Capture element timestamp in DoFnRunners so they do not access mutable fields

[kennknowles]

Reported in https://github.com/apache/beam/pull/34902/files#r2528479326 that there was a problem with the timestamp validation. A possible cause is validation against the timestamp of "current element" at a moment when the element has been mutated to hold a different value. By accessing the field once and storing the retrieved immutable value in the closure, that possibility should be eliminated.

Note: this PR is better practice even if it doesn't fix the problem, so it can be merged either way.

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Summary of Changes

Hello @kennknowles, 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 enhances the reliability of timestamp validation within Beam's DoFnRunners by introducing a practice of eagerly capturing and storing element timestamps. This ensures that validation logic consistently operates on the original timestamp of an element, mitigating risks associated with mutable element states and potential inconsistencies in timestamp checks, especially within asynchronous or deferred execution contexts like lambdas.

Highlights

• Timestamp Capture: Element timestamps are now captured immediately upon access and stored in an immutable local variable, preventing potential issues with mutable fields.
• Improved Validation Robustness: This change ensures that timestamp validation consistently uses the original, unmutated timestamp of the element, enhancing the reliability of data processing within DoFnRunners.
• Refactored checkTimestamp Method: The checkTimestamp method now explicitly accepts an inputTimestamp parameter, clarifying which timestamp is being validated against the allowed skew.

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[kennknowles]

R: @kellen

[github-actions]

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[reuvenlax]

Aren't elements expected to be immutable? DirectRunner even has mutability checks to fail pipelines that mutate elements.

[kennknowles]

Yes, the elements are immutable. But we mutate a local field FnApiDoFnRunner.currentElement, here:

beam/sdks/java/harness/src/main/java/org/apache/beam/fn/harness/FnApiDoFnRunner.java

Line 624 in b369dfa

currentElement = elem;

So I think if the lambda is called asynchronously after another element has started processing this field could be set to a different element.

[kennknowles]

But now that I look closer, SimpleDoFnRunner.DoFnProcessContext.elem is final so I don't think it can cause the problem. The stack traces in the problem indicate it is v1 and using SimpleDoFnRunner.