fix missing console.log messages (jestjs#3895)

* Ensure that console output is not lost in concurrent reporter

* console.log integration test + naming

integration_tests/__tests__/__snapshots__/console_log_output_when_run_in_band.test.js.snap

@@ -0,0 +1,24 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`prints console.logs when run with forceExit 1`] = `
+" PASS  __tests__/a-banana.js
+  ✓ banana
+
+"
+`;
+
+exports[`prints console.logs when run with forceExit 2`] = `
+"Test Suites: 1 passed, 1 total
+Tests:       1 passed, 1 total
+Snapshots:   0 total
+Time:        <<REPLACED>>
+Ran all test suites.
+"
+`;
+
+exports[`prints console.logs when run with forceExit 3`] = `
+"  console.log __tests__/a-banana.js:2
+    Hey
+
+"
+`;

integration_tests/__tests__/console_log_output_when_run_in_band.test.js

@@ -0,0 +1,41 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+'use strict';
+
+const path = require('path');
+const skipOnWindows = require('skipOnWindows');
+const {extractSummary, cleanup, writeFiles} = require('../utils');
+const runJest = require('../runJest');
+
+const DIR = path.resolve(__dirname, '../console_log_output_when_run_in_band');
+
+skipOnWindows.suite();
+
+beforeEach(() => cleanup(DIR));
+afterAll(() => cleanup(DIR));
+
+test('prints console.logs when run with forceExit', () => {
+  writeFiles(DIR, {
+    '__tests__/a-banana.js': `
+      test('banana', () => console.log('Hey'));
+    `,
+    'package.json': '{}',
+  });
+
+  const {stderr, stdout, status} = runJest(DIR, [
+    '-i',
+    '--ci=false',
+    '--forceExit',
+  ]);
+  const {rest, summary} = extractSummary(stderr);
+  expect(status).toBe(0);
+  expect(rest).toMatchSnapshot();
+  expect(summary).toMatchSnapshot();
+  expect(stdout).toMatchSnapshot();
+});

packages/jest-cli/src/reporters/DefaultReporter.js

@@ -24,6 +24,7 @@ import getConsoleOutput from './getConsoleOutput';
 import getResultHeader from './getResultHeader';
 
 type write = (chunk: string, enc?: any, cb?: () => void) => boolean;
+type FlushBufferedOutput = () => void;
 
 const TITLE_BULLET = chalk.bold('\u25cf ');
 
@@ -35,6 +36,7 @@ class DefaultReporter extends BaseReporter {
   _globalConfig: GlobalConfig;
   _out: write;
   _status: Status;
+  _bufferedOutput: Set<FlushBufferedOutput>;
 
   constructor(globalConfig: GlobalConfig) {
     super();
@@ -43,6 +45,7 @@ class DefaultReporter extends BaseReporter {
     this._out = process.stdout.write.bind(process.stdout);
     this._err = process.stderr.write.bind(process.stderr);
     this._status = new Status();
+    this._bufferedOutput = new Set();
     this._wrapStdio(process.stdout);
     this._wrapStdio(process.stderr);
     this._status.onChange(() => {
@@ -57,25 +60,28 @@ class DefaultReporter extends BaseReporter {
     let buffer = [];
     let timeout = null;
 
-    const doFlush = () => {
+    const flushBufferedOutput = () => {
       const string = buffer.join('');
       buffer = [];
       // This is to avoid conflicts between random output and status text
       this._clearStatus();
       originalWrite.call(stream, string);
       this._printStatus();
+      this._bufferedOutput.delete(flushBufferedOutput);
     };
 
-    const flush = () => {
+    this._bufferedOutput.add(flushBufferedOutput);
+
+    const debouncedFlush = () => {
       // If the process blows up no errors would be printed.
       // There should be a smart way to buffer stderr, but for now
       // we just won't buffer it.
       if (stream === process.stderr) {
-        doFlush();
+        flushBufferedOutput();
       } else {
         if (!timeout) {
           timeout = setTimeout(() => {
-            doFlush();
+            flushBufferedOutput();
             timeout = null;
           }, 100);
         }
@@ -85,11 +91,18 @@ class DefaultReporter extends BaseReporter {
     // $FlowFixMe
     stream.write = chunk => {
       buffer.push(chunk);
-      flush();
+      debouncedFlush();
       return true;
     };
   }
 
+  // Don't wait for the debounced call and flush all output immediately.
+  forceFlushBufferedOutput() {
+    for (const flushBufferedOutput of this._bufferedOutput) {
+      flushBufferedOutput();
+    }
+  }
+
   _clearStatus() {
     if (isInteractive) {
       this._out(this._clear);
@@ -116,6 +129,7 @@ class DefaultReporter extends BaseReporter {
   }
 
   onRunComplete() {
+    this.forceFlushBufferedOutput();
     this._status.runFinished();
     // $FlowFixMe
     process.stdout.write = this._out;
@@ -139,6 +153,7 @@ class DefaultReporter extends BaseReporter {
       test.context.config,
       testResult,
     );
+    this.forceFlushBufferedOutput();
   }
 
   _printTestFileSummary(