Allow top level playbook files to import other playbook files, added some basic

playbook structure checking.

examples/playbooks/nested_playbooks.yml

+---
+# it is possible to have top level playbook files import other playbook
+# files.  For example, a playbook called could include three
+# different playbooks, such as webservers, workers, dbservers, etc.
+#
+# Running the site playbook would run all playbooks, while individual
+# playbooks could still be run directly.  This is somewhat like
+# the tag feature and can be used in conjunction for very fine grained
+# control over what you want to target when running ansible.
+
+- name: this is a play at the top level of a file  
+  hosts: all
+  user: root
+  tasks:
+  - name: say hi
+    tags: foo
+    action: shell echo "hi..."
+
+# and this is how we include another playbook, be careful and
+# don't recurse infinitely or anything.  Note you can't use
+# any variables here.
+
+- include: intro_example.yml
+
+# and if we wanted, we can continue with more includes here,
+# or more plays inline in this file

lib/ansible/playbook/__init__.py

@@ -116,12 +116,46 @@ class PlayBook(object):
         if not self.inventory._is_script:
             self.global_vars.update(self.inventory.get_group_variables('all'))
 
-        self.basedir    = os.path.dirname(playbook)
-        self.playbook  = utils.parse_yaml_from_file(playbook)
+        self.basedir   = os.path.dirname(playbook)
+        self.playbook  = self._load_playbook_from_file(playbook)
 
         self.module_path = self.module_path + os.pathsep + os.path.join(self.basedir, "library")
 
     # *****************************************************
+
+    def _load_playbook_from_file(self, path):
+        '''
+        do some top level error checking on playbooks and allow them to include other
+        playbooks.
+        '''
+
+        playbook_data  = utils.parse_yaml_from_file(path)
+        accumulated_plays = []
+
+        if type(playbook_data) != list:
+           raise errors.AnsibleError(
+               "parse error: playbooks must be formatted as a YAML list"
+           )
+
+        for play in playbook_data:
+           if type(play) != dict:
+               raise errors.AnsibleError(
+                   "parse error: each play in a playbook must a YAML dictionary (hash), recieved: %s" % play
+               )
+           if 'include' in play:
+               if len(play.keys()) == 1:
+                   included_path = utils.path_dwim(self.basedir, play['include'])
+                   accumulated_plays.extend(self._load_playbook_from_file(included_path))
+               else:
+                   raise errors.AnsibleError(
+                       "parse error: top level includes cannot be used with other directives: %s" % play
+                   )
+           else:
+               accumulated_plays.append(play)
+
+        return accumulated_plays
+
+    # *****************************************************
         
     def run(self):
         ''' run all patterns in the playbook '''

lib/ansible/playbook/play.py

@@ -154,6 +154,10 @@ class Play(object):
 
     def should_run(self, tags):
         ''' does the play match any of the tags? '''
+
+        if len(self._tasks) == 0:
+            return False
+
         for task in self._tasks:
             for task_tag in task.tags:
                 if task_tag in tags: