import { userAgent } from '@nibling/sdk/user-agent';

declare const upstreamBrand: unique symbol;

/** `fetch` that names us to the service it calls. Only `upstream()` makes one, so every dependency that
 * takes an `Upstream` (the router, the gateway, backfill, ingest, identity) cannot be handed a bare fetch. */
export type Upstream = ((input: string | URL | Request, init?: RequestInit) => Promise<Response>) & { readonly [upstreamBrand]: true };

/** Wraps `base` so every request carries the nibling User-Agent, with `contact` as the address in it. */
export function upstream(base: typeof fetch, contact?: string): Upstream {
  const ua = userAgent(contact);
  const wrapped = (input: string | URL | Request, init?: RequestInit) => {
    const headers = new Headers(init?.headers ?? (input instanceof Request ? input.headers : undefined));
    headers.set('user-agent', ua);
    // workerd throws "Illegal invocation" when fetch is called as a method of another object.
    return base(input, { ...init, headers });
  };
  return wrapped as Upstream;
}