Alternator: DescribeTable can return spurious RANGE key for GSI

[nyh]

In the current implementation, when DescribeTable wants to list the table's key columns (KeySchema) we don't have the originally requested schema explicitly saved anywhere, so describe_key_schema() attempts to recreate this information from the Scylla table's schema.

But there is one problem with this when describing the KeySchema of a GSI (note that DescribeTable returns the schema of the base table and of all its indexes). The problem is in one specific case: In DynamoDB, one can have a base table with hash key "p", and create a GSI with a different hash key "x" and no range key. In Scylla, we had to, internally, also add "p" as a range key to the materialized view - because a view key must include the base's key columns. This means that with the current implementation, this extra range key "x" is returned by DescribeTable - yet it shouldn't have been. We have an xfailing test case - test_gsi_2_describe_table_schema - to reproduce this case.

[nyh]

@bartpeeters commented in #11455 (comment):

It is one of the things that makes Alternator not work with Terraform, after creating the table using Terraform and then applying the plan again it will pickup 'changes' to the index:

- global_secondary_index {
    - hash_key           = "ownerId" -> null
    - non_key_attributes = [] -> null
    - range_key          = "channelId" -> null
    - read_capacity      = 0 -> null
    - write_capacity     = 0 -> null
  }
+ global_secondary_index {
    + hash_key           = "ownerId"
    + name               = "ownerId-index"
    + non_key_attributes = []
    + projection_type    = "ALL"
  }

Terraform index resource code:

global_secondary_index {
  name            = "ownerId-index"
  hash_key        = "ownerId"
  projection_type = "ALL"
}

But I don't think this is the only thing tripping up Terraform, you also don't see the name or projection_type properties on the current table.