platform/src/servers/api/dev-proxy/router.ts

import { createRouter } from '../../create-router';
import { resolveAuthToken } from '../../auth-token';
import { isSuperAdmin } from '../../super-admin';
import { rewriteAssetPaths, rewriteHtml } from './html-rewrite';

/**
 * See a dev server running on THIS machine's loopback, from a browser that is somewhere else.
 *
 * `GET /api/dev-proxy/5173/…` forwards to `http://127.0.0.1:5173/…`. That is the whole feature.
 *
 * ── Why a proxy rather than pointing the iframe at the port ──
 *
 * Neither obvious approach works from a browser on `https://<platform>`. `http://localhost:5173` resolves
 * on the VIEWER's machine, not the server's. `http://<server>:5173` is mixed content inside an https page
 * and the browser blocks it. So the bytes have to arrive over the platform's own origin, which is what
 * this is — and it costs no certificate, no proxy-host entry, and no port opened to the outside.
 *
 * A dev server can therefore stay bound to 127.0.0.1. The alternative in use before this — a proxy host
 * per app — needs the server bound to 0.0.0.0 and a firewall rule for the docker bridge.
 *
 * ── The cost, stated plainly ──
 *
 * The app believes it is at `/`, and it is not. Absolute paths are rewritten on the way out
 * (`html-rewrite.ts`), which handles Bun's dev server and anything using ordinary relative or root-absolute
 * assets. It will not handle an app that builds URLs in ways the rewrite cannot see. For those, the answer
 * is the app's own `base` setting, or a real hostname — not more regexes here.
 *
 * ── Access ──
 *
 * This route reaches ANY port on the machine's loopback, so it cannot be open. An iframe cannot carry a
 * bearer token — it is a navigation, not a fetch — so the panel puts the token in the URL once, and this
 * exchanges it for a short-lived cookie that every sub-resource then carries automatically. Same origin as
 * the platform, so the cookie needs no domain games and never leaves it.
 */
export const devProxyRouter = createRouter();

/** Ports only, and only real ones. This string ends up in a URL that gets fetched. */
const PORT_RE = /^[0-9]{2,5}$/;

const COOKIE = 'officer_dev_proxy';

/**
 * How long a preview stays open without re-authenticating.
 *
 * Short, because it is a bearer of "read any loopback port". Long enough that a session of watching a dev
 * server does not blink. The panel re-issues it on every reload, so the ceiling is rarely reached.
 */
const COOKIE_TTL_S = 12 * 60 * 60;

const unauthorised = () =>
  new Response('Not authorised for the dev proxy. Open it from the Preview panel.', { status: 401 });

/**
 * Prove the caller is the OWNER, once, from the `?token=` the panel puts on the iframe's URL.
 *
 * Through `resolveAuthToken` — the one function that turns a bearer string into a caller, which every
 * other door already uses — rather than a second verification written here. This route is reachable
 * without the permission gate by construction, so its check has to be the real one.
 *
 * Owner only, and not because of a permission entry: reading an arbitrary loopback port is exactly the
 * `execution` power the registry says can never be granted to anyone else.
 */
async function isOwnerToken(token: string | null): Promise<boolean> {
  if (!token) return false;
  try {
    const identity = await resolveAuthToken(token);
    return identity ? await isSuperAdmin({ id: identity.id }) : false;
  } catch {
    // Every failure denies. A transient database error must not open a proxy to the whole loopback.
    return false;
  }
}

const hasCookie = (req: Request): boolean =>
  (req.headers.get('cookie') ?? '').split(';').some((part) => part.trim().startsWith(`${COOKIE}=`));

devProxyRouter.all('/:port/*', async (ctx) => {
  const port = ctx.req.param('port');
  if (!PORT_RE.test(port)) return new Response('Bad port', { status: 400 });

  const url = new URL(ctx.req.url);
  const token = url.searchParams.get('token');
  const authorised = hasCookie(ctx.req.raw) || (await isOwnerToken(token));
  if (!authorised) return unauthorised();

  const prefix = `/api/dev-proxy/${port}`;
  // Everything after the `/<port>` segment — what the dev server should see as its own path.
  //
  // Found rather than assumed. Slicing `prefix.length` off the pathname hardcodes the mount point, and a
  // router that is wrong when mounted somewhere else is a router that is wrong: the first version of this
  // returned the index page for every asset request, because the arithmetic was off by the length of the
  // prefix it had guessed.
  const marker = `/${port}`;
  const at = url.pathname.indexOf(marker);
  const path = at === -1 ? '/' : url.pathname.slice(at + marker.length) || '/';

  // The token never reaches the dev server, and never stays in the address the app sees.
  url.searchParams.delete('token');
  const search = url.searchParams.toString();

  let res: Response;
  try {
    const headers = new Headers(ctx.req.raw.headers);
    headers.delete('host');
    headers.delete('authorization');
    headers.delete('cookie');
    res = await fetch(`http://127.0.0.1:${port}${path}${search ? `?${search}` : ''}`, {
      method: ctx.req.method,
      headers,
      body: ctx.req.method === 'GET' || ctx.req.method === 'HEAD' ? undefined : ctx.req.raw.body,
      redirect: 'manual',
    });
  } catch {
    // The most common state by far: nothing is listening yet. Said in words rather than as a 502 with an
    // empty body, because this renders inside the panel and is the whole of what the user sees.
    return new Response(`Nothing is listening on 127.0.0.1:${port} on the server.`, {
      status: 502,
      headers: { 'content-type': 'text/plain' },
    });
  }

  const type = res.headers.get('content-type') ?? '';
  const isHtml = type.includes('text/html');
  const rewritable = isHtml || type.includes('javascript') || type.includes('text/css');

  const headers = new Headers(res.headers);
  // Anything framed here is the owner's own dev server; the platform frames it deliberately.
  headers.delete('x-frame-options');
  headers.delete('content-security-policy');

  // Set on the HTML response only — that is the navigation the panel drives, and every sub-resource
  // inherits it. `Lax` is enough: the frame and the platform are the same site.
  if (isHtml && token) {
    headers.append('set-cookie', `${COOKIE}=1; Path=/api/dev-proxy; Max-Age=${COOKIE_TTL_S}; HttpOnly; SameSite=Lax`);
  }

  if (!rewritable) return new Response(res.body, { status: res.status, headers });

  const text = await res.text();
  const body = isHtml ? rewriteHtml(text, prefix) : rewriteAssetPaths(text, prefix);
  // Both are wrong after a rewrite: the length changed, and the body was decoded on the way through.
  headers.delete('content-length');
  headers.delete('content-encoding');
  return new Response(body, { status: res.status, headers });
});