ignore/warn for undefined vars in unused handler names (#75244)

* Make undefined variables in handler names non-fatal if the handler is not used

* If the handler has no way to be notified (i.e. the name can't be templated and the handler has no listen topics), display a warning

* Add tests for variables in handler names

* changelog

changelogs/fragments/75244-fix-templated-handler-names.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - Give a warning instead of an error if a handler name contains undefined variables and has no listen topics (https://github.com/ansible/ansible/issues/58841).

lib/ansible/plugins/strategy/__init__.py

@@ -492,6 +492,7 @@ class StrategyBase:
             for handler_block in reversed(handler_blocks):
                 for handler_task in handler_block.block:
                     if handler_task.name:
+                        try:
                             if not handler_task.cached_name:
                                 if handler_templar.is_template(handler_task.name):
                                     handler_templar.available_variables = self._variable_manager.get_vars(play=iterator._play,
@@ -501,7 +502,6 @@ class StrategyBase:
                                     handler_task.name = handler_templar.template(handler_task.name)
                                 handler_task.cached_name = True
 
-                        try:
                             # first we check with the full result of get_name(), which may
                             # include the role name (if the handler is from a role). If that
                             # is not found, we resort to the simple name field, which doesn't
@@ -514,11 +514,17 @@ class StrategyBase:
 
                             if handler_name in candidates:
                                 return handler_task
-                        except (UndefinedError, AnsibleUndefinedVariable):
+                        except (UndefinedError, AnsibleUndefinedVariable) as e:
                             # We skip this handler due to the fact that it may be using
                             # a variable in the name that was conditionally included via
                             # set_fact or some other method, and we don't want to error
                             # out unnecessarily
+                            if not handler_task.listen:
+                                display.warning(
+                                    "Handler '%s' is unusable because it has no listen topics and "
+                                    "the name could not be templated (host-specific variables are "
+                                    "not supported in handler names). The error: %s" % (handler_task.name, to_text(e))
+                                )
                             continue
             return None
 

test/integration/targets/handlers/58841.yml

@@ -0,0 +1,9 @@
+---
+- hosts: localhost
+  gather_facts: no
+  tasks:
+    - include_role:
+        name: import_template_handler_names
+      tags:
+        - lazy_evaluation
+        - evaluation_time

test/integration/targets/handlers/roles/import_template_handler_names/tasks/main.yml

@@ -0,0 +1,11 @@
+- import_role:
+    name: template_handler_names
+    tasks_from: lazy_evaluation
+  tags:
+    - lazy_evaluation
+
+- import_role:
+    name: template_handler_names
+    tasks_from: evaluation_time
+  tags:
+    - evaluation_time

test/integration/targets/handlers/roles/template_handler_names/handlers/main.yml

@@ -0,0 +1,5 @@
+- name: handler name with {{ test_var }}
+  debug: msg='handler with var ran'
+
+- name: handler name
+  debug: msg='handler ran'

test/integration/targets/handlers/roles/template_handler_names/tasks/evaluation_time.yml

@@ -0,0 +1,5 @@
+- debug: msg='notify handler with variable in name'
+  notify: handler name with myvar
+  changed_when: True
+  tags:
+    - evaluation_time

test/integration/targets/handlers/roles/template_handler_names/tasks/lazy_evaluation.yml

@@ -0,0 +1,5 @@
+- debug: msg='notify handler'
+  notify: handler name
+  changed_when: True
+  tags:
+    - lazy_evaluation

test/integration/targets/handlers/runme.sh

@@ -96,3 +96,21 @@ result="$(ansible-playbook test_handlers_template_run_once.yml -i inventory.hand
 set -e
 grep -q "handler A" <<< "$result"
 grep -q "handler B" <<< "$result"
+
+# Test an undefined variable in another handler name isn't a failure
+ansible-playbook 58841.yml "$@" --tags lazy_evaluation 2>&1 | tee out.txt ; cat out.txt
+grep out.txt -e "\[WARNING\]: Handler 'handler name with {{ test_var }}' is unusable"
+[ "$(grep out.txt -ce 'handler ran')" = "1" ]
+[ "$(grep out.txt -ce 'handler with var ran')" = "0" ]
+
+# Test templating a handler name with a defined variable
+ansible-playbook 58841.yml "$@" --tags evaluation_time -e test_var=myvar | tee out.txt ; cat out.txt
+[ "$(grep out.txt -ce 'handler ran')" = "0" ]
+[ "$(grep out.txt -ce 'handler with var ran')" = "1" ]
+
+# Test the handler is not found when the variable is undefined
+ansible-playbook 58841.yml "$@" --tags evaluation_time 2>&1 | tee out.txt ; cat out.txt
+grep out.txt -e "ERROR! The requested handler 'handler name with myvar' was not found"
+grep out.txt -e "\[WARNING\]: Handler 'handler name with {{ test_var }}' is unusable"
+[ "$(grep out.txt -ce 'handler ran')" = "0" ]
+[ "$(grep out.txt -ce 'handler with var ran')" = "0" ]