fix: compatability check header shown if client is using cached responses (@fehmer) (monkeytypegame#6602)

If frontend and backend are deployed with a new COMPATABILITY_CHECK
header frontend might show the backend version is lower because of the
http header of a cached response.

Adding the COMPATABILITY_CHECK version as part of the etag fixes this.

Author: fehmer

backend/package.json

@@ -38,6 +38,7 @@
     "cron": "2.3.0",
     "date-fns": "3.6.0",
     "dotenv": "16.4.5",
+    "etag": "1.8.1",
     "express": "4.21.1",
     "express-rate-limit": "7.4.0",
     "firebase-admin": "12.0.0",

backend/src/app.ts

@@ -10,6 +10,9 @@ import {
 } from "./middlewares/rate-limit";
 import { compatibilityCheckMiddleware } from "./middlewares/compatibilityCheck";
 import { COMPATIBILITY_CHECK_HEADER } from "@monkeytype/contracts";
+import { createETagGenerator } from "./utils/etag";
+
+const etagFn = createETagGenerator({ weak: true });
 
 function buildApp(): express.Application {
   const app = express();
@@ -27,6 +30,8 @@ function buildApp(): express.Application {
   app.use(badAuthRateLimiterHandler);
   app.use(rootRateLimiter);
 
+  app.set("etag", etagFn);
+
   addApiRoutes(app);
 
   app.use(errorHandlingMiddleware);

backend/src/utils/etag.ts

@@ -0,0 +1,25 @@
+import { COMPATIBILITY_CHECK } from "@monkeytype/contracts";
+import { default as etag } from "etag";
+
+/**
+ * create etag generator, based on the express implementation https://github.com/expressjs/express/blob/9f4dbe3a1332cd883069ba9b73a9eed99234cfc7/lib/utils.js#L247
+ * Adds the api COMPATIBILITY_CHECK version in front of the etag.
+ * @param options
+ * @returns
+ */
+export function createETagGenerator(options: {
+  weak: boolean;
+}): (body: Buffer | string, encoding: BufferEncoding | undefined) => string {
+  return function generateETag(body, encoding) {
+    const buf = !Buffer.isBuffer(body) ? Buffer.from(body, encoding) : body;
+
+    // eslint-disable-next-line @typescript-eslint/no-unsafe-assignment, @typescript-eslint/no-unsafe-call
+    const generatedTag: string = etag(buf, options);
+
+    //custom code to add the version number
+    if (generatedTag.startsWith("W/")) {
+      return `W/"V${COMPATIBILITY_CHECK}-${generatedTag.slice(3)}`;
+    }
+    return `"V${COMPATIBILITY_CHECK}-${generatedTag.slice(1)}`;
+  };
+}