Decouple W3C lastParentId from shared propagation-tag mutation by dougqh · Pull Request #11702 · DataDog/dd-trace-java

dougqh · 2026-06-22T22:58:26Z

What Does This Do

Stop mutating the (trace-level, shared) PropagationTags with the injecting span's id at W3C inject time; supply it as a parameter instead.

Motivation

Correctness fix - but originally found while trying to optimize pTags allocation (see #11701)

Additional Notes

From Claude...

W3CHttpCodec.injectTraceState did:

propagationTags.updateLastParentId(DDSpanId.toHexStringPadded(context.getSpanId()));
Stringtracestate = propagationTags.headerValue(W3C);

But getPropagationTags() routes to the root (getRootSpanContextOrThis()), so the field is effectively shared across the segment. Concurrent sibling injects race on it — span A writes A, span B writes B, A reads back B and emits the wrong p: (W3C last-parent-id). It also thrashed the W3C header cache (invalidated on every differing id). Pre-existing; not introduced by the per-span sharing work (#11701).

How

New PropagationTags.headerValue(HeaderType, CharSequence lastParentIdOverride). W3CHttpCodec passes context.getSpanId(); W3CPTagsCodec uses the override for p:, falling back to the stored inbound last-parent-id when there's no override (span-link traceState, extraction reads).
No inject-time mutation of the shared tags for the last-parent-id → no race, no cross-talk between sibling injects, and the inbound last-parent-id is preserved for its ctor/extraction reads.
updateLastParentId is now unused and removed (it was the only inject-time mutator whose value varies per-span — hence the only one that raced; the OPM stamp is inject-time too but idempotent. The inbound value is set at construction).

Deliberately out of scope

OPM stamping is left as-is. It writes a process-constant local OPM, so concurrent injects are idempotent — no real race — and OPM is new; let it settle. Same-shape decouple is a clean future follow-up (or it rides the TraceSegment migration).

Test / validation

PropagationTagsLastParentIdTest — override supplies p:; sibling injects don't cross-talk; inbound p: preserved and unmutated by override use.
Full dd-trace-core propagation + span-build suite green (1947 tests, 0 failures), incl. W3CHttpInjectorTest.updateLastParentIdOnChildSpan, the 611 W3CPropagationTags cases, and span-link traceState.

Relationship

Step 2 of the PTags-per-segment work; pairs with #11701 (share parent's PTags for local children — the allocation win). Both are early steps toward a first-class TraceSegment.

🤖 Generated with Claude Code

Injecting a W3C tracestate wrote the injecting span's id into the (trace-level, shared via getRootSpanContextOrThis) PropagationTags via updateLastParentId, then read it back to build the `p:` member. Concurrent sibling injects race on that shared field -- A writes A, B writes B, A can emit B as its last-parent. Supply the injecting span's id as a parameter instead: PropagationTags.headerValue(HeaderType, lastParentIdOverride). W3CHttpCodec passes context.getSpanId(); the W3C codec uses the override for `p:`, falling back to the stored inbound last-parent-id for non-inject callers (span-link traceState). No inject-time mutation of the shared tags -> no race, no cross-talk between sibling injects, and the inbound last-parent-id is preserved for its ctor/extraction reads. updateLastParentId is now unused and removed (it was the only inject-time mutator; the inbound value is set at construction). OPM stamping is left as-is on purpose: it writes a process-constant local OPM, so concurrent injects are idempotent (no race). Full dd-trace-core propagation + span-build suite passes (1947 tests). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dd-octo-sts · 2026-06-22T23:22:02Z

🟢 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	14.01 s	13.97 s	[-0.4%; +1.0%] (no difference)
startup:insecure-bank:tracing:Agent	12.93 s	12.97 s	[-0.9%; +0.3%] (no difference)
startup:petclinic:appsec:Agent	16.95 s	16.74 s	[+0.2%; +2.3%] (maybe worse)
startup:petclinic:iast:Agent	16.98 s	16.95 s	[-0.7%; +1.0%] (no difference)
startup:petclinic:profiling:Agent	16.67 s	16.40 s	[-2.9%; +6.1%] (no difference)
startup:petclinic:sca:Agent	16.91 s	16.83 s	[-0.5%; +1.5%] (no difference)
startup:petclinic:tracing:Agent	16.02 s	16.11 s	[-1.6%; +0.5%] (no difference)

Commit:734f641c · 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.

datadog-datadog-prod-us1-2 · 2026-06-22T23:22:23Z

🎯 Code Coverage (details)
• Patch Coverage: 66.67%
• Overall Coverage: 57.07% (-0.21%)

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: 734f641 | Docs | Datadog PR Page | Give us feedback!}

datadog-datadog-prod-us1-2 · 2026-06-23T10:52:16Z

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Bits Code status: ✅ Done

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dd-octo-sts · 2026-07-20T18:12:12Z

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dd-octo-sts · 2026-07-20T19:20:43Z

/merge

gh-worker-devflow-routing-ef8351 · 2026-07-20T19:20:48Z

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2026-07-20 19:20:48 UTC ℹ️ Start processing command /merge

2026-07-20 19:20:52 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in master is approximately 2h (p90).

2026-07-20 20:28:19 UTC ℹ️ MergeQueue: This merge request was merged

dougqh mentioned this pull request Jun 24, 2026
Share parent's PropagationTags for local child spans #11701
Draft

Merge branch 'master' into dougqh/ptags-decouple-lastparentid
2dd6a30

dougqh marked this pull request as ready for review July 20, 2026 18:11

Merge branch 'master' into dougqh/ptags-decouple-lastparentid
734f641

dougqh requested review from a team as code owners July 20, 2026 18:11

dougqh requested review from mcculls and removed request for a teamJuly 20, 2026 18:11

dd-octo-stsBot added the tag: ai generated Largely based on code generated by an AI or LLM label Jul 20, 2026

dougqh added comp: context propagation Trace context propagation type: bug fix Bug fix labels Jul 20, 2026

mhlidd approved these changes Jul 20, 2026
View reviewed changes

dougqh added this pull request to the merge queue Jul 20, 2026

github-merge-queueBot removed this pull request from the merge queue due to failed status checks Jul 20, 2026

gh-worker-dd-mergequeue-cf854dBot merged commit 057a33f into masterJul 20, 2026
590 of 596 checks passed

gh-worker-dd-mergequeue-cf854dBot deleted the dougqh/ptags-decouple-lastparentid branch July 20, 2026 20:28

github-actionsBot added this to the 1.65.0 milestone Jul 20, 2026

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Decouple W3C lastParentId from shared propagation-tag mutation - #11702

Decouple W3C lastParentId from shared propagation-tag mutation#11702
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What Does This Do

Motivation

Additional Notes

How

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Test / validation

Relationship

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🟢 Java Benchmark SLOs — All performance SLOs passed

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