Bug: tryaddvaluesasync-wall-clock-timeout-flaky #780

• Work Mode: full-bug

Summary

DictionaryExtensions.TryAddValuesAsync cancels its inner Task.Run after a hard-coded 500 ms wall-clock window. Under parallel test execution with coverage instrumentation the thread pool can delay the task start past that window, so DictionaryExtensions_Tests.TryAddValuesAsync_UpdatesExistingValue fails intermittently with TaskCanceledException even though the operation itself completes in about 2 ms when run alone.

Environment

• OS/version: Windows 11 Pro 10.0.26200, .NET Framework 4.8.1 test host (VSTest 18.9.0, MSTest 4.4.0)
• Python version: n/a
• Command/flags used: vstest.console.exe UtilitiesCS.Test.dll VBFunctions.Test.dll /EnableCodeCoverage /InIsolation /Logger:trx "/TestCaseFilter:TestCategory!=LiveOutlook" (MSTest assembly-level parallelization enabled: 24 workers, class-level scope)
• Data source or fixture: none; in-memory ConcurrentDictionary<string,int>

Steps to Reproduce

1. Build the solution in Debug.
2. Run UtilitiesCS.Test.dll together with another assembly under /EnableCodeCoverage on a machine where the thread pool is saturated (24 parallel class workers).
3. Observe TryAddValuesAsync_UpdatesExistingValue occasionally reported as failed after roughly 20 s.
4. Re-run /TestCaseFilter:FullyQualifiedName~DictionaryExtensions_Tests alone: all 14 tests pass, the affected test in about 2 ms.

Expected Behavior

TryAddValuesAsync returns true and updates the value regardless of thread-pool scheduling latency, and the unit test is deterministic under parallel execution.

Actual Behavior

Intermittent failure (observed 2026-09-04 during PR #779 verification):

Test method UtilitiesCS.Test.Extensions.DictionaryExtensions_Tests.TryAddValuesAsync_UpdatesExistingValue threw exception:
System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at UtilitiesCS.DictionaryExtensions.<TryAddValuesAsync>d__10`2.MoveNext() in UtilitiesCS\Extensions\DictionaryExtensions.cs:line 179

Logs / Screenshots

• Attached minimal logs or screenshot
• Snippet: see Actual Behavior. Local run summary: 4767 tests, 4766 passed, 1 failed (this test), total time 35 s; the failing test alone took 21 s.

Impact / Severity

• Blocker
• High
• Medium
• Low

Flaky CI failures on unrelated pull requests; production callers may receive a spurious TaskCanceledException under thread-pool starvation.

Source

From: docs/features/potential/2026-09-04-tryaddvaluesasync-wall-clock-timeout-flaky.md