GH-50239: [R] Data race issue from R API requests in parallel region

[thisisnic]

Rationale for this change

When use_threads = TRUE, parallel tasks are submitted to the thread pool immediately during RTasks::Append(). This means worker threads start reading R vector headers (via INTEGER(), REAL(), etc.) while the main thread is still assigning vectors into the output list via SET_VECTOR_ELT, which writes to the same header memory. ThreadSanitizer detects this as a data race.

What changes are included in this PR?

Delay submission of parallel tasks until RTasks::Finish(), matching the existing pattern for serial tasks. All R-side allocations and list assignments complete on the main thread before any worker threads start.

Are these changes tested?

Existing tests cover the affected code path.
I also ran some local benchmarks to see if this change affected performance, but seems fine.

Are there any user-facing changes?

No.

• GitHub Issue: [R] Data race issue from R API requests in parallel region #50239

[github-actions]

⚠️ GitHub issue #50239 has been automatically assigned in GitHub to PR creator.

[thisisnic]

@ursabot please benchmark

[rok]

Benchmark runs are scheduled for commit 824f648. Watch https://buildkite.com/apache-arrow and https://conbench.arrow-dev.org for updates. A comment will be posted here when the runs are complete.

[Copilot]

Pull request overview

This PR addresses a ThreadSanitizer-detected data race in the R bindings when use_threads = TRUE by preventing worker threads from touching R vectors until after all R-side allocations and list assignments are complete on the main thread.

Changes:

• Delay scheduling of parallel RTasks work until RTasks::Finish() instead of submitting immediately in RTasks::Append().
• Add storage for delayed parallel tasks and ensure it is cleared on Reset().

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
r/src/RTasks.cpp	Queues parallel tasks in Append(), then submits them in Finish() after serial work completes; clears the new queue in Reset().
r/src/r_task_group.h	Adds delayed_parallel_tasks_ to hold parallel tasks until Finish().