Native Jinja2: raise undefined error immediately (#52237)

Fixes #52158

changelogs/fragments/52158-jinja2-native-types-contructed-keyed-groups.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - Fix unexpected error when using Jinja2 native types with non-strict constructed keyed_groups (https://github.com/ansible/ansible/issues/52158).

lib/ansible/template/native_helpers.py

@@ -12,6 +12,7 @@ import types
 
 from jinja2._compat import text_type
 
+from jinja2.runtime import StrictUndefined
 from ansible.parsing.yaml.objects import AnsibleVaultEncryptedUnicode
 
 
@@ -37,6 +38,17 @@ def ansible_native_concat(nodes):
         if isinstance(out, AnsibleVaultEncryptedUnicode):
             return out.data
 
+        if isinstance(out, StrictUndefined):
+            # A hack to raise proper UndefinedError/AnsibleUndefinedVariable exception.
+            # We need to access the AnsibleUndefined(StrictUndefined) object by either of the following:
+            # __iter__, __str__, __len__, __nonzero__, __eq__, __ne__, __bool__, __hash__
+            # to actually raise the exception.
+            # (see Jinja2 source of StrictUndefined to get up to date info)
+            # Otherwise the undefined error would be raised on the next access which might not be properly handled.
+            # See https://github.com/ansible/ansible/issues/52158
+            # We do that only here because it is taken care of by text_type() in the else block below already.
+            str(out)
+
         # short circuit literal_eval when possible
         if not isinstance(out, list):
             return out

test/units/template/test_native_concat.py

@@ -0,0 +1,28 @@
+# Copyright: (c) 2019, Ansible Project
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+# Make coding more python3-ish
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+import pytest
+
+from ansible import constants as C
+from ansible.errors import AnsibleUndefinedVariable
+
+# need to mock DEFAULT_JINJA2_NATIVE here so native modules are imported
+# correctly within the template module
+C.DEFAULT_JINJA2_NATIVE = True
+from ansible.template import Templar
+
+from units.mock.loader import DictDataLoader
+
+
+# https://github.com/ansible/ansible/issues/52158
+def test_undefined_variable():
+    fake_loader = DictDataLoader({})
+    variables = {}
+    templar = Templar(loader=fake_loader, variables=variables)
+
+    with pytest.raises(AnsibleUndefinedVariable):
+        templar.template("{{ missing }}")