[dougqh]

What Does This Do

A null value given to a TagMap setter now removes the tag, matching what setTag(tag, null) already does on a span.

• TagMap.put, set and getAndSet, by name or by id, remove the tag and return its prior value. putAll and fromMap go through set, so they follow.
• A null Entry or EntryReader is still a no-op, as before.
• In payload tagging, a JSON null in a request or response body now produces no tag. Before, it produced a tag with a null value.

Motivation

TagMap and the span API disagreed about null. Writers that bypass setTag and write to a TagMap directly could leave null-valued tags on a span, and the v0.4 serializer then sent them as a msgpack nil under meta. Payload tagging was the one writer that did this.

This is split out of the TagMap store work (#12918) so that the behavior change is reviewed on its own and the optimization PRs stacked on it change no behavior.

Behavior change

Payload tagging only:

• A JSON null (for example p.j3.3 from [1, 2, 3.14, null, true]) no longer emits a tag.
• It also no longer counts toward the max-tags limit, so the next value can take its place. In respectMaxTagsLimit, that's p.j3.4.

Additional Notes

• New TagMapNullToleranceTest cases cover set by name, set by id, put, getAndSet and fromMap.
• Local runs: internal-api and dd-trace-core pass. The only failures are the two TracerConnectionReliabilityTest cases, which need Docker.

Where this is going

This PR is one step in replacing the span's single TagMap with storage that knows each tag's kind. The goal is to cut what every span allocates on the application thread, without changing behavior.

The end state

• A tag is classified once, at the boundary. A known tag becomes an id (from the tag registry, Resolve direction-scoped OpenTelemetry names at the OpenTelemetry boundary (otlp - tag registry - phase 3) #12731), and a custom tag stays a name. Instrumentation sets known tags by id (Route intercepted tags by id instead of by name (otlp - tag registry - phase 3) #12817), so the JIT can constant-fold the routing.
• The span owns its tag stores directly. Known tags are kept by id and custom tags by name. TagMap becomes a thin view for code outside the span.
• Known tags live in a dense store by default. Parallel arrays of serials and values replace an Entry object per tag, and each span's store is presized from a per-operation hint. With the flag on, a web-request-shaped trace allocates about 30% less on the application thread than on master.
• Core reads and writes a span's stored tags through SpanStorage, not a TagMap. That covers post-processors now and serializers later, so no TagMap view gets built for them. SpanStorage has its own verbs, load and store, apart from TagMap's get/set and the span's intercepted getTag/setTag, so DDSpanContext can offer both. It starts with four methods and grows id, typed-load and primitive-store variants. Later, an intercepted counterpart (the setTag vocabulary) becomes the type setTagsFrom, TagContributor and TagExtractor take, with one lock per batch.

Later, not in this stack: cheaper read-through to parent spans, trace-level tags in a per-trace holder (TraceSegment), and primitive tag values without boxing.

The stack. This replaces #12918, split into pieces that can each be reviewed, and measured, on their own.

PR	Step	Allocation per 7-span trace (flag off unless noted)
#12999	A null tag value removes the tag in TagMap. This is the stack's only behavior change, kept separate on purpose.	unchanged
#12992	Bucket known tags by slot in TagMap, and add TraceShapeBenchmark	−96 B vs master
#13001	SpanStorage for the eager post-processors	unchanged
#13002	Extract TagMap's bucket mechanics into HashSupport (a pure move)	unchanged
#13006	Add the id and name tag stores, not yet used	unchanged
#13007	Make TagMap a facade over the two stores	unchanged
#13003	Have the span own its tag stores directly	+104 B (+1.3%)
#13004	Dense id store behind trace.experimental.dense.tags.enabled (off by default)	flag on: −30.6%
#12926	Turn the dense store on by default, add sizing hints and bulk prototype seeding	flag on: about 3% less again

Every PR after #12999 is behavior-preserving.

The pieces' allocation figures come from a deterministic per-thread probe (ThreadMXBean allocated bytes) that builds the same 7-span trace as TraceShapeBenchmark. The #12992 and #12926 figures come from TraceShapeBenchmark itself.

🤖 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	13.98 s	14.05 s	[-1.3%; +0.3%] (no difference)
startup:insecure-bank:tracing:Agent	12.95 s	13.04 s	[-1.3%; +0.0%] (no difference)
startup:petclinic:appsec:Agent	17.18 s	17.18 s	[-1.1%; +1.0%] (no difference)
startup:petclinic:iast:Agent	16.93 s	17.18 s	[-2.3%; -0.7%] (maybe better)
startup:petclinic:profiling:Agent	16.57 s	16.92 s	[-2.9%; -1.2%] (significantly better)
startup:petclinic:sca:Agent	17.12 s	16.92 s	[+0.3%; +2.1%] (maybe worse)
startup:petclinic:tracing:Agent	16.18 s	16.13 s	[-0.6%; +1.2%] (no difference)

Commit: fa9b4418 · 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.