run was aborted. Reason: Test host process crashed".
3. Re-run the failed job on the same commit: attempt 2 passed all checks, so the crash is intermittent.

Expected Behavior

The MSTest run completes deterministically; no test leaves an unhandled exception on a background or dispatcher thread that can terminate the test host.

Actual Behavior

Intermittent test host crash. The last tests reported before the crash were GetSnapshotAsync_WorkerOriginatedColdBuild_UsesCapturedStaDispatcher, Dispose_DuringRefresh_LeavesDisposedWithoutPublicationOrNotification, FolderChanged_DuringInFlightBuild_SchedulesOneFollowUpRefresh, NotificationRefreshAndDispose_RunOnTheCapturedDispatcher and NotificationRefresh_RunsOnCapturedDispatcher.

Logs / Screenshots

• Attached minimal logs or screenshot
• Snippet: The active test run was aborted. Reason: Test host process crashed : Process path: ...\TestPlatform\Extensions\..\testhost.exe / Test Run Aborted. Total tests: Unknown Passed: 4622

Impact / Severity

• Blocker
• High
• Medium
• Low

Source

From: docs/features/potential/2026-10-22-ci-mstest-testhost-crash-in-folder-tree-service-tests.md