Allow field attributes to have priorities

So that roles are loaded ahead of all other play fields, meaning any
plugins contained within the roles are loaded first before tasks.

Fixes #11858

lib/ansible/playbook/attribute.py

@@ -21,13 +21,17 @@ __metaclass__ = type
 
 class Attribute:
 
-    def __init__(self, isa=None, private=False, default=None, required=False, listof=None):
+    def __init__(self, isa=None, private=False, default=None, required=False, listof=None, priority=0):
 
        self.isa = isa
        self.private = private
        self.default = default
        self.required = required
        self.listof = listof
+       self.priority = priority
+
+    def __cmp__(self, other):
+       return cmp(other.priority, self.priority)
 
 class FieldAttribute(Attribute):
     pass

lib/ansible/playbook/base.py

@@ -20,6 +20,7 @@ from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
 import itertools
+import operator
 import uuid
 
 from functools import partial
@@ -153,24 +154,18 @@ class Base:
         else:
             self._loader = DataLoader()
 
-        # FIXME: is this required anymore? This doesn't seem to do anything
-        #        helpful, and was added in very early stages of the base class
-        #        development.
-        #if isinstance(ds, string_types) or isinstance(ds, FileIO):
-        #    ds = self._loader.load(ds)
-
         # call the preprocess_data() function to massage the data into
         # something we can more easily parse, and then call the validation
         # function on it to ensure there are no incorrect key values
         ds = self.preprocess_data(ds)
         self._validate_attributes(ds)
 
-        # Walk all attributes in the class.
+        # Walk all attributes in the class. We sort them based on their priority
-        #
+        # so that certain fields can be loaded before others, if they are dependent.
         # FIXME: we currently don't do anything with private attributes but
         #        may later decide to filter them out of 'ds' here.
-
+        base_attributes = self._get_base_attributes()
-        for name in self._get_base_attributes():
+        for name, attr in sorted(base_attributes.items(), key=operator.itemgetter(1)):
             # copy the value over unless a _load_field method is defined
             if name in ds:
                 method = getattr(self, '_load_%s' % name, None)

lib/ansible/playbook/play.py

@@ -69,15 +69,15 @@ class Play(Base, Taggable, Become):
     _vars_prompt         = FieldAttribute(isa='list', default=[])
     _vault_password      = FieldAttribute(isa='string')
 
+    # Role Attributes
+    _roles               = FieldAttribute(isa='list', default=[], priority=100)
+
     # Block (Task) Lists Attributes
     _handlers            = FieldAttribute(isa='list', default=[])
     _pre_tasks           = FieldAttribute(isa='list', default=[])
     _post_tasks          = FieldAttribute(isa='list', default=[])
     _tasks               = FieldAttribute(isa='list', default=[])
 
-    # Role Attributes
-    _roles               = FieldAttribute(isa='list', default=[])
-
     # Flag/Setting Attributes
     _any_errors_fatal    = FieldAttribute(isa='bool', default=False)
     _force_handlers      = FieldAttribute(isa='bool')