[darkweak]

Closes #14

Problem

A relative include (<esi:include src="/my-esi-content/whatever"/>) is always resolved against the host of the incoming request. When one proxy (e.g. Traefik in Kubernetes) routes several apps, the fragments live on an internal service, so the host had to be hardcoded in the HTML and the app rebuilt per environment.

Change

• writer: new ParseBaseURL(string) and WithBaseURL(*url.URL) option. NewWriter(rw, rq, opts ...Option) is variadic, so existing callers are unaffected.
  • Accepts host[:port] (scheme defaults to http) or http(s)://host[:port]. Rejects paths, queries, userinfo and other schemes.
  • The include request takes the base scheme and host but keeps the incoming path, so src="fragment" still resolves relative to the page. The incoming request is not modified, and $(HTTP_HOST) still reports the original host.
  • Absolute includes are unchanged.
• Traefik: defaultHost plugin setting:

  spec:
    plugin:
      esi:
        defaultHost: static-webserver.my-app.svc.cluster.local

• Caddy: esi { default_host backend:8080 }
• RoadRunner: http.esi.default_host
• README updated.

Notes

• Cookie/Authorization are forwarded to the default host, since it becomes the same-origin target for relative includes (the same as before, when they were forwarded to the request host). Absolute includes pointing at the public host no longer count as same-origin when a default host is set.
• The issue's "full" service + namespace form isn't included: a plugin can't resolve Kubernetes references, and service.namespace.svc.cluster.local[:port] does the same thing.

Testing

• Unit tests for ParseBaseURL and WithBaseURL (include routing, relative path, cookie forwarding, original request unchanged), plus one integration test each for the Traefik, Caddy (caddytest) and RoadRunner middlewares.
• go test -race ./... passes in the root module and all three middleware modules, and golangci-lint reports no new issues.
• Checked by hand in traefik:latest (Yaegi) with defaultHost: whoami: the relative include went to the whoami service with Host: whoami.

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