fix handler include_tasks templating (#85015)

* Add test for a handler including tasks from a variable filename

* Add FieldAttributeBase attribute to indicate if the object should be post validated

Co-authored-by: Matt Davis <6775756+nitzmahone@users.noreply.github.com>

changelogs/fragments/85015-fix-handler-include_tasks-templating.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - include_tasks - fix templating options when used as a handler (https://github.com/ansible/ansible/pull/85015).

lib/ansible/playbook/base.py

@@ -83,6 +83,11 @@ class _ClassProperty:
 
 class FieldAttributeBase:
 
+    _post_validate_object = False
+    """
+    `False` skips FieldAttribute post-validation on intermediate objects and mixins for attributes without `always_post_validate`.
+    Leaf objects (e.g., `Task`) should set this attribute `True` to opt-in to post-validation.
+    """
     fattributes = _ClassProperty()
 
     @classmethod
@@ -566,8 +571,8 @@ class FieldAttributeBase:
             # only import_role is checked here because import_tasks never reaches this point
             return Sentinel
 
-        # FIXME: compare types, not strings
+        # Skip post validation unless always_post_validate is True, or the object requires post validation.
-        if not attribute.always_post_validate and self.__class__.__name__ not in ('Task', 'Handler', 'PlayContext', 'IncludeRole', 'TaskInclude'):
+        if not attribute.always_post_validate and not self._post_validate_object:
             # Intermediate objects like Play() won't have their fields validated by
             # default, as their values are often inherited by other objects and validated
             # later, so we don't want them to fail out early

lib/ansible/playbook/play_context.py

@@ -71,6 +71,8 @@ class PlayContext(Base):
     connection/authentication information.
     """
 
+    _post_validate_object = True
+
     # base
     module_compression = FieldAttribute(isa='string', default=C.DEFAULT_MODULE_COMPRESSION)
     shell = FieldAttribute(isa='string')

lib/ansible/playbook/task.py

@@ -65,6 +65,8 @@ class Task(Base, Conditional, Taggable, CollectionSearch, Notifiable, Delegatabl
        Task.something(...)
     """
 
+    _post_validate_object = True
+
     # =================================================================================
     # ATTRIBUTES
     # load_<attribute_name> and

test/integration/targets/handlers/test_handlers_including_task.yml

@@ -9,8 +9,15 @@
       debug:
         msg: notifying handler
       changed_when: yes
-      notify: include a task from the handlers section
+      notify:
+        - include a task from the handlers section
+        - include a task in a loop from the handlers section
 
   handlers:
     - name: include a task from the handlers section
       include_tasks: handlers.yml
+
+    - name: include a task in a loop from the handlers section
+      include_tasks: "{{ item }}"
+      with_first_found:
+        - handlers.yml