Add opt-in instrumentation support for generated lambda classes

[amarziali]

What Does This Do?

Adds opt-in infrastructure for applying Datadog instrumentations to classes generated by LambdaMetafactory.

Generated lambda classes are normally defined without passing through the standard Java agent transformation path. This change instruments InnerClassLambdaMetafactory at the byte-generation point and routes
eligible lambda bytecode through the existing transformer before class definition.

The infrastructure is disabled by default and can be enabled with:

dd.trace.lambda.enabled=true

Enabling it alone does not select any production lambda classes. An enabled instrumentation must explicitly implement Instrumenter.ForLambda and register the exact functional interface it supports.

Notable Changes

• Introduces Instrumenter.ForLambda for instrumentations targeting generated implementations of an exact functional interface.
• Instruments java.lang.invoke.InnerClassLambdaMetafactory immediately after lambda bytecode generation.
• Routes eligible generated classes through the existing matching, context-store, helper-injection, and transformation pipeline.
• Supports the metafactory layouts used by JDK 8, JDK 11+, and the ClassFile API path used by newer JDKs.
• Uses the module-aware ClassFileTransformer entry point on JDK 9+.
• Applies only transformations registered for the generated lambda’s functional interface.
• Avoids hidden-lambda cache pollution when symbolic class names are reused.
• Skips the mechanism during GraalVM Native Image builds.

Compatibility and Limitations

Lambda transformation is intentionally best-effort:

• The feature is disabled by default.
• Reentrant lambda generation during transformation is skipped to prevent recursion.
• CDS/AppCDS lambda classes that bypass runtime metafactory generation are not covered.
• GraalVM Native Image builder JVMs are excluded. Instrumented lambda shapes conflict with Graal’s lambda-renaming analysis.
• If a future JDK changes the metafactory byte-generation layout, the agent leaves the metafactory unchanged and emits a warning.
• Security restrictions that prevent access to the defining loader, module, or protection domain cause transformation to fall back to the original bytes.

Motivation

Additional Notes

Contributor Checklist

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Jira ticket: [PROJ-IDENT]

[amarziali]

@DataDog review

[datadog-prod-us1-6]

On JDK 8 and JDK 11, the new structure matcher requires a field that does not exist. The lambda transformer does not run, and Runnable lambdas still use wrappers.

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[amarziali]

@DataDog review

[datadog-prod-us1-6]

More details

The PR instruments generated Runnable lambdas so executor propagation keeps task identity, with supported JDK paths and a Graal build guard. This update fixes the earlier JDK 8 and 11 samBase mismatch and adds no new finding; the documented concurrent-reuse limitation remains, and no review finding is open.

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[amarziali]

@codex review

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-09T13:28:01.820141Z	490090a	Draft marked ready

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

Reviewed commit: 5655a6ff32

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[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	14.04 s	13.96 s	[-0.1%; +1.2%] (no difference)
startup:insecure-bank:tracing:Agent	12.86 s	13.00 s	[-1.9%; -0.4%] (maybe better)
startup:petclinic:appsec:Agent	16.99 s	16.85 s	[-0.3%; +2.0%] (no difference)
startup:petclinic:iast:Agent	16.92 s	16.58 s	[-2.4%; +6.5%] (no difference)
startup:petclinic:profiling:Agent	16.34 s	16.88 s	[-7.7%; +1.3%] (no difference)
startup:petclinic:sca:Agent	16.92 s	16.29 s	[-0.5%; +8.3%] (no difference)
startup:petclinic:tracing:Agent	16.25 s	16.16 s	[-0.5%; +1.6%] (no difference)

Commit: ec52a02c · 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-prod-us1-6]

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[dd-octo-sts]

Hi! 👋 Thanks for your pull request! 🎉

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[amarziali]

@DataDog review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: a721063dff

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[datadog-prod-us1-6]

A hidden Runnable lambda can put its type description in the shared name cache. A later real class with the same legal binary name can receive wrong instrumentation on JDK 21 and later.

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[amarziali]

@DataDog review

[datadog-prod-us1-6]

The new module has no Gradle lock file. Its dependency graph can change between builds.

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[datadog-prod-us1-6]

More details

The opt-in lambda path limits transformation to registered functional interfaces and keeps generated-class metadata out of shared caches.

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