Integration with next-auth fails, routing issue

🐛 Bug Report

I want to integrate next-auth with my fastify-nextjs project. This worked until the next.auth library does the login/POST callback - somehow this call does not succeed and just times out. I ported the exact same code to a vanilla Next.js project (using npx create-next-app) and there it works flawlessly!

To Reproduce

• Fastify Next.js example which does not work: https://github.com/neophob/nextjs-fastify-auth
• Next.js only example which does work: https://github.com/neophob/nextjs-auth

Expected behavior

call is terminated correct / callback is called. I quickly added some console log into the plugin to figure out where the code hangs:

    function handler (req, reply) {
      if (callback) {
        return callback(app, req, reply)
      }
      console.log("handle next request", req.params)
      return handleNextRequests(req.raw, reply.raw)
        .then(() => {
          console.log("...handled")
          reply.sent = true
        })

• when the login POST request is hanging, the handleNextRequests function is running
• when I cancel the POST request (using postman), then the ...handled console output is visible…

the console shows this output:

{"level":30,"time":1613721952594,"pid":94353,"hostname":"hostname","reqId":29,"req":{"method":"POST","url":"/api/auth/callback/credentials","hostname":"localhost:3333","remoteAddress":"::1","remotePort":55873},"msg":"incoming request"}
handle next request { '*': 'auth/callback/credentials' }
event - build page: /api/auth/[...nextauth]
wait  - compiling...
event - compiled successfully

To summarize, I think the POST call to /api/auth/callback/credentials is either not routed correctly OR the return value is invalid (promise vs. value maybe?)

Your Environment

• node version: 12 and 14
• fastify version: 3.12
• os: Mac

[simoneb]

Not exactly, we would like to include this feature in the plugin so it doesn’t have to be done in user code.

[neophob]

The user-land fix works for me! Thanks again!