Display debug messages after playbook run

This patch adds a callback handler for displaying debug messages at the end of
a playbook run. This is helpful in situations where a debug message might be
lost in the middle of the Ansible output while a playbook runs.

You can view the output from a small example playbook here:

  https://gist.github.com/major/9583c97e335262228c2a9ede50d86458

Change-Id: I962b586751069a16adb073d05b33d310e0afc5af

callback/debug_message_collector.py

@@ -0,0 +1,63 @@
+#!/usr/bin/env python
+# Copyright 2016, Rackspace US, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Callback for displaying debug messages after a playbook run."""
+
+from ansible.plugins.callback import CallbackBase
+
+
+class CallbackModule(CallbackBase):
+    """
+    Callback plugin for collecting messages from debug tasks.
+
+    This plugin watches for debug tasks, collects the messages from those
+    tasks, and displays them at the end of the playbook run.
+    """
+
+    # Ansible constants for the plugin
+    CALLBACK_VERSION = 2.0
+    CALLBACK_TYPE = 'aggregate'
+    CALLBACK_NAME = 'debug_message_collector'
+    CALLBACK_NEEDS_WHITELIST = False
+
+    def __init__(self):
+        """Constructor for the plugin class."""
+        self.debug_tasks = []
+
+    def v2_playbook_on_task_start(self, task, is_conditional):
+        """
+        Collect debug messages from tasks.
+
+        After each task runs, this function checks to see if the task's action
+        is 'debug'. If so, we collect the name of the task as well as its
+        debug message here.
+        """
+        if task.action == 'debug':
+            self.debug_tasks.append({
+                'name': task.name,
+                'msg': task.args['msg']
+            })
+
+    def v2_playbook_on_stats(self, stats):
+        """
+        Print debug messages when the playbook has finished.
+
+        Print all of the collected debug messages (if any exist) and display
+        them at the end of the Ansible task output.
+        """
+        if len(self.debug_tasks) > 0:
+            print("DEBUG MESSAGE RECAP ".ljust(80, '*'))
+        for task in self.debug_tasks:
+            print("DEBUG: [{0}] ".format(task['name']).ljust(80, '*'))
+            print("{0}\n".format(task['msg']))