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Contributing to BOSH Linux Stemcell Builder

Branches are named for the Ubuntu release on which they are based:

  • ubuntu-<short_name>

As of 2026-06-09 the following stemcell lines / branches are supported:

  • Ubuntu Jammy / ubuntu-jammy
  • Ubuntu Noble / ubuntu-noble
  • Ubuntu Resolute / ubuntu-resolute

Merge-Forward Strategy

This repository uses a merge-forward strategy to keep stemcell branches in sync.

Branch order (oldest → newest):

ubuntu-jammy → ubuntu-noble → ubuntu-resolute

How it works

  1. Open your PR against the oldest applicable branch (e.g. ubuntu-jammy if the change applies to all lines).
  2. Merge it.
  3. A maintainer manually dispatches .github/workflows/merge-forward.yml with source_branch set to the merged branch. Optionally, set validated_sha to the commit that passed Concourse validation (aggregate-candidate-stemcells) — when provided, the workflow merges from that exact SHA rather than the branch tip.
  4. Merge the forwarded PR, then repeat from step 3 for the next branch in the chain.

No labels are needed. After each relevant merge, a maintainer dispatches the workflow to create the forward PR.

Clean vs. conflict merge-forwards

  • Clean merge — the merge-forward PR is opened ready for review.
  • Conflict — the merge-forward PR is opened as a draft. Check out the branch, perform the merge manually, resolve the conflicts, and mark the PR ready for review.

Re-runs

The workflow is idempotent: if the merge-forward branch already exists but has no associated PR (i.e. the branch was pushed but PR creation failed on a previous run), re-running retries opening the PR without re-doing the merge. If a PR already exists (open), the run skips silently.

CI on merge-forward PRs

GitHub suppresses workflow runs triggered by a GITHUB_TOKEN-pushed branch. This means the merge-forward PR will not have CI results initially. A maintainer can start CI by pushing a trivial commit to the forward branch.