Preview mentions (#5571)

Co-authored-by: Tom Moor <tom.moor@gmail.com>

server/routes/api/urls/urls.ts

@@ -0,0 +1,64 @@
+import Router from "koa-router";
+import parseDocumentSlug from "@shared/utils/parseDocumentSlug";
+import parseMentionUrl from "@shared/utils/parseMentionUrl";
+import { NotFoundError } from "@server/errors";
+import auth from "@server/middlewares/authentication";
+import validate from "@server/middlewares/validate";
+import { Document, User } from "@server/models";
+import { authorize } from "@server/policies";
+import { presentDocument, presentMention } from "@server/presenters/unfurls";
+import { APIContext } from "@server/types";
+import * as T from "./schema";
+
+const router = new Router();
+
+router.post(
+  "urls.unfurl",
+  auth(),
+  validate(T.UrlsUnfurlSchema),
+  async (ctx: APIContext<T.UrlsUnfurlReq>) => {
+    const { url, documentId } = ctx.input.body;
+    const { user: actor } = ctx.state.auth;
+    const urlObj = new URL(url);
+
+    if (urlObj.protocol === "mention:") {
+      const { modelId: userId } = parseMentionUrl(url);
+
+      const [user, document] = await Promise.all([
+        User.findByPk(userId),
+        Document.findByPk(documentId!, {
+          userId,
+        }),
+      ]);
+      if (!user) {
+        throw NotFoundError("Mentioned user does not exist");
+      }
+      if (!document) {
+        throw NotFoundError("Document does not exist");
+      }
+      authorize(actor, "read", user);
+      authorize(actor, "read", document);
+
+      ctx.body = presentMention(user, document);
+      return;
+    }
+
+    const previewDocumentId = parseDocumentSlug(url);
+    if (!previewDocumentId) {
+      ctx.response.status = 204;
+      return;
+    }
+
+    const document = previewDocumentId
+      ? await Document.findByPk(previewDocumentId)
+      : undefined;
+    if (!document) {
+      throw NotFoundError("Document does not exist");
+    }
+    authorize(actor, "read", document);
+
+    ctx.body = presentDocument(document, actor);
+  }
+);
+
+export default router;