[vjfridge]

Motivation

A customer asked about flushing feature-flag telemetry from Python Lambda functions (FFL-3412). Short invocations can return before background writers send evaluation and exposure events, leaving telemetry delayed by Lambda freezing or lost on termination.

This change automatically attempts a flush before the invocation finishes, without customers adding handler code. Providers remain available for warm invocations.

Changes

• Flush feature-flag telemetry during invocation cleanup, including handler errors and failures in other cleanup steps.
• Cap the wait at DD_FEATURE_FLAG_FLUSH_TIMEOUT_MS (default 500 ms) and remaining invocation time minus a 100 ms safety margin.
• Preserve the handler's response or exception; safely skip older tracers and unused OpenFeature.
• Add compatibility tests, configuration docs, and a smoke handler/runbook.

Testing

• After merging main: 361 tests plus 15 subtests passed with the rebuilt candidate tracer; 360 plus 15 subtests passed with ddtrace 4.1.1 (candidate-only test skipped). Flake8 passed.
• HTTP patch tests passed separately; running them after the wrapper tests exposed shared instrumentation state.
• Earlier local Lambda emulator smoke tests covered Agent/direct delivery, handler errors, timeouts, and warm recovery using local receivers and pre-merge builds.
• Still outstanding: deployed AWS freeze/thaw and Datadog ingestion validation; I don't have access to the sandbox.

Release coordination

Companion: dd-trace-py #21135. Either PR can merge first; ship the tracer capability before the Lambda library/layer containing both changes. Older tracers safely skip the flush. Delivery remains best effort; timeout stops waiting, not the underlying send.

PR checklist

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[datadog-official]

More details

Static review identified no actionable defect in cleanup placement, timeout budgeting, or older-tracer compatibility. Runtime behavior and deployed AWS freeze/thaw delivery remain unverified.

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