preserve same order as inventory manager when using host lookup (#55331)

* preserve same order as inventory manager when using inventory_hostnames lookup

add a test

* move generic code

lib/ansible/inventory/manager.py

@@ -36,6 +36,7 @@ from ansible.module_utils.six import string_types
 from ansible.module_utils._text import to_bytes, to_text
 from ansible.parsing.utils.addresses import parse_address
 from ansible.plugins.loader import inventory_loader
+from ansible.utils.helpers import deduplicate_list
 from ansible.utils.path import unfrackpath
 from ansible.utils.display import Display
 
@@ -369,8 +370,7 @@ class InventoryManager(object):
                     # exclude hosts mentioned in any restriction (ex: failed hosts)
                     hosts = [h for h in hosts if h.name in self._restriction]
 
-                seen = set()
-                self._hosts_patterns_cache[pattern_hash] = [x for x in hosts if x not in seen and not seen.add(x)]
+                self._hosts_patterns_cache[pattern_hash] = deduplicate_list(hosts)
 
             # sort hosts list if needed (should only happen when called from strategy)
             if order in ['sorted', 'reverse_sorted']:

lib/ansible/plugins/lookup/inventory_hostnames.py

@@ -36,6 +36,7 @@ RETURN = """
 
 from ansible.inventory.manager import split_host_pattern, order_patterns
 from ansible.plugins.lookup import LookupBase
+from ansible.utils.helpers import deduplicate_list
 
 
 class LookupModule(LookupBase):
@@ -70,4 +71,4 @@ class LookupModule(LookupBase):
                     host_list.extend(that)
 
         # return unique list
-        return list(set(host_list))
+        return deduplicate_list(host_list)

lib/ansible/utils/helpers.py

@@ -41,3 +41,11 @@ def object_to_dict(obj, exclude=None):
     if exclude is None or not isinstance(exclude, list):
         exclude = []
     return dict((key, getattr(obj, key)) for key in dir(obj) if not (key.startswith('_') or key in exclude))
+
+
+def deduplicate_list(original_list):
+    """
+    Creates a deduplicated list with the order in which each item is first found.
+    """
+    seen = set()
+    return [x for x in original_list if x not in seen and not seen.add(x)]

test/integration/targets/lookup_inventory_hostnames/aliases

@@ -0,0 +1 @@
+shippable/posix/group2

test/integration/targets/lookup_inventory_hostnames/inventory

@@ -0,0 +1,6 @@
+[group01]
+test01
+test05
+test03
+test02
+test04

test/integration/targets/lookup_inventory_hostnames/main.yml

@@ -0,0 +1,13 @@
+---
+- hosts: localhost
+  gather_facts: no
+  tasks:
+  - set_fact:
+      hosts_a: "{{ lookup('inventory_hostnames', 'group01', wantlist=true) }}"
+
+  - set_fact:
+      hosts_b: "{{ groups['group01'] }}"
+
+  - assert:
+      that:
+        - hosts_a == hosts_b

test/integration/targets/lookup_inventory_hostnames/runme.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+set -eux
+
+ansible-playbook main.yml -i inventory -e "$@"