[gatsby develop]: cleanup port detection

Author: Moocar

packages/gatsby/src/commands/develop.js

@@ -13,6 +13,7 @@ const { formatError } = require(`graphql`)
 const request = require(`request`)
 const rl = require(`readline`)
 const webpack = require(`webpack`)
+const { promisify } = require(`util`)
 const webpackConfig = require(`../utils/webpack.config`)
 const bootstrap = require(`../bootstrap`)
 const { store } = require(`../redux`)
@@ -32,6 +33,7 @@ const slash = require(`slash`)
 const { initTracer } = require(`../utils/tracer`)
 const apiRunnerNode = require(`../utils/api-runner-node`)
 const telemetry = require(`gatsby-telemetry`)
+const detectPort = promisify(require(`detect-port`))
 
 // const isInteractive = process.stdout.isTTY
 
@@ -260,7 +262,6 @@ module.exports = async (program: any) => {
   telemetry.trackCli(`DEVELOP_START`)
   telemetry.startBackgroundUpdate()
 
-  const detect = require(`detect-port`)
   const port =
     typeof program.port === `string` ? parseInt(program.port, 10) : program.port
 
@@ -284,35 +285,27 @@ module.exports = async (program: any) => {
     })
   }
 
-  let compiler
-  await new Promise(resolve => {
-    detect(port, (err, _port) => {
-      if (err) {
-        report.panic(err)
-      }
-
-      if (port !== _port) {
-        // eslint-disable-next-line max-len
-        const question = `Something is already running at port ${port} \nWould you like to run the app at another port instead? [Y/n] `
-
-        rlInterface.question(question, answer => {
-          if (answer.length === 0 || answer.match(/^yes|y$/i)) {
-            program.port = _port // eslint-disable-line no-param-reassign
-          }
+  const readlinePort = port => {
+    const question = `Something is already running at port ${port} \nWould you like to run the app at another port instead? [Y/n] `
+    return new Promise(resolve => {
+      rlInterface.question(question, answer => {
+        resolve(answer.length === 0 || answer.match(/^yes|y$/i))
+      })
+    })
+  }
 
-          startServer(program).then(([c, l]) => {
-            compiler = c
-            resolve()
-          })
-        })
-      } else {
-        startServer(program).then(([c, l]) => {
-          compiler = c
-          resolve()
-        })
+  const selectPort = async port => {
+    let foundPort = port
+    const detectedPort = await detectPort(port)
+    if (port !== detectedPort) {
+      if (await readlinePort(port)) {
+        foundPort = detectedPort
       }
-    })
-  })
+    }
+    return foundPort
+  }
+  program.port = await selectPort(port)
+  const [compiler] = await startServer(program)
 
   function prepareUrls(protocol, host, port) {
     const formatUrl = hostname =>