Draft: POC: Public CI Catalog server

What does this MR do and why?

This is a Proof of Concept (PoC) for enabling CI Catalog components to be used across different Cells.

Problem

In a multi-Cell GitLab deployment, components published in one Cell cannot currently be used by projects in other Cells. This blocks the Cells architecture from supporting cross-Cell CI component sharing.

Solution

This PoC introduces the concept of a "Public CI Catalog Server" where:

1. New public API endpoint (GET /projects/:id/components/:path) - Exposes component content for public catalog resources without authentication
2. ExternalPath class - Handles component addresses pointing to external servers (other Cells)
3. Cell detection - InstancePath.match? now checks if the component's project is on the same Cell before treating it as local

sequenceDiagram
    participant P as Project CI (Cell 2)
    participant F as FetchService (Cell 2)
    participant E as ExternalPath (Cell 2)
    participant HR as HTTP Router
    participant API as Components API (Cell 1)
    participant C as CI Catalog (Cell 1)

    P->>F: include: component: cell1/org/project/comp@1.0
    F->>F: InstancePath.match?() → false (different cell)
    F->>F: ExternalPath.match?() → true (allowed host)
    F->>E: fetch_content!
    E->>HR: GET gitlab.com/projects/:id/components/:path?version=1.0
    HR->>API: Route to Cell 1
    API->>C: InstancePath.fetch_content!
    C-->>API: ComponentData {content, path, version, sha}
    API-->>HR: JSON response
    HR-->>E: Proxy response
    E-->>F: ComponentData
    F-->>P: Component content for pipeline

References

How to use components from different Cells (#456843 - closed)

How to set up and validate locally

1. Set up Cells
   • gdk config set cells.enabled true and gdk config set cells.instance_count 1
   • gdk reconfigure
   • gdk cells up
2. Run gdk cells start
3. New cell installation failed in my case. I entered /Users/USER/gdk/gitlab-cells/cell-2 and ran gdk update. Then, ran gdk cells start again.

• Now, in http://gdk.test:3333/, I have my main cell.
• In http://gdk.test:12002/, I have another cell. I entered the URL and signed up as a new user. It will require an admin approval so you need to sign in as admin too.
• WARNING: Use different browser user or incognito mode for different cells. If there is any confusion about the Cells on your browser tabs, then remove the session cookie.

Current setup:

4. Have a published component in your main cell.

5. Use this component in the new Cell's project 🎉

MR acceptance checklist

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