fix searched paths in DataLoader.path_dwim_relative (avoid AnsibleFileNotFound) (#26729)

* add unit test: nested dynamic includes

* nested dynamic includes: avoid AnsibleFileNotFound error

Error was:
Unable to retrieve file contents
Could not find or access 'include2.yml'

Before 8f758204cf, at the end of
'path_dwim_relative' method, the 'search' variable contained amongst
others paths:
'/tmp/roles/testrole/tasks/tasks/included.yml' and
'/tmp/roles/testrole/tasks/included.yml'.
The commit mentioned before removed the last one despite the method
docstrings specify 'with or without explicitly named dirname subdirs'.

* add integration test: nested includes

test/units/parsing/test_dataloader.py

@@ -19,12 +19,17 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
+import os
+
 from ansible.compat.tests import unittest
 from ansible.compat.tests.mock import patch, mock_open
 from ansible.errors import AnsibleParserError, yaml_strings
+from ansible.module_utils._text import to_text
 from ansible.module_utils.six import PY3
 from ansible.parsing.dataloader import DataLoader
 
+from units.mock.path import mock_unfrackpath_noop
+
 
 class TestDataLoader(unittest.TestCase):
 
@@ -69,6 +74,45 @@ class TestDataLoader(unittest.TestCase):
         self.assertIn(yaml_strings.YAML_COMMON_LEADING_TAB_ERROR, str(cm.exception))
         self.assertIn('foo: bar', str(cm.exception))
 
+    @patch('ansible.parsing.dataloader.unfrackpath', mock_unfrackpath_noop)
+    @patch.object(DataLoader, '_is_role')
+    def test_path_dwim_relative(self, mock_is_role):
+        """
+        simulate a nested dynamic include:
+
+        playbook.yml:
+        - hosts: localhost
+          roles:
+            - { role: 'testrole' }
+
+        testrole/tasks/main.yml:
+        - include: "include1.yml"
+          static: no
+
+        testrole/tasks/include1.yml:
+        - include: include2.yml
+          static: no
+
+        testrole/tasks/include2.yml:
+        - debug: msg="blah"
+        """
+        mock_is_role.return_value = False
+        with patch('os.path.exists') as mock_os_path_exists:
+            mock_os_path_exists.return_value = False
+            self._loader.path_dwim_relative('/tmp/roles/testrole/tasks', 'tasks', 'included2.yml')
+
+            # Fetch first args for every call
+            # mock_os_path_exists.assert_any_call isn't used because os.path.normpath must be used in order to compare paths
+            called_args = [os.path.normpath(to_text(call[0][0])) for call in mock_os_path_exists.call_args_list]
+
+            # 'path_dwim_relative' docstrings say 'with or without explicitly named dirname subdirs':
+            self.assertIn('/tmp/roles/testrole/tasks/included2.yml', called_args)
+            self.assertIn('/tmp/roles/testrole/tasks/tasks/included2.yml', called_args)
+
+            # relative directories below are taken in account too:
+            self.assertIn('tasks/included2.yml', called_args)
+            self.assertIn('included2.yml', called_args)
+
 
 class TestDataLoaderWithVault(unittest.TestCase):