gerbeur/api/middleware/og.ts

/**
 * Answers a link to a dump, a profile or a public playlist with `index.html`
 * carrying that page's Open Graph and Twitter card tags, for the link previews
 * of chat apps and social sites, which don't run the app. main.ts installs it
 * before the routers.
 *
 * @module
 */

import { Context, Next } from "@oak/oak";
import { getDump } from "../services/dump-service.ts";
import { getUserByUsername } from "../services/user-service.ts";
import { getPlaylistById } from "../services/playlist-service.ts";
import { FRONTEND_URL, OG_SITE_NAME, PUBLIC_URL } from "../config.ts";
import { loadIndexHtml } from "../lib/index-html.ts";

interface OGMeta {
  title: string;
  description?: string;
  imageUrl?: string;
  /**
   * Set for a dump's artwork only. It asks for the large card, which would
   * stretch a square icon or avatar.
   */
  imageIsWide?: boolean;
  url: string;
}

function escapeAttr(s: string): string {
  return s
    .replace(/&/g, "&")
    .replace(/"/g, """)
    .replace(/</g, "&lt;")
    .replace(/>/g, "&gt;");
}

function buildTags(meta: OGMeta): string {
  const card = meta.imageUrl && meta.imageIsWide
    ? "summary_large_image"
    : "summary";
  const tags = [
    `<title>${escapeAttr(meta.title)}</title>`,
    `<meta property="og:site_name" content="${escapeAttr(OG_SITE_NAME)}" />`,
    `<meta property="og:type" content="website" />`,
    `<meta property="og:url" content="${escapeAttr(meta.url)}" />`,
    `<meta property="og:title" content="${escapeAttr(meta.title)}" />`,
    `<meta name="twitter:card" content="${card}" />`,
  ];
  if (meta.description) {
    tags.push(
      `<meta name="description" content="${escapeAttr(meta.description)}" />`,
      `<meta property="og:description" content="${
        escapeAttr(meta.description)
      }" />`,
    );
  }
  if (meta.imageUrl) {
    tags.push(
      `<meta property="og:image" content="${escapeAttr(meta.imageUrl)}" />`,
    );
  }
  return tags.join("\n    ");
}

function inject(html: string, meta: OGMeta): string {
  return html
    .replace(/<title>[^<]*<\/title>/, "")
    .replace("</head>", `    ${buildTags(meta)}\n  </head>`);
}

/**
 * Returns the filled `index.html` of dist/, or else the source one, which is
 * all there is in development. Returns null when neither can be read.
 */
async function findIndexHtml(): Promise<string | null> {
  for (const root of [`${Deno.cwd()}/dist`, Deno.cwd()]) {
    try {
      return await loadIndexHtml(root);
    } catch {
      continue;
    }
  }
  return null;
}

const DUMP_RE = /^\/dumps\/([^/]+)$/;
/** Matches a profile, and its tabs under `/~/<tab>`. */
const USER_RE = /^\/users\/([^/]+)(?:\/~\/[^/]+)?$/;
const PLAYLIST_RE = /^\/playlists\/([^/]+)$/;

/**
 * Serves `index.html` with the tags of the dump, user or public playlist a
 * path names, and passes the request on when it names none that exists or
 * the visitor may see. The page's URL is on `FRONTEND_URL` and the images' on
 * `PUBLIC_URL`, not on the request's origin, which a reverse proxy may have
 * rewritten to an internal host or plain http.
 */
export async function ogMiddleware(
  ctx: Context,
  next: Next,
): Promise<unknown> {
  const { pathname } = ctx.request.url;

  if (pathname.startsWith("/api/") || pathname.includes(".")) {
    return await next();
  }

  const origin = PUBLIC_URL;
  const pageUrl = `${FRONTEND_URL}${pathname}`;

  let meta: OGMeta | null = null;

  const dumpMatch = pathname.match(DUMP_RE);
  const userMatch = pathname.match(USER_RE);
  const playlistMatch = pathname.match(PLAYLIST_RE);

  if (dumpMatch) {
    try {
      const dump = getDump(dumpMatch[1]);
      let imageUrl: string | undefined;
      if (dump.kind === "file" && dump.fileMime?.startsWith("image/")) {
        imageUrl = `${origin}/api/files/${dump.id}`;
      } else if (dump.richContent?.thumbnailUrl) {
        imageUrl = dump.richContent.thumbnailUrl;
      }
      // Without artwork, the site's icon: the linked site's favicon is too
      // small for a card, and may refuse hotlinking.
      meta = {
        title: dump.title,
        description: dump.comment,
        imageUrl: imageUrl ?? `${origin}/apple-touch-icon.png`,
        imageIsWide: !!imageUrl,
        url: pageUrl,
      };
    } catch { /* Unknown, or private. */ }
  } else if (userMatch) {
    try {
      const user = getUserByUsername(userMatch[1]);
      const imageUrl = user.avatarMime
        ? `${origin}/api/avatars/${user.id}`
        : undefined;
      meta = {
        title: user.username,
        description: user.description,
        imageUrl: imageUrl ?? `${origin}/apple-touch-icon.png`,
        url: pageUrl,
      };
    } catch { /* Unknown. */ }
  } else if (playlistMatch) {
    try {
      const playlist = getPlaylistById(playlistMatch[1]);
      // getPlaylistById doesn't check visibility, unlike getDump.
      if (playlist.isPublic) {
        const imageUrl = playlist.imageMime
          ? `${origin}/api/playlists/${playlist.id}/image`
          : undefined;
        meta = {
          title: playlist.title,
          description: playlist.description,
          imageUrl: imageUrl ?? `${origin}/apple-touch-icon.png`,
          url: pageUrl,
        };
      }
    } catch { /* Unknown. */ }
  }

  if (!meta) return await next();

  const html = await findIndexHtml();
  if (!html) return await next();

  ctx.response.headers.set("Content-Type", "text/html; charset=utf-8");
  ctx.response.body = inject(html, meta);
}