[cloud] Add check mode for cloudformation module (#23483)

* cloudformation: dummy check mode

* cloudformation: use changesets to implement check mode

* cloudformation: wait at most 5min for change set

* cloudformation: handle stack creation and deletion in check mode

* cloudformation: standardize output format in check mode

msg is a string, meta is a list

* cloudformation: use same naming convention in get_changeset as create_changeset

also add comment about code duplication between said functions

* Remove unused imports

* PEP8 whitespace fix

* Fix CI, convert success=True check to for/else

lib/ansible/modules/cloud/amazon/cloudformation.py

@@ -250,7 +250,6 @@ from ansible.module_utils.basic import AnsibleModule
 from ansible.module_utils._text import to_bytes
 
 
-
 def get_stack_events(cfn, stack_name):
     '''This event data was never correct, it worked as a side effect. So the v2.3 format is different.'''
     ret = {'events':[], 'log':[]}
@@ -297,36 +296,28 @@ def create_stack(module, stack_params, cfn):
 
 def list_changesets(cfn, stack_name):
     res = cfn.list_change_sets(StackName=stack_name)
-    changesets = []
+    return [cs['ChangeSetName'] for cs in res['Summaries']]
-    for cs in res['Summaries']:
+
-        changesets.append(cs['ChangeSetName'])
-    return changesets
 
 def create_changeset(module, stack_params, cfn):
     if 'TemplateBody' not in stack_params and 'TemplateURL' not in stack_params:
         module.fail_json(msg="Either 'template' or 'template_url' is required.")
 
     try:
-        if not 'ChangeSetName' in stack_params:
+        changeset_name = build_changeset_name(stack_params)
-            # Determine ChangeSetName using hash of parameters.
+        stack_params['ChangeSetName'] = changeset_name
-            json_params = json.dumps(stack_params, sort_keys=True)
-
-            changeset_name = 'Ansible-' + stack_params['StackName'] + '-' + sha1(to_bytes(json_params, errors='surrogate_or_strict')).hexdigest()
-            stack_params['ChangeSetName'] = changeset_name
-        else:
-            changeset_name = stack_params['ChangeSetName']
 
         # Determine if this changeset already exists
         pending_changesets = list_changesets(cfn, stack_params['StackName'])
         if changeset_name in pending_changesets:
-            warning = 'WARNING: '+str(len(pending_changesets))+' pending changeset(s) exist(s) for this stack!'
+            warning = 'WARNING: %d pending changeset(s) exist(s) for this stack!' % len(pending_changesets)
-            result = dict(changed=False, output='ChangeSet ' + changeset_name + ' already exists.', warnings=[warning])
+            result = dict(changed=False, output='ChangeSet %s already exists.' % changeset_name, warnings=[warning])
         else:
             cs = cfn.create_change_set(**stack_params)
             result = stack_operation(cfn, stack_params['StackName'], 'UPDATE')
-            result['warnings'] = [('Created changeset named ' + changeset_name + ' for stack ' + stack_params['StackName']),
+            result['warnings'] = ['Created changeset named %s for stack %s' % (changeset_name, stack_params['StackName']),
-                ('You can execute it using: aws cloudformation execute-change-set --change-set-name ' + cs['Id']),
+                'You can execute it using: aws cloudformation execute-change-set --change-set-name %s' % cs['Id'],
-                ('NOTE that dependencies on this stack might fail due to pending changes!')]
+                'NOTE that dependencies on this stack might fail due to pending changes!']
     except Exception as err:
         error_msg = boto_exception(err)
         if 'No updates are to be performed.' in error_msg:
@@ -407,11 +398,49 @@ def stack_operation(cfn, stack_name, operation):
     return {'failed': True, 'output':'Failed for unknown reasons.'}
 
 
+def build_changeset_name(stack_params):
+    if 'ChangeSetName' in stack_params:
+        return stack_params['ChangeSetName']
+
+    json_params = json.dumps(stack_params, sort_keys=True)
+
+    return 'Ansible-{0}-{1}'.format(
+        stack_params['StackName'],
+        sha1(to_bytes(json_params, errors='surrogate_or_strict')).hexdigest()
+    )
+
+
+def check_mode_changeset(module, stack_params, cfn):
+    """Create a change set, describe it and delete it before returning check mode outputs."""
+    stack_params['ChangeSetName'] = build_changeset_name(stack_params)
+    try:
+        change_set = cfn.create_change_set(**stack_params)
+        for i in range(60): # total time 5 min
+            description = cfn.describe_change_set(ChangeSetName=change_set['Id'])
+            if description['Status'] in ('CREATE_COMPLETE', 'FAILED'):
+                break
+            time.sleep(5)
+        else:
+            # if the changeset doesn't finish in 5 mins, this `else` will trigger and fail
+            module.fail_json(msg="Failed to create change set %s" % stack_params['ChangeSetName'])
+
+        cfn.delete_change_set(ChangeSetName=change_set['Id'])
+
+        reason = description.get('StatusReason')
+
+        if description['Status'] == 'FAILED' and "didn't contain changes" in description['StatusReason']:
+            return {'changed': False, 'msg': reason, 'meta': description['StatusReason']}
+        return {'changed': True, 'msg': reason, 'meta': description['Changes']}
+
+    except (botocore.exceptions.ValidationError, botocore.exceptions.ClientError) as err:
+        error_msg = boto_exception(err)
+        module.fail_json(msg=error_msg, exception=traceback.format_exc())
+
+
 def get_stack_facts(cfn, stack_name):
     try:
         stack_response = cfn.describe_stacks(StackName=stack_name)
         stack_info = stack_response['Stacks'][0]
-    #except AmazonCloudFormationException as e:
     except (botocore.exceptions.ValidationError,botocore.exceptions.ClientError) as err:
         error_msg = boto_exception(err)
         if 'does not exist' in error_msg:
@@ -451,6 +480,7 @@ def main():
     module = AnsibleModule(
         argument_spec=argument_spec,
         mutually_exclusive=[['template_url', 'template']],
+        supports_check_mode=True
     )
     if not HAS_BOTO3:
         module.fail_json(msg='boto3 and botocore are required for this module')
@@ -509,6 +539,16 @@ def main():
 
     stack_info = get_stack_facts(cfn, stack_params['StackName'])
 
+    if module.check_mode:
+        if state == 'absent' and stack_info:
+            module.exit_json(changed=True, msg='Stack would be deleted', meta=[])
+        elif state == 'absent' and not stack_info:
+            module.exit_json(changed=False, msg='Stack doesn\'t exist', meta=[])
+        elif state == 'present' and not stack_info:
+            module.exit_json(changed=True, msg='New stack would be created', meta=[])
+        else:
+            module.exit_json(**check_mode_changeset(module, stack_params, cfn))
+
     if state == 'present':
         if not stack_info:
             result = create_stack(module, stack_params, cfn)