Index templates are not compatible with file-based component templates

[jeanfabrice]

Elasticsearch Version

8.16.1

Installed Plugins

none

Java Version

bundled

OS Version

Kubernetes 1.30.4 on Ubuntu 22.04

Problem Description

Elasticsearch returns HTTP/400 when creating an index template via API call where one of the associated component templates is a file-based component template

This, for example, prevents integration from upgrading when it ships with an updated index templates.
E.g:

(I'm sorry I can't copy-paste the error as the message is not persistent when I try to move my mouse to select it.)

Additional context in these Slack threads:

• https://elastic.slack.com/archives/CJZ9U5G9X/p1732801746634399
• https://elastic.slack.com/archives/C0D8ST60Y/p1680259171010499

Steps to Reproduce

1. Create and apply test@custom component template via ECK stackconfigpolicy

apiVersion: stackconfigpolicy.k8s.elastic.co/v1alpha1
kind: StackConfigPolicy
metadata:
  name: elasticsearch
spec:
  elasticsearch:
    indexTemplates:
      componentTemplates:
        test@custom:
          template:
            settings:
              refresh_interval: "10s"

2. Wait for the stackconfigpolicy to be in ready phase (change propagated)

$ k get stackconfigpolicies.stackconfigpolicy.k8s.elastic.co elasticsearch 
NAME            READY   PHASE   AGE
elasticsearch   2/2     Ready   1d

3. Go to Kibana developer tools and create a test index template this way

PUT _index_template/test
{
  "index_patterns": ["test*"],
  "template": {
    "settings": {},
    "mappings": {}
  },
  "priority": 200,
  "composed_of": ["test@custom","test2@custom"],
  "ignore_missing_component_templates": ["test@custom","test2@custom"]
}

4. See the error returned

{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": """Failed to process request [PutTemplateV2Request[name=test, cause=api, create=false, index_template={"index_patterns":["test*"],"template":{"settings":{}},"composed_of":["test@custom","test2@custom"],"priority":200,"ignore_missing_component_templates":["test@custom","test2@custom"]}]] with errors: [[component_template:test@custom] is reserved by [file_settings]]"""
      }
    ],
    "type": "illegal_argument_exception",
    "reason": """Failed to process request [PutTemplateV2Request[name=test, cause=api, create=false, index_template={"index_patterns":["test*"],"template":{"settings":{}},"composed_of":["test@custom","test2@custom"],"priority":200,"ignore_missing_component_templates":["test@custom","test2@custom"]}]] with errors: [[component_template:test@custom] is reserved by [file_settings]]"""
  },
  "status": 400
}

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)