[Spark Runner] Prepare Spark 3 structured-streaming to shared base, adopt Flink-style version overrides

[tkaymak]

What

Pure refactor of runners/spark/. Two changes:

1. Hoist runners/spark/3/src/.../structuredstreaming/ (the only sources Spark 3 ships) into the shared runners/spark/src/. runners/spark/3/src/ is removed entirely.
2. Replace the existing copySourceBase / useCopiedSourceSet block in runners/spark/spark_runner.gradle with the per-version source-overrides layering already used by runners/flink/flink_runner.gradle:
   • the lowest spark_major (currently 3) builds straight from the shared base;
   • higher majors get a Copy task with DuplicatesStrategy.INCLUDE that merges shared + previous majors + ./src so per-version files override.

runners/spark/3/build.gradle now sets spark_major = '3' instead of the removed copySourceBase = false flag, and gradle.properties gains spark_versions=3 (mirrors the existing flink_versions).

Why

Prepares the tree for the Spark 4 runner (#36841 / #38212), so it can land as a small overrides layer on top of this restructuring instead of duplicating the entire structured-streaming source tree.

Compatibility

Pure refactor — Spark 3's compiled output and test classpath are unchanged. The hoisted files are git-renames (verified with git diff --find-renames).

Next Step

Spark 4 follow-up branch ready: https://github.com/tkaymak/beam/tree/spark4-runner-slim

[gemini-code-assist]

Summary of Changes

Hello, 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 performs a structural refactor of the Spark runner to support a more scalable multi-version build architecture. By centralizing the core source code and implementing a layered override system for version-specific logic, the repository is now better prepared for the upcoming integration of Spark 4 without requiring extensive code duplication.

Highlights

• Source Code Hoisting: Moved all source files from runners/spark/3/ to the shared runners/spark/src/ directory, effectively removing the version-specific subdirectory.
• Build System Refactoring: Replaced the legacy copySourceBase mechanism in spark_runner.gradle with a Flink-style layering approach that merges shared and version-specific sources.
• Configuration Updates: Updated gradle.properties to include spark_versions and adjusted runners/spark/3/build.gradle to use the new spark_major property.

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[tkaymak]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements a version-specific source override mechanism for Spark runners, similar to the existing Flink runner implementation. It introduces logic in spark_runner.gradle to merge shared base sources with version-specific overrides into a single directory for compilation. The review feedback suggests improving the robustness of version comparison to handle multi-digit version numbers, simplifying the override detection logic using list indices, and ensuring that local source directories are consistently included even for the base Spark version.

[tkaymak]

Force-pushed the updated commit after review comments by Gemini.

[gemini-code-assist]

Code Review

This pull request introduces a per-version source override mechanism for Spark runners, enabling higher Spark versions to merge shared base code with version-specific overrides. Key changes involve updates to gradle.properties and the Spark runner's build configuration to manage merged source sets. A review comment recommends a more robust and idiomatic Gradle configuration by using TaskProvider directly within srcDirs to automatically manage task dependencies.

[tkaymak]

Followed the last open comment by @gemini-code-assist — applied the fix.

[gemini-code-assist]

Thanks for the update, @tkaymak. The revised Gradle configuration looks much cleaner and correctly leverages Gradle's task dependency auto-wiring. It's great to see the removal of the manual dependsOn blocks and null-guards. I agree that aligning runners/flink/flink_runner.gradle is a good candidate for a follow-up, but keeping it out of scope for this PR is the right call to maintain focus. The changes look solid.

[tkaymak]

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