Stabilize TaskNodesDieAfterBuild: bump WaitForExit timeout + add process-identity telemetry

[jankratochvilcz]

The TaskNodesDieAfterBuild theory in src/Build.UnitTests/BackEnd/TaskHostFactory_Tests.cs flakes intermittently on slow CI agents because the post-build WaitForExit(3000) budget is too tight for the task host child process to drain stdio and exit.

Change

• WaitForExit budget bumped from 3 s -> 15 s.
• ProcessName and StartTime are captured up front so the failure message reveals a PID-reuse race (where the OS recycled the pid to an unrelated process between build-end and GetProcessById) instead of looking like the task host hung.
• elapsedMs is logged via Stopwatch so a follow-up PR can shrink the budget back to a tight-but-safe value once CI data accumulates.

Risk

Test-only change. No production code touched.

Repro signal

The flaky version was observed on multiple distinct branches in dnceng-public pipeline 75 over a 7-day window. The fix has been validated locally (dotnet test ... --filter-method *TaskNodesDieAfterBuild* -> 4/4 passing).

[Copilot]

Pull request overview

Stabilizes the TaskNodesDieAfterBuild unit test by increasing the post-build task host exit wait budget and adding diagnostic output intended to disambiguate slow-exit vs PID-reuse scenarios on CI.

Changes:

• Increased WaitForExit timeout from 3s to 15s in TaskNodesDieAfterBuild.
• Captured process identity fields (name/start time) best-effort and logged elapsed wait time via Stopwatch.
• Added SafeGetProcessField helper to prevent diagnostic reads from failing the test.

[github-actions]

Review Summary — 24 Dimensions Evaluated

✅ 24/24 dimensions clean — no findings.

This is a well-crafted test improvement that:

• Correctly addresses the flaky test issue by increasing the timeout to a generous 15s (5× the observed maximum)
• Adds excellent diagnostic information (process name, start time, elapsed ms) that will help distinguish PID-reuse races from genuine hangs
• Handles edge cases gracefully via SafeGetProcessField (process may exit before diagnostics are captured)
• Documents the reasoning clearly with "why" comments and a TELEMETRY note for future tuning
• Maintains the same core assertion (task host must exit after build)

Key dimensions verified clean:

Dimension	Result
Concurrency & Thread Safety	✅ Single-threaded test, inter-process race acknowledged and handled
Correctness & Edge Cases	✅ HasExited access in assertion message is safe (handle valid after WaitForExit)
Resource & Memory Management	✅ No new resource issues (Process not disposed is pre-existing)
Test Coverage & Completeness	✅ Assertion preserved, diagnostics improved, flakiness addressed
Scope & PR Discipline	✅ Single concern: fix flaky test + improve diagnostics
Documentation Accuracy	✅ Comments explain rationale, not just mechanics

Note

🔒 Integrity filter blocked 2 items

The following items were blocked because they don't meet the GitHub integrity level.

• #13828 pull_request_read: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
• #13828 pull_request_read: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".

To allow these resources, lower min-integrity in your GitHub frontmatter:

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    min-integrity: approved  # merged | approved | unapproved | none

Generated by Expert Code Review (on open) for issue #13828 · ● 4M

[jankratochvilcz]

Thanks — reviewer flagged 24/24 dimensions clean with no findings, so nothing to address here. This PR is test-only (src/Build.UnitTests/BackEnd/TaskHostFactory_Tests.cs) so the standard production guard rails don't apply.

[AlesProkop]

LGTM

[jankratochvilcz]

24h post-merge telemetry from main (dnceng-public pipeline 75, builds 1430301, 1430551, 1430938, 1431045, 1431261):

TFM/arch	Runs	min ms	p50 ms	p95 ms	max ms	Failed
net10.0/x64	37	6	8	44	191	0
net472/x86	20	98	105	129	136	0

The 15 000 ms budget gave ~80x headroom on the worst observed (191 ms). Zero failures.

Follow-up to honor the "shrink the budget back" promise from the PR body: draft #13845 lowers it to 2 000 ms (~10x of worst observed, comfortably above the 3 000 ms that originally flaked).