feat: add listen async callback warning (fastify#6011)

Author: Eomm

lib/server.js

@@ -6,6 +6,7 @@ const dns = require('node:dns')
 const os = require('node:os')
 
 const { kState, kOptions, kServerBindings } = require('./symbols')
+const { FSTWRN003 } = require('./warnings')
 const { onListenHookRunner } = require('./hooks')
 const {
   FST_ERR_HTTP2_INVALID_VERSION,
@@ -29,6 +30,10 @@ function createServer (options, httpHandler) {
     cb = undefined
   ) {
     if (typeof cb === 'function') {
+      if (cb.constructor.name === 'AsyncFunction') {
+        FSTWRN003('listen method')
+      }
+
       listenOptions.cb = cb
     }
     if (listenOptions.signal) {

lib/warnings.js

@@ -8,6 +8,7 @@ const { createWarning } = require('process-warning')
  *   - FSTSEC001
  *
  * Deprecation Codes FSTDEP001 - FSTDEP021 were used by v4 and MUST NOT not be reused.
+ * Warning Codes FSTWRN001 - FSTWRN002 were used by v4 and MUST NOT not be reused.
  */
 
 const FSTWRN001 = createWarning({
@@ -17,6 +18,13 @@ const FSTWRN001 = createWarning({
   unlimited: true
 })
 
+const FSTWRN003 = createWarning({
+  name: 'FastifyWarning',
+  code: 'FSTWRN003',
+  message: 'The %s mixes async and callback styles that may lead to unhandled rejections. Please use only one of them.',
+  unlimited: true
+})
+
 const FSTSEC001 = createWarning({
   name: 'FastifySecurity',
   code: 'FSTSEC001',
@@ -26,5 +34,6 @@ const FSTSEC001 = createWarning({
 
 module.exports = {
   FSTWRN001,
+  FSTWRN003,
   FSTSEC001
 }

test/listen.5.test.js

@@ -4,6 +4,7 @@ const { test } = require('node:test')
 const net = require('node:net')
 const Fastify = require('../fastify')
 const { once } = require('node:events')
+const { FSTWRN003 } = require('../lib/warnings.js')
 
 function createDeferredPromise () {
   const promise = {}
@@ -97,3 +98,25 @@ test('same port conflict and success should not fire callback multiple times - p
   await fastify.listen()
   await fastify.close()
 })
+
+test('should emit a warning when using async callback', (t, done) => {
+  t.plan(2)
+
+  process.on('warning', onWarning)
+  function onWarning (warning) {
+    t.assert.strictEqual(warning.name, 'FastifyWarning')
+    t.assert.strictEqual(warning.code, FSTWRN003.code)
+  }
+
+  const fastify = Fastify()
+
+  t.after(async () => {
+    await fastify.close()
+    process.removeListener('warning', onWarning)
+    FSTWRN003.emitted = false
+  })
+
+  fastify.listen({ port: 0 }, async function doNotUseAsyncCallback () {
+    done()
+  })
+})