adr-sml/infra/cloudfront/router.js

/**
 * CloudFront Function, VIEWER REQUEST, on the default cache behaviour only.
 *
 * ⚠️ THE SITE DOES NOT WORK WITHOUT THIS. `astro.config.mjs` sets
 * `trailingSlash: 'always'` and `build.format: 'directory'`, so every route is
 * `<dir>/index.html`. CloudFront forwards the viewer path to the S3 REST origin
 * unchanged, S3 has no key `about/`, and the request fails. Measured on the live
 * distribution 2026-09-01, before this function existed: `/about/` and
 * `/definitely-not-a-page/` both returned **403 with an 111-byte
 * `application/xml` body** — S3's AccessDenied, served raw to the reader. Only
 * `/` worked, via the distribution's default root object. That is 22 of the 23
 * pages.
 *
 * ⚠️ DO NOT ASSOCIATE IT WITH THE `/api/*` BEHAVIOUR. The intake path
 * `/api/intake` has no extension and no trailing slash, so the redirect branch
 * below would answer a form POST with a 301 — and a 301 turns a POST into a GET,
 * which would lose the submission body silently. The association is per
 * behaviour and `/api/*` gets none.
 *
 * Two rules, and the second is a `docs/04` requirement rather than a nicety:
 *
 *   /about/   -> rewrite to /about/index.html      (the origin has that key)
 *   /about    -> 301 to /about/                    (one canonical URL per page)
 *
 * Anything with a file extension in its last segment is left alone —
 * `robots.txt`, `sitemap-0.xml`, `/_astro/*`, `/fonts/*`, `/og/*.jpg`,
 * `favicon.ico`, `pouya-lajevardi-bio.pdf`, and `404.html` itself.
 *
 * Written to the `cloudfront-js-2.0` runtime and deliberately conservative: no
 * arrow functions, no `String.prototype.endsWith`, no template literals. The
 * runtime supports more than this; a viewer-request function runs on every
 * request to the site and is the wrong place to be clever.
 */
/* The header-injection surface, and nothing else: C0 controls, DEL, space, and
   WHATWG's query percent-encode set (`"`, `#`, `<`, `>`). `#` is in because it
   changes the STRUCTURE of the Location — left in, `?a=x#&b=y` drops `&b=y` into
   a fragment. `| ^ ` { }` are NOT in, and must not be added: browsers send them
   raw and `|` is routine in tracking values. Strip rather than encode — these
   values arrive percent-encoded, so encoding again makes `%20` into `%2520`. */
function safe(part) {
  // eslint-disable-next-line no-control-regex
  return String(part).replace(/[\u0000-\u0020\u007f"<>#]/g, '');
}

function handler(event) {
  var request = event.request;

  /* ⚠️ NORMALISE, THEN REDIRECT IF ANYTHING CHANGED. Leading `//` and `\` are
     collapsed because CloudFront forwards duplicate slashes verbatim (it resolves
     dot-segments; it does not collapse `//`) and `Location: //host/x` is a
     network-path reference that REPLACES THE AUTHORITY — RFC 3986 s4.2. `\` does
     the same, because the URL Standard maps it to `/` in special schemes.
     Redirect rather than rewrite, or `//about/` serves the About page at a second
     URL with a 200. Only the leading run: an interior `//` is a key that does not
     exist. */
  var uri = request.uri.replace(/\\/g, '/').replace(/^\/+/, '/');
  var normalised = uri !== request.uri;
  var lastSlash = uri.lastIndexOf('/');
  var lastSegment = uri.substring(lastSlash + 1);

  // A file, not a route.
  if (lastSegment.indexOf('.') !== -1) {
    if (normalised) return moved(uri, request);
    return request;
  }

  // A directory-style route: hand the origin the key it actually holds.
  if (lastSegment === '') {
    if (normalised) return moved(uri, request);
    request.uri = uri + 'index.html';
    return request;
  }

  /* Extensionless and no trailing slash. Redirect rather than rewrite, so the
     page has ONE address: serving it at both would put two indexable URLs on the
     same content, which `docs/04` treats as its primary concern. */
  return moved(uri + '/', request);
}

/**
 * 301 to a path on this origin, carrying the query string. `location` is always
 * built from an already-normalised path, which is what keeps it same-origin.
 */
function moved(path, request) {
  var qs = '';
  var names = Object.keys(request.querystring);
  for (var i = 0; i < names.length; i++) {
    var name = names[i];
    var value = request.querystring[name];
    if (value.multiValue) {
      for (var j = 0; j < value.multiValue.length; j++) {
        qs +=
          (qs === '' ? '' : '&') +
          safe(name) +
          '=' +
          safe(value.multiValue[j].value);
      }
    } else {
      /* Always `name=value`, so `?ref` and `?ref=` normalise to one form rather
         than the function guessing which the viewer meant. */
      qs += (qs === '' ? '' : '&') + safe(name) + '=' + safe(value.value);
    }
  }
  return {
    statusCode: 301,
    statusDescription: 'Moved Permanently',
    headers: {
      location: { value: path + (qs === '' ? '' : '?' + qs) },
      'cache-control': { value: 'public, max-age=0, must-revalidate' },
    },
  };
}