🚱 Prevent stdout/stderr data listener leaks

Old listeners were not cleaned up before new ones were being added
causing the following console message:

warning: possible EventEmitter memory leak detected.

Refs #7033

spec/task-spec.coffee

@@ -57,3 +57,16 @@ describe "Task", ->
       expect(deprecations.length).toBe 1
       expect(deprecations[0].getStacks()[0][1].fileName).toBe handlerPath
       jasmine.restoreDeprecationsSnapshot()
+
+  it "adds data listeners to standard out and error to report output", ->
+    task = new Task(require.resolve('./fixtures/task-spec-handler'))
+    {stdout, stderr} = task.childProcess
+
+    task.start()
+    task.start()
+    expect(stdout.listeners('data').length).toBe 1
+    expect(stderr.listeners('data').length).toBe 1
+
+    task.terminate()
+    expect(stdout.listeners('data').length).toBe 0
+    expect(stderr.listeners('data').length).toBe 0

src/task.coffee

@@ -100,11 +100,12 @@ class Task
     @childProcess.removeAllListeners()
     @childProcess.on 'message', ({event, args}) =>
       @emit(event, args...) if @childProcess?
+
     # Catch the errors that happened before task-bootstrap.
-    @childProcess.stdout.on 'data', (data) ->
-      console.log data.toString()
-    @childProcess.stderr.on 'data', (data) ->
-      console.error data.toString()
+    @childProcess.stdout.removeAllListeners()
+    @childProcess.stdout.on 'data', (data) -> console.log data.toString()
+    @childProcess.stderr.removeAllListeners()
+    @childProcess.stderr.on 'data', (data) -> console.error data.toString()
 
   # Public: Starts the task.
   #
@@ -152,6 +153,8 @@ class Task
     return unless @childProcess?
 
     @childProcess.removeAllListeners()
+    @childProcess.stdout.removeAllListeners()
+    @childProcess.stderr.removeAllListeners()
     @childProcess.kill()
     @childProcess = null