[SPARK-57077][INFRA] Prevent merging draft PRs in merge script

[zhengruifeng]

What changes were proposed in this pull request?

Add a fail-hard check in dev/merge_spark_pr.py that aborts when the target PR is a draft (pr["draft"] == True). The check sits next to the existing [WIP]/[DO-NOT-MERGE] title guard.

Why are the changes needed?

A draft PR is, by definition, not ready for merge. The script today only inspects the title for [WIP]/[DO-NOT-MERGE], so a committer who runs it against a draft PR whose title looks final can still kick off the merge flow. Checking the GitHub API's draft flag closes that gap.

Does this PR introduce any user-facing change?

No. Committer tooling only.

How was this patch tested?

Existing doctests in dev/merge_spark_pr.py still pass (python3 -m doctest dev/merge_spark_pr.py — 57/57).

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.7)

[zhengruifeng]

use the updated script to merge this draft PR gets:

(spark-dev-313) ➜  spark git:(merge-script-block-draft-pr-dev3) dev/merge_spark_pr.py
git rev-parse --abbrev-ref HEAD
Which pull request would you like to merge? (e.g. 34): 56120
Cannot merge a draft PR #56120: [SPARK-57077][INFRA] Prevent merging draft PRs in merge script
Restoring head pointer to merge-script-block-draft-pr-dev3
git checkout merge-script-block-draft-pr-dev3
Already on 'merge-script-block-draft-pr-dev3'
git branch
Restoring head pointer to merge-script-block-draft-pr-dev3
git checkout merge-script-block-draft-pr-dev3
Already on 'merge-script-block-draft-pr-dev3'
git branch

[dongjoon-hyun]

Since this is simple, could you merge this into the above existing logic like the following? I guess this new message is more proper to cover all WIP cases.

if pr.get("draft", False) or "[WIP]" in title or "[DO-NOT-MERGE]" in title:
    fail("Cannot merge a draft PR #%s: %s" % (pr_num, title))

[zhengruifeng]

sounds good

[zhengruifeng]

thanks, merged to master