Share parent's PropagationTags for local child spans#11701
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CoreTracer span-build allocates a fresh propagationTagsFactory.empty() per local child span; only the distributed ExtractedContext path reuses the parent's. Before optimizing that per-span allocation away, this test pins the load-bearing behavior: that a non-root (local child) span still carries the inbound distributed _dd.p.* tags when it injects. Verdict (both tests pass): it does. DDSpanContext.getPropagationTags() routes to the root (getRootSpanContextOrThis), so a non-root child's own propagationTags field is never read for injection. The per-span empty() is pure allocation waste, not a latent correctness bug. This test is the gate + safety net for the planned "share the parent's PropagationTags for local children" allocation fix. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
CoreTracer span-build allocated a fresh propagationTagsFactory.empty() for every local child span, reused from the parent only on the distributed ExtractedContext path — N+1 PropagationTags per N-span trace, N of them needless empties. A non-root child's own PropagationTags is never read for injection: DDSpanContext.getPropagationTags() routes to the root (getRootSpanContextOrThis), confirmed by PropagationTagsChildSpanTest (inbound _dd.p.* survive child injection). So local children can share the parent's (trace-level) instance instead of allocating their own, collapsing N+1 -> 1 per trace. Safe: the ctor's updateTraceIdHighOrderBits stamp is guard-idempotent and a no-op on the already-stamped shared root instance (same trace => same high-order bits). Full dd-trace-core propagation + span-build suite passes. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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dougqh
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Jun 23, 2026
| ciVisibilityContextData = null; | ||
| } | ||
| propagationTags = tracer.propagationTagsFactory.empty(); | ||
| // Local child: share the parent's (trace-level) PropagationTags instead of allocating a |
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To Claude - can we simplify this a bit?
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What This Does
Local child spans now share the parent's (trace-level)
PropagationTagsinstead of each allocating a freshempty(). Collapses N+1 → 1PropagationTagsper N-span trace.Motivation
CoreTracerspan-build allocatedpropagationTagsFactory.empty()for every local child span (the common in-process case); only the distributedExtractedContextpath reused the parent's. A non-root child's ownPropagationTags, however, is never read —DDSpanContext.getPropagationTags()routes to the root (getRootSpanContextOrThis().propagationTags). So the per-spanempty()was pure allocation waste.Additional Notes
Correctness — gated by a characterization test
First commit adds
PropagationTagsChildSpanTest, which pins the load-bearing behavior: a non-root (local child) span still carries the inbound distributed_dd.p.*tags on injection. Both tests pass, confirming this is pure allocation waste, not a latent correctness bug. The test is the gate + safety net for the change.Safety
updateTraceIdHighOrderBitsstamp is guard-idempotent (if (traceIdHighOrderBits != highOrderBits)) and a no-op on the already-stamped shared root instance (same trace ⇒ same high-order bits).dd-trace-corepropagation + span-build suite passes (W3C/Datadog extractors+injectors,W3CPropagationTags, OPM round-trip, OrgGuard,CoreSpanBuilderTest,DDSpanContextTest) — 0 failures.Misc
The
lastParentId/OPM inject-time mutation race (a transient per-injection value parked in the shared object) is pre-existing — injection already routes to the root's shared instance viagetPropagationTags()— so it is a separate concurrency concern, not introduced or worsened by this change. Tracked for a follow-up.🤖 Generated with Claude Code