[ENH] Cancellation safety for append_batch.

[rescrv]

Description of changes

In a commit back in March I took out the cancellation safety from append batch.

This re-adds that so that the batch only waits for itself, not until all work is done.

Test plan

CI

Documentation Changes

N/A

[propel-code-bot]

Restore Cancellation Safety to append_batch Processing

This PR reintroduces cancellation safety for batch appends in the WAL writer. Now, the batch processing only waits for its own completion instead of until all work is done, by spawning each append_batch using tokio tasks. Minimal wording updates were made to clarify a comment in batch_manager.rs.

Key Changes:
• Revised OnceLogWriter to spawn append_batch via tokio::task::spawn, ensuring cancellation safety.
• Modified code to ensure each batch append waits only on its own completion, not global completion.
• Minor comment update in batch_manager.rs for clarity on notification.

Affected Areas:
• rust/wal3/src/writer.rs
• rust/wal3/src/batch_manager.rs

This summary was automatically generated by @propel-code-bot

[github-actions]

Reviewer Checklist

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Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[Sicheng-Pan]

Note: previously append_batch may continue to do more work, but after the fix it will only wait for its own work to be done.