Two changes to relieve macOS arm64 runner contention:
1. `changes` job: runs on `ubuntu-latest` instead of
`[self-hosted, macos, arm64]`. This job does a plain `git diff`
— it has zero macOS dependencies. Moving it off the runner frees
the slot immediately on every workflow trigger.
2. Add workflow-level concurrency to `ci.yml`:
`concurrency: group: ci-${{ github.ref }}; cancel-in-progress: true`
Without this, every new push to a PR or main queues a full new
workflow run, each competing for the same single runner. With
`cancel-in-progress: true`, stale in-flight CI runs are cancelled
when a newer commit arrives — the runner always runs the latest
state, not a backlog of old ones.
Context: the self-hosted macOS arm64 runner is shared by ci.yml,
e2e-api.yml, canary-verify.yml, and publish-*.yml. The combination of
(1) the `changes` job holding the runner during `fetch-depth: 0`
checkout on every trigger, and (2) no workflow-level cancellation
caused 100+ queued runs with 0 in-progress.
Follow-up candidates (need verification before changing):
- platform-build: Go build may work on ubuntu-latest (no macOS deps)
- canvas-build: Next.js build may work on ubuntu-latest
- python-lint: needs `setup-python` instead of Homebrew Python
Co-authored-by: Molecule AI Infra-SRE <infra-sre@agents.moleculesai.app>
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