`date`'s `format` updateability is trappy

[jpountz]

Here is a recreation:

DELETE index

PUT index
{
  "mappings": {
    "doc": {
      "properties": {
        "my_date": {
          "type": "date", 
          "format": "yyyy/MM/dd"
        }
      }
    }
  }
}

PUT index/_mapping/doc
{
  "doc": {
    "properties": {
      "my_date": {
        "type": "date", 
        "format": "yyyy-MM-dd"
      }
    }
  }
}

GET index/_mapping

date allows its format to be updated. This is trappy because then it could be changed to a format that doesn't work with already indexed documents, or even worse: a format that parses to a different date.

I think we need to ensure that the format either cannot be changed, or make it a list that is append-only so that it is only possible to append date formats.

[s1monw]

++ to move to an append-able list! we discussed this in fixit friday and we are convinced that we should not move to a list due to the ordering aspects. If we'd allow only adding to it but not sending the list is that we need to barf if the order changes which is weird. The right fix here is to make the date immutable and reject any updates to it.