_score should not be a reserved attribute in ES|QL

[tteofili]

Drop checks in ES|QL Verifier that ensured _score is a reserved attribute.
These checks are too restrictive, in addition to that relaxing them would make handling of _score consistent with other (metadata) attributes and fields.

fixes #118460

[github-actions]

Documentation preview:

• ✨ Changed pages

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[elasticsearchmachine]

Hi @tteofili, I've created a changelog YAML for you.

[ioanatia]

The changes look right with me!
Can we get a CSV test just to prevent any future regressions with allowing _score to be overwritten?
Something like what you had in the test before: "from foo metadata _score | where qstr(\"bar\") | eval _score = _score + 1"
and another one where _score receives a different type:

from foo metadata _score | where qstr(\"bar\") | eval _score = "foobar"

with that, I think this should be good to go!

[ioanatia]

also what would happen if _score is an actual indexed field? have we tested that?
we don't have to fix it here, but can be a follow up in the meta issue #116599

[tteofili]

also what would happen if _score is an actual indexed field? have we tested that?

added an item in #116599

an we get a CSV test just to prevent any future regressions with allowing _score to be overwritten?

added a couple of tests for score manipulation and override in 0ff6fb3

[ioanatia]

an we get a CSV test just to prevent any future regressions with allowing _score to be overwritten?

added a couple of tests for score manipulation and override in 0ff6fb3

I did not realize this would actually create a problem for bwc 8.18 tests - feel free to remove them and then we can add them later after the changes make it in 8.18.
The alternatives are:

• mute the tests you just added and unmute once these changes make it in 8.18
• add a new EsqlCapability to signal that _score is no longer reserved - this is too heavy, let's not do it

[elasticsearchmachine]

Hi @tteofili, I've updated the changelog YAML for you.