Class loader issue with Groovy Module

[nknize]

Uncovered this issue on master while investigating #17506

It appears the Groovy module may be using a different class loader which is causing class resolution errors during script compilation.

Reproduction:

Try the following in a script_field:

"script": "import org.elasticsearch.common.Strings; org.elasticsearch.common.Strings.hasLength(\"foo\")"

Error:

{
  "type": "script_exception",
  "reason": "Error compiling script d8fc6d7a038969a64764034a31e17fd0f336e955",
  "script_stack": [
    "d8fc6d7a038969a64764034a31e17fd0f336e955: 1: unable to resolve class org.elasticsearch.common.Strings\n @ line 1, column 1.\n   import org.elasticsearch.common.Strings; org.elasticsearch.common.Strings.hasLength(\"foo\")\n   ^\n"
  ],
  "script": "import org.elasticsearch.common.Strings; org.elasticsearch.common.Strings.hasLength(\"foo\")",
  "lang": "groovy"
}

This looks highly problematic. Upgrading from 2.x to 5.x will potentially break existing groovy scripts?

[nknize]

@rjernst pointed me to the ClassPermission policy lock down in 2.2 so I'm closing this as a non-issue.