[Drain] TimerData causedByDrain field

[stankiewicz]

This is PR is adding new field to TimerData. Noop for most of implementation apart from runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/WindmillTimerInternals.java that propagates draining information to TimerData.

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[stankiewicz]

R: @kennknowles

[gemini-code-assist[bot]]

Summary of Changes

Hello @stankiewicz, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant enhancement to the Apache Beam TimerData structure by adding a causedByDrain boolean field. This new field is designed to provide crucial context, allowing the system to differentiate between timers that are fired as part of a normal pipeline execution and those specifically triggered due to a draining operation. While the TimerData definition is updated globally, the specific logic for propagating the draining status is primarily implemented within the Dataflow worker's WindmillTimerInternals, with other timer instantiations defaulting to false. This change lays the groundwork for more robust and nuanced handling of timers during pipeline draining scenarios.

Highlights

• New Field in TimerData: A new boolean field, causedByDrain, has been added to the TimerData class. This field indicates whether a timer was triggered as a result of a draining operation.
• Dataflow Worker Integration: The WindmillTimerInternals in the Dataflow worker (runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/WindmillTimerInternals.java) has been updated to propagate the actual draining status to the new causedByDrain field when creating TimerData instances.
• Default Behavior for Existing Timers: For most other existing timer creation sites across the codebase, the causedByDrain field is explicitly set to false, ensuring backward compatibility and a clear default value where draining status is not yet relevant or propagated.

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[stankiewicz]

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