Correct HMR port when switching ports fixes #457

lib/utils/develop.js

@@ -25,9 +25,8 @@ import globPages from './glob-pages'
 import webpackConfig from './webpack.config'
 const debug = require('debug')('gatsby:application')
 
-function startServer (program, launchPort) {
+function startServer (program) {
   const directory = program.directory
-  const serverPort = launchPort || program.port
 
   // Load pages for the site.
   return globPages(directory, (err, pages) => {
@@ -58,7 +57,7 @@ function startServer (program, launchPort) {
 
       server.connection({
         host: program.host,
-        port: serverPort,
+        port: program.port,
       })
 
       server.route({
@@ -170,7 +169,7 @@ function startServer (program, launchPort) {
           if (e) {
             if (e.code === 'EADDRINUSE') {
               // eslint-disable-next-line max-len
-              console.log(`Unable to start Gatsby on port ${serverPort} as there's already a process listing on that port.`)
+              console.log(`Unable to start Gatsby on port ${program.port} as there's already a process listing on that port.`)
             } else {
               console.log(e)
             }
@@ -200,12 +199,11 @@ module.exports = (program) => {
       const question = `Something is already running at port ${program.port} \nWould you like to run the app at another port instead? [Y/n] `
 
       return rlInterface.question(question, (answer) => {
-        let launchPort = program.port
         if (answer.length === 0 || answer.match(/^yes|y$/i)) {
-          launchPort = _port
+          program.port = _port
         }
 
-        return startServer(program, launchPort)
+        return startServer(program)
       })
     }