feat(runner): Buffer stdout and stderr for output when errors occur

When it is determined that an error occured during the run of a launch
it's useful to have output from the process.
We just need to know what was going on in order to debug (if necessary)

Related to #2663 Complete browser log output when process isn't started

lib/launchers/process.js

@@ -6,6 +6,11 @@ var ProcessLauncher = function (spawn, tempDir, timer, processKillTimeout) {
   var self = this
   var onExitCallback
   var killTimeout = processKillTimeout || 2000
+  // Will hold output from the spawned child process
+  var streamedOutputs = {
+    stdout: '',
+    stderr: ''
+  }
 
   this._tempDir = tempDir.getPath('/karma-' + this.id.toString())
 
@@ -46,6 +51,14 @@ var ProcessLauncher = function (spawn, tempDir, timer, processKillTimeout) {
     return path.normalize(cmd)
   }
 
+  this._onStdout = function (data) {
+    streamedOutputs.stdout += data
+  }
+
+  this._onStderr = function (data) {
+    streamedOutputs.stderr += data
+  }
+
   this._execCommand = function (cmd, args) {
     if (!cmd) {
       log.error('No binary for %s browser on your platform.\n  ' +
@@ -61,9 +74,12 @@ var ProcessLauncher = function (spawn, tempDir, timer, processKillTimeout) {
 
     log.debug(cmd + ' ' + args.join(' '))
     self._process = spawn(cmd, args)
-
     var errorOutput = ''
 
+    self._process.stdout.on('data', self._onStdout)
+
+    self._process.stderr.on('data', self._onStderr)
+
     self._process.on('exit', function (code) {
       self._onProcessExit(code, errorOutput)
     })
@@ -98,7 +114,14 @@ var ProcessLauncher = function (spawn, tempDir, timer, processKillTimeout) {
       error = 'crashed'
     }
 
+    if (error) {
+      log.error('%s stdout: %s', self.name, streamedOutputs.stdout)
+      log.error('%s stderr: %s', self.name, streamedOutputs.stderr)
+    }
+
     self._process = null
+    streamedOutputs.stdout = ''
+    streamedOutputs.stderr = ''
     if (self._killTimer) {
       timer.clearTimeout(self._killTimer)
       self._killTimer = null

test/unit/launchers/process.spec.js

@@ -22,6 +22,7 @@ describe('launchers/process.js', () => {
 
     mockSpawn = sinon.spy(function (cmd, args) {
       var process = new EventEmitter()
+      process.stdout = new EventEmitter()
       process.stderr = new EventEmitter()
       process.kill = sinon.spy()
       process.exitCode = null
@@ -137,16 +138,30 @@ describe('launchers/process.js', () => {
 
     // when the browser fails to get captured in default timeout, it should restart
     it('start -> timeout -> restart', (done) => {
+      var stdOutSpy = sinon.spy(launcher, '_onStdout')
+      var stdErrSpy = sinon.spy(launcher, '_onStderr')
+
       // start
       launcher.start('http://localhost/')
 
       // expect starting the process
       expect(mockSpawn).to.have.been.calledWith(BROWSER_PATH, ['http://localhost/?id=fake-id'])
       var browserProcess = mockSpawn._processes.shift()
 
+      var expectedStdoutString = 'starting...'
+      var expectedStderrString = 'Oops...there was a problem'
+      browserProcess.stdout.emit('data', expectedStdoutString)
+      browserProcess.stderr.emit('data', expectedStderrString)
+
       // timeout
       mockTimer.wind(101)
 
+      // We must've caught some output
+      expect(stdOutSpy).to.have.been.called
+      expect(stdErrSpy).to.have.been.called
+
+      stdOutSpy.calledWith(expectedStdoutString)
+
       // expect killing browser
       expect(browserProcess.kill).to.have.been.called
       browserProcess.emit('exit', 0)