[WIP] Drain in element metadata v1

[stankiewicz]

Extension of #36373 - implementation of https://s.apache.org/beam-drain-mode

1. bubbles up drainMode from backend up to doFn signature of onTimer event and downstream (e.g. aggregation) via WindowedValue's draining flag
2. encodes draining information so information is not lost

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[gemini-code-assist]

Summary of Changes

Hello @stankiewicz, I'm Gemini Code Assist¹! 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 extends Apache Beam's capabilities to handle 'drain mode' signals more effectively across distributed execution environments. It establishes a standardized way to communicate whether a pipeline is in a draining state, allowing user-defined functions and internal processing logic to react appropriately. This ensures that data processing can gracefully conclude during pipeline shutdowns or reconfigurations without losing critical state information.

Highlights

• Drain Mode Propagation: Introduced a 'drainMode' flag that is bubbled up from the backend to DoFn onTimer events and propagated downstream via WindowedValue's draining flag.
• Element Metadata Encoding: Implemented encoding of draining information within element metadata (specifically in beam_fn_api.proto and PaneInfo) to ensure this state is not lost during serialization and deserialization.
• API and Internal Changes: Modified core Beam APIs and internal runner components (e.g., DoFnRunner, ReduceFn, TimerInternals, WindowedValue) to accept, store, and propagate the new 'draining' boolean flag.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩