Fix NullPointerException from activating a null span in scope manager (quick fix)

[dougqh]

What Does This Do

• ContinuableScopeManager.activate now returns the existing INVALID_SCOPE (noop) sentinel when given a null span, instead of silently pushing a ContinuableScope with a null Context onto the scope stack.
• TracingSendHandler.onResult (JSR-356 websocket async send instrumentation) skips activateSpan entirely when the websocket span is null, matching the existing null-check already used in TracingOutputStream.close() for the same HandlerContext.
• Added ScopeManagerForkedTest#activatingNullSpanReturnsNoopScopeAndDoesNotCorruptStack, which reproduces the original bug: activating a null span returns a noop scope, and a subsequent real activation on the same thread no longer NPEs.

Motivation

Fixes a production NullPointerException surfaced via Datadog error tracking (issue 6d4f45fa-b461-11f0-8716-da7ad0900002) in ContinuableScopeManager.activate.

Root cause: HandlerContext.getWebsocketSpan() can race with HandlerContext.reset() clearing that field from another thread, so TracingSendHandler.onResult could call activateSpan with a null span. ContinuableScopeManager.activate had no guard for this — it pushed a ContinuableScope with a null Context. The corruption didn't throw immediately; it only surfaced as an NPE on the next activation on that thread, when top.context.with(span) dereferenced the poisoned null context. The pre-existing assert span != null never caught this in practice, since assertions are never enabled (-ea) in production JVMs.

Additional Notes

• Low customer impact (11 total occurrences, single customer) and not a crash — the exception was already caught by the instrumentation's own exception guard — but it silently corrupted the scope stack for the affected thread, which is worse than the NPE itself.
• ./gradlew :dd-trace-core:forkedTest --tests "datadog.trace.core.scopemanager.ScopeManagerForkedTest" passes.
• ./gradlew spotlessApply produces no additional 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
• Update the CODEOWNERS file on source file addition, migration, or deletion — not applicable, no files added/removed
• Update public documentation with any new configuration flags or behaviors — not applicable, no config change
• Once approved, use merge queue to merge the PR

Jira ticket: N/A — originated from Datadog Error Tracking issue 6d4f45fa-b461-11f0-8716-da7ad0900002

🤖 Generated with Claude Code

[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	14.76 s	14.65 s	[-0.2%; +1.6%] (no difference)
startup:insecure-bank:tracing:Agent	13.61 s	13.67 s	[-1.2%; +0.3%] (no difference)
startup:petclinic:appsec:Agent	17.16 s	17.05 s	[-0.2%; +1.5%] (no difference)
startup:petclinic:iast:Agent	17.04 s	16.93 s	[-0.2%; +1.5%] (no difference)
startup:petclinic:profiling:Agent	16.80 s	16.80 s	[-1.1%; +1.1%] (no difference)
startup:petclinic:sca:Agent	17.04 s	16.99 s	[-0.6%; +1.2%] (no difference)
startup:petclinic:tracing:Agent	16.05 s	16.08 s	[-1.1%; +0.8%] (no difference)

Commit: 51e86248 · CI Pipeline · Benchmarking Platform UI

---

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

[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 2026-10-07T20:21:53.284867Z	9624bfc	Draft marked ready

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

[datadog-prod-us1-5]

More details

Null-span activation now leaves the scope stack untouched, preventing the poisoned context that caused subsequent activations to fail. The websocket callback’s error and cleanup paths remain compatible with a missing span.

Was this helpful? React 👍 or 👎

_{Open Bits AI session}

_{🤖 Bits Code Review · Commit 9624bfc · @DataDog review to ask questions}

[datadog-prod-us1-5]

More details

Null activation now returns before touching the scope stack, preventing corruption of subsequent activations. The websocket fallback preserves callback delivery when its span has been cleared.

Was this helpful? React 👍 or 👎

_{Open Bits AI session}

_{🤖 Bits Code Review · Commit 51e8624}

[dd-octo-sts]

/merge

[gh-worker-devflow-routing-ef8351]

View all feedbacks in Devflow UI.

2026-10-09 20:20:58 UTC ℹ️ Start processing command /merge

---

2026-10-09 20:21:01 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in master is approximately 1h (p90).

---

2026-10-09 22:21:29 UTC ❌ MergeQueue: The build pipeline has timeout

The merge request has been interrupted because the build 5666697513913241170 took longer than expected. The current limit for the base branch 'master' is 120 minutes.

[dd-octo-sts]

/merge

[gh-worker-devflow-routing-ef8351]

View all feedbacks in Devflow UI.

2026-10-10 01:43:18 UTC ℹ️ Start processing command /merge

---

2026-10-10 01:43:22 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in master is approximately 1h (p90).

---

2026-10-10 02:53:11 UTC ℹ️ MergeQueue: This merge request was merged