Elasticsearch version (bin/elasticsearch --version):
Version: 8.1.0-SNAPSHOT, Build: default/tar/e38cadb5283feeea06f09d555dbe47fb1d5e8b39/2021-11-22T22:35:22.825882Z, JVM: 17.0.1

OS version (uname -a if on a Unix-like system):
Darwin 20.6.0 Darwin Kernel Version 20.6.0: Mon Aug 30 06:12:21 PDT 2021; root:xnu-7195.141.6~3/RELEASE_X86_64 x86_64

Description of the problem including expected versus actual behavior:

I expect ES to reject snapshot restore requests that restore a data stream alias with the same name as an existing data stream or index.

However, ES currently accepts these requests. This cluster now has a data stream/index and alias with the same name. A user may not be sure which resource a search request targeting the name will hit.

Steps to reproduce:

1. Register a snapshot repository

PUT /_snapshot/my_repository
{
  "type": "fs",
  "settings": {
    "location": "my_backup_location"
  }
}

2. Create a data stream.

POST logs-my_app-default/_doc
{
  "@timestamp": "2099-05-06T16:21:15.000Z"
}

3. Add a logs alias to the data stream.

POST _aliases
{
  "actions": [
    {
      "add": {
        "index": "logs-my_app-default",
        "alias": "logs"
      }
    }
  ]
}

4. Create a snapshot containing the data stream and its alias

PUT _snapshot/my_repository/my_snapshot?wait_for_completion=true

5. Delete the data stream. This also deletes the alias.

DELETE _data_stream/logs-my_app-default

6. Create another data stream with same name as the deleted alias (logs).

PUT _index_template/my-index-template
{
  "index_patterns": ["logs"],
  "data_stream": { },
  "priority": 500
}

POST logs/_doc
{
  "@timestamp": "2099-05-07T16:21:15.000Z"
}

7. Restore the data stream and logs alias from the snapshot.

POST /_snapshot/my_repository/my_snapshot/_restore?wait_for_completion=true
{
  "indices": "logs-my_app-default",
  "include_aliases": true
}

I expected this to return an error due to a conflict between the restored logs
alias and the pre-existing logs data stream. However, the request was
accepted, and both resources now exist.

Retrieve the logs data stream:

GET _data_stream/logs

Retrieve the logs alias:

GET _alias/logs

A search on logs only hits the logs data stream:

GET logs/_search

However, a search on log* hits both the alias and the data stream:

GET log*/_search