template: fix KeyError: 'undefined variable: 0 (#27972)

* template: fix KeyError: 'undefined variable: 0

For compatibility with the Context.get_all() implementation
in jinja 2.9, make AnsibleJ2Vars implement collections.Mapping.
Also, make AnsibleJ2Template.newcontext() handle dict type
for the 'vars' parameter.

See: d67f0fd4cc
Fixes: https://github.com/ansible/ansible/issues/20494

* add units/template/test_vars

* intg tests for jinja-2.9 issues like 20494

test cases here are based on
https://github.com/ansible/ansible/issues/20494#issue-202108318

lib/ansible/template/template.py

@@ -33,4 +33,11 @@ class AnsibleJ2Template(jinja2.environment.Template):
     '''
 
     def new_context(self, vars=None, shared=False, locals=None):
-        return self.environment.context_class(self.environment, vars.add_locals(locals), self.name, self.blocks)
+        if vars is not None:
+            if isinstance(vars, dict):
+                vars = vars.copy()
+                if locals is not None:
+                    vars.update(locals)
+            else:
+                vars = vars.add_locals(locals)
+        return self.environment.context_class(self.environment, vars, self.name, self.blocks)

lib/ansible/template/vars.py

@@ -19,6 +19,8 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
+from collections import Mapping
+
 from jinja2.utils import missing
 
 from ansible.module_utils.six import iteritems
@@ -28,7 +30,7 @@ from ansible.module_utils._text import to_native
 __all__ = ['AnsibleJ2Vars']
 
 
-class AnsibleJ2Vars:
+class AnsibleJ2Vars(Mapping):
     '''
     Helper class to template all variable content before jinja2 sees it. This is
     done by hijacking the variable storage that jinja2 uses, and overriding __contains__
@@ -69,6 +71,16 @@ class AnsibleJ2Vars:
             return True
         return False
 
+    def __iter__(self):
+        keys = set()
+        keys.update(self._templar._available_variables, self._locals, self._globals, *self._extras)
+        return iter(keys)
+
+    def __len__(self):
+        keys = set()
+        keys.update(self._templar._available_variables, self._locals, self._globals, *self._extras)
+        return len(keys)
+
     def __getitem__(self, varname):
         if varname not in self._templar._available_variables:
             if varname in self._locals: