Cancel superseded pull request CI runs

[rzo1]

Pushing a new commit to a PR currently leaves the run already in flight to finish. Nobody reads that result — the newer commit supersedes it — but it still occupies runners to completion.

Java CI runs nine jobs per run: a 3 OS × 2 JDK build matrix (fail-fast: false) plus three shell-test jobs, and macOS and Windows runners bill at a multiple of Linux. So a superseded run is not a rounding error.

How often this happens

Sampling the last 100 maven.yml pull_request runs (52 branches), 9 had a newer commit arrive while they were still running — ~9%. Median PR run is ~11 minutes of wall-clock across those nine jobs. Branches with the most, unsurprisingly, are the ones under active review.

The change

Both workflows that react to pull_request get:

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
  cancel-in-progress: ${{ github.event_name == 'pull_request' }}

What is deliberately not cancelled

• Pushes to main. They fall to the github.ref group and cancel-in-progress evaluates to false, so every commit on main still gets a full build. Cancelling those would leave commits without a result and could interfere with Regenerate NOTICE and Publish snapshot artifacts.
• license.yml and publish-snapshots.yml are untouched — both are push-to-main-only, and license.yml commits back to the repo, so cancelling it mid-run is exactly what we don't want. (Serialising snapshot publishes with cancel-in-progress: false might be worth a separate look, but it is a different problem from this one.)

Note on the required status check

ASF Allowlist Check is a required check via .asf.yaml. Cancellation only ever hits runs for superseded commits; the run for the head commit always completes and reports. So the required check keeps behaving as before.

concurrency is a core Actions feature, not an action reference, so the ASF allowlist doesn't apply to it.

[krickert]

Good idea, straightforward.. thanks for doing this.

[krickert]

Good idea, straightforward.. thanks for doing this.