Don't use the task for a cache, return a special cache var

changelogs/fragments/delegate_to_loop_hostvars.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+- delegate_to - When templating ``delegate_to`` in a loop, don't use the task for a cache, return a special cache through ``get_vars`` allowing looping over a hostvar (https://github.com/ansible/ansible/issues/47207)

lib/ansible/executor/task_executor.py

@@ -210,7 +210,12 @@ def _get_loop_items(self):
 
         templar = Templar(loader=self._loader, shared_loader_obj=self._shared_loader_obj, variables=self._job_vars)
         items = None
-        if self._task.loop_with:
+        loop_cache = self._job_vars.get('_ansible_loop_cache')
+        if loop_cache is not None:
+            # _ansible_loop_cache may be set in `get_vars` when calculating `delegate_to`
+            # to avoid reprocessing the loop
+            items = loop_cache
+        elif self._task.loop_with:
             if self._task.loop_with in self._shared_loader_obj.lookup_loader:
                 fail = True
                 if self._task.loop_with == 'first_found':

lib/ansible/vars/manager.py

@@ -430,7 +430,7 @@ def plugins_by_groups():
         # if we have a task and we're delegating to another host, figure out the
         # variables for that host now so we don't have to rely on hostvars later
         if task and task.delegate_to is not None and include_delegate_to:
-            all_vars['ansible_delegated_vars'] = self._get_delegated_vars(play, task, all_vars)
+            all_vars['ansible_delegated_vars'], all_vars['_ansible_loop_cache'] = self._get_delegated_vars(play, task, all_vars)
 
         # 'vars' magic var
         if task or play:
@@ -594,16 +594,15 @@ def _get_delegated_vars(self, play, task, existing_variables):
                 include_hostvars=False,
             )
 
+        _ansible_loop_cache = None
         if has_loop and cache_items:
-            # delegate_to templating produced a change, update task.loop with templated items,
+            # delegate_to templating produced a change, so we will cache the templated items
+            # in a special private hostvar
             # this ensures that delegate_to+loop doesn't produce different results than TaskExecutor
             # which may reprocess the loop
-            # Set loop_with to None, so we don't do extra unexpected processing on the cached items later
-            # in TaskExecutor
-            task.loop_with = None
-            task.loop = items
+            _ansible_loop_cache = items
 
-        return delegated_host_vars
+        return delegated_host_vars, _ansible_loop_cache
 
     def clear_facts(self, hostname):
         '''

test/integration/targets/delegate_to/runme.sh

@@ -9,6 +9,8 @@ ansible-playbook test_loop_control.yml -v "$@"
 
 ansible-playbook test_delegate_to_loop_randomness.yml -v "$@"
 
-ansible-playbook delegate_and_nolog.yml -v "$@"
+ansible-playbook delegate_and_nolog.yml -i ../../inventory -v "$@"
 
-ansible-playbook delegate_facts_block.yml -v "$@"
+ansible-playbook delegate_facts_block.yml -i ../../inventory -v "$@"
+
+ansible-playbook test_delegate_to_loop_caching.yml -i ../../inventory -v "$@"

test/integration/targets/delegate_to/test_delegate_to_loop_caching.yml

@@ -0,0 +1,45 @@
+- hosts: testhost,testhost2
+  gather_facts: false
+  vars:
+    delegate_to_host: "localhost"
+  tasks:
+    - set_fact:
+        gandalf:
+          shout: 'You shall not pass!'
+      when: inventory_hostname == 'testhost'
+
+    - set_fact:
+        gandalf:
+          speak: 'Run you fools!'
+      when: inventory_hostname == 'testhost2'
+
+    - name: works correctly
+      debug: var=item
+      delegate_to: localhost
+      with_dict: "{{ gandalf }}"
+      register: result1
+
+    - name: shows same item for all hosts
+      debug: var=item
+      delegate_to: "{{ delegate_to_host }}"
+      with_dict: "{{ gandalf }}"
+      register: result2
+
+    - debug:
+        var: result2.results[0].item.value
+
+    - assert:
+        that:
+          - result1.results[0].item.value == 'You shall not pass!'
+          - result2.results[0].item.value == 'You shall not pass!'
+      when: inventory_hostname == 'testhost'
+
+    - assert:
+        that:
+          - result1.results[0].item.value == 'Run you fools!'
+          - result2.results[0].item.value == 'Run you fools!'
+      when: inventory_hostname == 'testhost2'
+
+    - assert:
+        that:
+          - _ansible_loop_cache is undefined