fix(deploy): build the previous release so rollback works across a cutover

[corrin]

Follow-up to #407 (merged). Surfaced while rehearsing the legacy→new cutover on msm-uat.

Problem

deploy.sh records PREVIOUS_SHA in deploy-state.env and tells operators to roll back with predeploy_rollback.sh <inst> <prev-sha>, but it never ensured that previous release was built. On a first legacy→new cutover the previous (legacy) SHA has no releases/<sha>, so predeploy_rollback.sh fails its release_complete guard — the supported rollback path is impossible on the first cutover. The cutover itself succeeds cleanly; only rollback was broken.

Fix

A guarded ensure_release "$previous_sha" before the destructive steps in the per-instance loop:

• No-op on a normal deploy (previous release already complete) — zero steady-state overhead.
• On a cutover, builds the old release from the shared repo before the switch, so the rollback target always exists (and the migrate-failure rollback hint becomes valid).
• Fatal if it can't build — don't cut over without a rollback target.

Docs (docs/updating.md, scripts/server/README.md) updated.

Verification

To be confirmed on the msm-uat rehearsal: reset msm-uat to legacy from the on-disk snapshot, pull this fix, re-run deploy.sh msm-uat, confirm releases/<legacy-sha>/.complete builds automatically during the cutover and predeploy_rollback.sh msm-uat <legacy-sha> then completes end-to-end. shellcheck clean.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Improved rollback reliability during deployment cutovers by ensuring the previous release is available before switching instances.
  • Added a safeguard so rollback targets exist even when transitioning from an older deployment setup.

• Documentation

  • Updated deployment and rollback instructions to reflect the new behavior and clarify when the extra build step occurs.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 7994d72 and 0773bce.

📒 Files selected for processing (3)

• docs/updating.md
• scripts/server/README.md
• scripts/server/deploy.sh

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📝 Walkthrough

Walkthrough

The deploy script now builds the prior release SHA before per-instance cutover when one exists. The rollback guidance in the docs was updated to describe that behavior.

Changes

Rollback target provisioning

Layer / File(s)	Summary
Previous release provisioning scripts/server/deploy.sh, scripts/server/README.md, docs/updating.md	deploy.sh now calls ensure_release for the prior SHA before the backup and cutover sequence, and the rollback documentation and deploy instructions describe that behavior.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

A bunny hopped through deploy night air 🐰
Built the old release with gentle care
Cutover twitched, then rolled just right
Now rollback paths gleam soft and bright ✨

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch shared-disk-space-saving

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