infra/docs/atlas-gitea-migration.md

Gitea migration from Prometheus to Atlas

This is a staged migration plan, not a cutover authorization. Keep the source Gitea, its data, both NPM Proxy Hosts, and public DNS unchanged until the target and rollback have been tested. Gitea is temporary on Atlas until Uranus; NPM remains on Prometheus.

Observed source and chosen topology (2026-10-01)

• Prometheus runs the rootful docker.gitea.com/gitea:1.25.2 image in its managed Compose stack. /opt/gitea/data is about 280 MiB, uses SQLite, and contains 33 repositories. A live read-only SQLite quick_check passed. /home/git/.ssh is a separate small bind mount; /opt/gitea/data/ssh contains the existing SSH host keys. Neither tree may be discarded.
• Gitea answers HTTP 200 on Prometheus port 3000. NPM currently forwards git.fscotto.duckdns.org and git.ov-ad3410.infomaniak.ch to the Compose hostname gitea:3000. Public DNS resolves to Prometheus. The container's SSH port is bound only to 127.0.0.1:222; this is not a public Gitea SSH listener. Prometheus' public port 22 remains administrative SSH.
• Atlas has a healthy pool and a verified, private Prometheus backup under /zpool/backup/hosts/prometheus/latest. The 2026-10-01 scheduled export and pull succeeded. The intended target is a separate /zpool/services/data/gitea dataset, not Archive or the backup dataset.
• The approved cutover keeps NPM on Prometheus, changes the two HTTP Proxy Hosts to Atlas over the Prometheus--Aegis gateway, and offers public Gitea SSH on port 2222 via the same gateway. Prometheus port 22 is unchanged. HTTPS and SSH must be validated together before declaring cutover.
• Run Gitea as a rootless user Quadlet under a dedicated, non-login Atlas account, using the pinned 1.25.2-rootless image. This is an explicit rootful-to-rootless data-layout conversion, not a drop-in image swap: the target mounts /var/lib/gitea and /etc/gitea, and uses Gitea's built-in SSH server instead of the source image's OpenSSH daemon. Keep the application version unchanged until the conversion has passed an isolated restore test. The host's rootful Quadlet directory must not be used.

Phase 1: prepare without traffic changes

1. Provision a dedicated target dataset and non-login service identity via Ansible, keeping UID/GID distinct from Atlas' reserved Immich 1100. Install the user Quadlet in that identity's ~/.config/containers/systemd/, without an [Install] section; do not enable, start, or expose it yet.
2. Verify the selected Atlas backup SHA-256 and metadata, then extract only opt/gitea/data and home/git/.ssh to private staging. Never unpack NPM, WireGuard, or other host configuration from this sensitive tarball into a live namespace. Convert the rootful /data tree on a disposable copy: place application data under /var/lib/gitea, move app.ini to /etc/gitea, and rewrite every absolute /data/... path for the new layout. Enable START_SSH_SERVER, use internal SSH port 2222, and retain the source host-key pairs for the built-in server only after verifying their fingerprints and compatibility. Do not rely on the old /home/git/.ssh OpenSSH mount in the rootless image. Set only the target copy's ownership and path-scoped SELinux labels.
3. Validate SQLite integrity, repository count and representative git fsck, LFS/attachment presence, permissions, and an isolated rootless test container with no production ingress or outbound network. Because the source stays active, this is a rehearsal copy, not the final cutover copy. Regenerate Git hooks if the changed installation path requires it.
4. Confirm that Atlas snapshots, Borg, and offline USB include the new dataset; test at least one independent restore before user traffic is accepted.

Phase 2: explicit final cutover

1. Agree on an outage and record source/target versions, pool health, the latest backups, SSH host-key fingerprints, and both current NPM routes. Stop the Prometheus export timer for the change window so it cannot restart the old Compose stack unexpectedly.
2. Quiesce source writes. Run one final consistent Prometheus export, pull it to Atlas, verify checksum and timestamp, then stop the source Gitea. Keep /opt/gitea/data and /home/git/.ssh intact for rollback. Do not allow source Gitea to restart after accepting writes on Atlas.
3. Restore the final Gitea-only payload to the target and repeat integrity checks. Set Gitea's advertised SSH port to 2222 while retaining its existing HTTPS ROOT_URL and verified host keys. Start the pinned rootless Atlas user Quadlet, initially without public ingress; validate local HTTP, SQLite, repositories, LFS/attachments, and SSH host-key identity.
4. Permit only Aegis' source-NAT address to reach Atlas' Gitea HTTP and SSH ports. Enable the public TCP/2222 forward on Prometheus to Atlas over Aegis without changing administrative TCP/22. Update both NPM Proxy Hosts from gitea:3000 to Atlas' HTTP endpoint. Do not change public DNS.
5. Test HTTPS login, representative clone/push, LFS, and public SSH clone/push on port 2222 from outside the Atlas LAN. Record the last source write and first healthy target service times; do not claim RPO/RTO without measuring.
6. Only after successful traffic validation, remove Gitea from Prometheus' desired Compose stack and its backup-export path/container checks, leaving NPM and its backups operational. Do not delete the old data. Verify the next Atlas snapshot/Borg run covers Gitea and test a restored target copy.

Rollback gate

Before Atlas accepts writes, revert the two NPM routes, disable the public 2222 forward, and restart the unchanged source Gitea if target validation fails. After Atlas accepts writes, do not blindly restart the source: its SQLite database and repositories are stale. Quiesce Atlas, capture its new data, and decide a reverse migration or an extended outage explicitly.

Upstream references: rootful container layout, rootless image layout and incompatibility, rootless Podman Quadlet, standard-image conversion, and restore and hook regeneration.