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Akka HTTP rejects malformed requests such as GET /%% HTTP/1.1 before invoking the application handler, bypassing server-span creation. This change traces the generated response without duplicating spans for requests already delivered to the handler.

• Instrumentation: Identify pre-delivery rejections and record the actual response status through the shared HTTP instrumenter, preserving sampling and response customization without inventing unavailable request attributes.
• Lifecycle: Restore temporary rejection state on exceptional exits without opening context scopes.
• Regression coverage: Assert HTTP 400 and exactly one server span using raw sockets on Akka HTTP 10.0 and 10.2; cover ordinary application-generated 400s and entity-stream failures.

Fixes #5139.

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Waiting on the author · refreshed 2026-10-23 22:18 UTC

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[Copilot]

🟢 Approval recommended

The supplied review evidence supports rejection detection, state cleanup, and focused regression coverage, with no unresolved blocking findings.

No open issues 🎉

What changed in this PR

Addresses #5139 by extending Akka HTTP server instrumentation to trace parser-generated error responses that bypass application handlers, without duplicating spans for requests already delivered.

Changes:

• Detect pre-handler parsing failures and trace their response status through the shared HTTP instrumenter.
• Restore temporary rejection state on exceptional exits and reuse response customization.
• Add regression coverage for malformed requests, application-generated 400s, and entity-stream failures.

File	Description
AkkaHttpServerRouteTest.scala	Adds application-generated 400 coverage.
AkkaHttpServerInstrumentationTest.scala	Verifies malformed requests produce a 400 and one server span.
AbstractHttpServerInstrumentationTest.scala	Clarifies the comment explaining unavailable request methods.
MessageStartErrorInstrumentation.java	Marks parser-generated request-start errors.
HttpServerControllerInstrumentation.java	Intercepts rejection responses and restores temporary state.
AkkaHttpServerInstrumentationModule.java	Registers the new instrumentation classes.
AkkaHttpResponseMutator.java	Extracts shared response customization.
AkkaHttpParsingErrorSingletons.java	Tracks parsing errors and manages response-span creation.
AkkaHttpParsingErrorAttributesGetter.java	Extracts response attributes without inventing request details.
AkkaFlowWrapper.java	Reuses the shared response customizer.
AkkaHttpServerJavaRouteTest.java	Covers malformed targets and entity failures through raw sockets.

_{Review effort: Balanced}