Plugin Fails Behind Reverse Proxy with Subpath

[Imtehariya]

Problem

The GitLab plugin doesn't work when Backstage runs behind a reverse proxy with a subpath (e.g., /my-app/dev). The pathRewrite patterns can't match because the subpath is stripped before reaching the plugin.

Environment:

• Backstage: 1.44.2
• Backend URL: https://domain.com/my-app/dev

What Happens

Request sent: /my-app/dev/api/gitlab/rest/gitlab.com/api/v4/projects/123

Plugin receives: /rest/gitlab.com/api/v4/projects/123

Result: The subpath is lost, causing pathRewrite to fail → GitLab returns 404

Root Cause

Backstage's httpRouter mounts plugins at /api/{pluginId}, removing the upstream subpath before plugin middleware runs.

Workaround

Create a custom httpRouter service factory that restores the full path (with subpath) into req.originalUrl.

Proposed Solutions

1. Auto-detect base path from backend.baseUrl config
2. Add config option:

   gitlab:
     basePath: /my-app/dev

3. Use relative paths in pathRewrite (no subpath needed)

Impact

Affects all Backstage deployments using reverse proxies (Nginx, Istio, etc.) with URL subpaths.