fix(server): fix setupExitSignals usage

bin/webpack-dev-server.js

@@ -19,8 +19,14 @@ const getVersions = require('../lib/utils/getVersions');
 const options = require('./options');
 
 let server;
-
-setupExitSignals(server);
+const serverData = {
+  server: null,
+};
+// we must pass an object that contains the server object as a property so that
+// we can update this server property later, and setupExitSignals will be able to 
+// recognize that the server has been instantiated, because we will set
+// serverData.server to the new server object.
+setupExitSignals(serverData);
 
 // Prefer the local installation of webpack-dev-server
 if (importLocal(__filename)) {
@@ -98,6 +104,7 @@ function startDevServer(config, options) {
 
   try {
     server = new Server(compiler, options, log);
+    serverData.server = server;
   } catch (err) {
     if (err.name === 'ValidationError') {
       log.error(colors.error(options.stats.colors, err.message));

lib/utils/setupExitSignals.js

@@ -2,11 +2,11 @@
 
 const signals = ['SIGINT', 'SIGTERM'];
 
-function setupExitSignals(server) {
+function setupExitSignals(serverData) {
   signals.forEach((signal) => {
     process.on(signal, () => {
-      if (server) {
-        server.close(() => {
+      if (serverData.server) {
+        serverData.server.close(() => {
           // eslint-disable-next-line no-process-exit
           process.exit();
         });

test/server/utils/setupExitSignals.test.js

@@ -0,0 +1,57 @@
+'use strict';
+
+const setupExitSignals = require('../../../lib/utils/setupExitSignals');
+
+describe('setupExitSignals', () => {
+  let server;
+  let exitSpy;
+  const signals = ['SIGINT', 'SIGTERM'];
+
+  beforeAll(() => {
+    exitSpy = jest.spyOn(process, 'exit').mockImplementation(() => { });
+    server = {
+      close: jest.fn((callback) => {
+        callback();
+      }),
+    };
+  });
+
+  afterEach(() => {
+    exitSpy.mockReset();
+    server.close.mockClear();
+    signals.forEach((signal) => {
+      process.removeAllListeners(signal);
+    });
+  });
+
+  signals.forEach((signal) => {
+    it(`should exit process (${signal}, server never defined)`, (done) => {
+      setupExitSignals({
+        server: null,
+      });
+      process.emit(signal);
+      setTimeout(() => {
+        expect(exitSpy.mock.calls.length).toEqual(1);
+        done();
+      }, 1000);
+    });
+
+    it(`should close server, then exit process (${signal}, server defined before signal)`, (done) => {
+      const serverData = {
+        server: null,
+      };
+      setupExitSignals(serverData);
+
+      setTimeout(() => {
+        serverData.server = server;
+        process.emit(signal);
+      }, 500);
+
+      setTimeout(() => {
+        expect(server.close.mock.calls.length).toEqual(1);
+        expect(exitSpy.mock.calls.length).toEqual(1);
+        done();
+      }, 1500);
+    });
+  });
+});