Prevent smoke-test teardown race

[AlexeyKuznetsov-DD]

What Does This Do

Use the number of threads actually returned by ThreadGroup.enumerate() when joining smoke-test output readers during teardown.

Add a deterministic regression test that models an output thread disappearing between activeCount() and enumerate().

Motivation

ThreadGroup.activeCount() is only an estimate. The teardown code sized an array from that estimate, ignored the smaller count returned by enumerate(), and attempted to join every array entry. This could dereference an unused null entry after an output reader exited.

Additional Notes

The failure was flaky because teardown first interrupts the output-reader thread. If that thread exited after activeCount() reported it but before enumerate() populated the array, the array retained a null tail entry:

java.lang.NullPointerException
    at datadog.smoketest.OutputThreads.close(OutputThreads.java:36)
    at datadog.smoketest.AbstractSmokeApp.afterAll(AbstractSmokeApp.java:265)

Whether the reader exited inside that narrow window depended on JVM thread scheduling, so the same test could pass on retry. Joining only indexes below the count returned by enumerate() follows the API contract and removes the race.

Validation:

• ./gradlew :dd-smoke-tests:test -PtestJvm=11 --tests datadog.smoketest.ErrorLogSmokeTest --tests datadog.smoketest.OutputThreadsTest — 2 passed
• ./gradlew :dd-smoke-tests:test -PtestJvm=11 — 60 passed
• ./gradlew :dd-smoke-tests:spotlessJavaCheck — passed

No public API or user-facing behavior changes.

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
  Use solves instead, and assign the PR milestone to the issue
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• Update public documentation with any new configuration flags or behaviors
• Once approved, use merge queue to merge the PR

Jira ticket: N/A

[AlexeyKuznetsov-DD]

@codex review

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-10T22:14:38.064002Z	da93a6f	Draft marked ready
🔒 Security Review	✅ Completed 2026-09-10T22:15:24.749718Z	da93a6f	Draft marked ready

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

Reviewed commit: da93a6feec

ℹ️ About Codex in GitHub

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• Open a pull request for review
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• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

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[datadog-official]

More details

The teardown loop now uses the count from ThreadGroup.enumerate(). It does not read null array entries when a thread stops during teardown.

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[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.81 s	14.68 s	[-0.1%; +1.9%] (no difference)
startup:insecure-bank:tracing:Agent	13.70 s	13.69 s	[-0.7%; +0.7%] (no difference)
startup:petclinic:appsec:Agent	17.51 s	17.40 s	[-0.1%; +1.4%] (no difference)
startup:petclinic:iast:Agent	17.38 s	17.05 s	[-2.8%; +6.7%] (no difference)
startup:petclinic:profiling:Agent	17.11 s	17.43 s	[-3.1%; -0.6%] (maybe better)
startup:petclinic:sca:Agent	17.50 s	17.43 s	[-0.8%; +1.6%] (no difference)
startup:petclinic:tracing:Agent	16.52 s	16.62 s	[-1.4%; +0.3%] (no difference)

Commit: 4d5c7891 · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.