Decouple W3C lastParentId from shared propagation-tag mutation

[dougqh]

What

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

Why

W3CHttpCodec.injectTraceState did:

propagationTags.updateLastParentId(DDSpanId.toHexStringPadded(context.getSpanId()));
String tracestate = 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 → 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).

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

[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	13.92 s	13.93 s	[-0.8%; +0.7%] (no difference)
startup:insecure-bank:tracing:Agent	12.91 s	12.99 s	[-1.5%; +0.4%] (no difference)
startup:petclinic:appsec:Agent	16.68 s	16.65 s	[-1.0%; +1.3%] (no difference)
startup:petclinic:iast:Agent	16.79 s	16.53 s	[-2.6%; +5.9%] (no difference)
startup:petclinic:profiling:Agent	16.72 s	16.67 s	[-0.7%; +1.3%] (no difference)
startup:petclinic:sca:Agent	15.86 s	16.21 s	[-9.7%; +5.3%] (unstable)
startup:petclinic:tracing:Agent	15.91 s	15.64 s	[-2.4%; +5.8%] (no difference)

Commit: 6f422485 · CI Pipeline · Benchmarking Platform UI

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Bits is fixing CI

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View in Datadog | Commit 6f42248