The index mode validation doesn't always kick in when updating settings

[martijnvg]

The index.mode setting is an index setting that is somewhat special. Besides that it validates itself, it also validation other index settings. Unfortunately this doesn't always kick in.

When creating a new index it does kick in:

PUT index1
{
  "settings": {
    "index.time_series.start_time": "2000-01-01T00:00Z",
    "index.time_series.end_time": "2000-01-01T00:00Z"
  }
}

However when creating a new index and then updating the index settings the index.mode validation does not kick in:

PUT index1

PUT /_settings
{
  "settings": {
    "index.time_series.end_time": "2000-01-02T00:00Z"
  }
}

GET index1

This leaves the cluster in a bad state, as the validation does kick in later during unrelated operations. Like updating another unrelated index setting or when a new node joins the cluster.

Note that if index.mode index setting is configured to standard, then the index mode validation does kick in

The expect outcome is that when updating index settings that index.mode validation is also performed.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)