Ensure implicit flush_handlers have a parent block (#84709)

* Ensure implicit flush_handlers have a parent block

To avoid getting tracebacks when calling methods like ``get_play()`` on
them.

* task needs to be copied

* copy only when necessary

changelogs/fragments/implicit_flush_handlers_parents.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - "Implicit ``meta: flush_handlers`` tasks now have a parent block to prevent potential tracebacks when calling methods like ``get_play()`` on them internally."

lib/ansible/playbook/play.py

@@ -298,7 +298,7 @@ class Play(Base, Taggable, CollectionSearch):
         # of the playbook execution
         flush_block = Block(play=self)
 
-        t = Task()
+        t = Task(block=flush_block)
         t.action = 'meta'
         t.resolved_action = 'ansible.builtin.meta'
         t.args['_raw_params'] = 'flush_handlers'
@@ -318,6 +318,9 @@ class Play(Base, Taggable, CollectionSearch):
         else:
             flush_block.block = [t]
 
+        # NOTE keep flush_handlers tasks even if a section has no regular tasks,
+        #      there may be notified handlers from the previous section
+        #      (typically when a handler notifies a handler defined before)
         block_list = []
         if self.force_handlers:
             noop_task = Task()
@@ -327,18 +330,33 @@ class Play(Base, Taggable, CollectionSearch):
             noop_task.set_loader(self._loader)
 
             b = Block(play=self)
-            b.block = self.pre_tasks or [noop_task]
+            if self.pre_tasks:
+                b.block = self.pre_tasks
+            else:
+                nt = noop_task.copy(exclude_parent=True)
+                nt._parent = b
+                b.block = [nt]
             b.always = [flush_block]
             block_list.append(b)
 
             tasks = self._compile_roles() + self.tasks
             b = Block(play=self)
-            b.block = tasks or [noop_task]
+            if tasks:
+                b.block = tasks
+            else:
+                nt = noop_task.copy(exclude_parent=True)
+                nt._parent = b
+                b.block = [nt]
             b.always = [flush_block]
             block_list.append(b)
 
             b = Block(play=self)
-            b.block = self.post_tasks or [noop_task]
+            if self.post_tasks:
+                b.block = self.post_tasks
+            else:
+                nt = noop_task.copy(exclude_parent=True)
+                nt._parent = b
+                b.block = [nt]
             b.always = [flush_block]
             block_list.append(b)