SSR caching example is broken (UnhandledPromiseRejectionWarning: TypeError: argument entity is required)

Bug report

Describe the bug

The SSR caching login is broken with the error below.

(node:14465) UnhandledPromiseRejectionWarning: TypeError: argument entity is required
    at etag (/home/omar/Desktop/ssr-caching-app/node_modules/etag/index.js:72:11)
    at /home/omar/Desktop/ssr-caching-app/node_modules/cacheable-response/index.js:95:32
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
(node:14465) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:14465) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

To Reproduce

Steps to reproduce the behavior, please provide code snippets or a repository:

1. Go to https://github.com/vercel/next.js/tree/canary/examples/ssr-caching
2. Run locally
3. Access the website
4. See error in the terminal (notice the speed and the missing headers in the response)

Expected behavior

It should cache pages normally.

System information

• OS: Ubuntu 18.04
• Browser (if applies): Chrome
• Version of Next.js: 9.5.2(latest)
• Version of Node.js: 12.16.2

Additional context

It was working fine on our project, until it didn’t when I upgraded to v9.5.2 from v9.5.1 So I thought of testing the official example, just to find out the same issue!

[ppbl]

https://github.com/vercel/next.js/issues/14737#issuecomment-680092517

Refer to @Charioteer‘s ideas

Modify the get method of ssr-caching demo

get: async ({ req, res }) => {
      const rawResEnd = res.end
      const data = await new Promise((resolve) => {
          res.end = (payload) => {
              if (res.statusCode === 200) {
                  resolve(payload)
              } else {
                  resolve()
              }
          }
          app.render(req, res, req.path, {
              ...req.query,
              ...req.params,
          })
      })
      res.end = rawResEnd
      return { data }
},     

Currently we can write like this first 😊

Since the response needs to be terminated correctly when the send is executed, the res.end modified in get needs to be restored

[fabioespinosa]

@leerob seems like example is still broken:

/var/task/next.js/examples/ssr-caching/node_modules/etag/index.js:72
    throw new TypeError('argument entity is required')
          ^

TypeError: argument entity is required
    at etag (/var/task/next.js/examples/ssr-caching/node_modules/etag/index.js:72:11)