profiling(ddprof): migrate context bridge to the all-native API (Phase 2)

[rkennke]

What

Migrates the profiler context bridge off the deprecated DirectByteBuffer (DBB) context API onto java-profiler's all-native API (setTraceContext / clearTraceContext / setContextValue / clearContextValue).

This is Phase 2 of the all-native context work (Phase 1 = DataDog/java-profiler#631).

Why

• Fixes the virtual-thread use-after-free: the DBB path cached a DirectByteBuffer over the native OTEP record; on a carrier that a mounted virtual thread later migrated off, the record was freed while the cached buffer kept being written. The all-native API resolves the current carrier's record inside each JNI call — no cached buffer to dangle.
• Per-activation perf: activation collapses from setContext + 2×setContextValue (3 JNI calls) into one setTraceContext (~28% faster on the combined cycle per the design-note benchmark).

Changes

• DatadogProfilingIntegration.activate → one setTraceContext(...) carrying trace/span context + operation & resource attributes; close/clearContext → clearTraceContext().
• DatadogProfiler: setContextValue/clearContextValue/reapplyAppContext/syncNativeAppContext are now all-native. reapplyAppContext uses a per-slot native setContextValue loop — native setContextValue publishes the record (valid=1) even with no active span, so app context stays visible between spans (preserves Restore app context attributes wiped by ddprof setContext on span activation #11646). A native batch reapply is deferred to a measured follow-up (java-profiler PROF-15361).
• AppContextSnapshot simplified to strings-only (the native path resolves each value's encoding via the process-wide cache — no cached id/utf8/snapshotTags DBB read).
• snapshot() uses the new native copyContextTags read (no ThreadContext/DBB, so it observes native writes without resetting the record).
• ContextSetter kept only for offsetOf + size (pure Java); no DBB usage remains.

Behavior notes

• Per-slot reapply is not atomic across app attributes. reapplyAppContext publishes each app-managed attribute with its own native setContextValue (detach→attach), so a profiler sample landing mid-reapply can observe a partial app-attribute set on activation — a window the old batch fast-path (setContextValuesByIdAndBytes, single attach) did not have. This is deliberate for the expected small app-attribute cardinality (0–2); the single-call batch reapply is the measured follow-up (PROF-15361). Trace/span + operation/resource stay atomic (one setTraceContext).
• Defensive zero-span guard. The native setTraceContext rejects spanId==0 (it is the activation path; clearing is clearTraceContext). Span ids are non-zero by construction (IdGenerationStrategy never yields 0, DDSpanId.ZERO means "no span", and DDSpanContext is the only ProfilerContext), so the bridge routes a stray zero span to clearTraceContext() rather than letting the IllegalArgumentException be swallowed over stale context.

Dependency / how to build

Requires java-profiler with the all-native API — DataDog/java-profiler#631 (ddprof ≥ the Phase-1 release). Until that ships, build/test against a local snapshot:

# in java-profiler (PR #631 branch):
./gradlew publishToMavenLocal          # com.datadoghq:ddprof:1.47.0-SNAPSHOT
# here:
./gradlew :dd-java-agent:agent-profiling:profiling-ddprof:test -PddprofUseSnapshot=true

Status / verification

• Draft — gated on the java-profiler Phase-1 release (Start implementation of new tracer API #631) so the ddprof pin can move to a published 1.47.0. CI here stays red until then because it compiles against the pinned 1.46.0, which predates the all-native API (cannot find symbol on the new calls) — not a code defect.
• Verified GREEN on Linux against local 1.47.0-SNAPSHOT:
  • ddprof unit suites DatadogProfilerTest / DatadogProfilerConfigTest / DatadogProfilerRecordingTest — 51 tests, skipped=0, 0 failures.
  • dd-smoke-tests:profiling-integration-tests (CodeHotspotsTest, JFRBasedProfilingIntegrationTest) — 20 tests, 0 failures (end-to-end agent + profiler + JFR, exercising span/op/resource context).

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