[SPARK-49913][SQL] Add check for unique label names in nested labeled scopes

[miland-db]

What changes were proposed in this pull request?

We are introducing checks for unique label names.
New rules for label names:

• Labels can't have the same name as some of the labels in scope surrounding them
• Labels can have the same name as other labels in the same scope

Valid code:

BEGIN
  lbl: BEGIN
    SELECT 1;
  END;

  lbl: BEGIN
    SELECT 2;
  END;

  BEGIN
    lbl: WHILE 1=1 DO
      LEAVE lbl;
    END WHILE;
  END;
END

Invalid code:

BEGIN
  lbl: BEGIN
    lbl: BEGIN
      SELECT 1;
    END;
  END;
END

Design explanation:

Even though there are Listeners with enterRule and exitRule methods to check labels before and remove them from seenLabels after visiting node, we favor this approach because minimal changes were needed and code is more compact to avoid dependency issues.

Additionally, generating label text would need to be done in 2 places and we wanted to avoid duplicated logic:

• enterRule
• visitRule

Why are the changes needed?

It will be needed in future when we release Local Scoped Variables for SQL Scripting so users can target variables from outer scopes if they are shadowed.

How was this patch tested?

New unit tests in 'SqlScriptingParserSuite.scala'.

Was this patch authored or co-authored using generative AI tooling?

No.

[miland-db]

Adding: @cloud-fan @srielau @davidm-db @dejankrak-db @dusantism-db

[davidm-db]

Please make seenLabels thread local as Wenchen suggested, other than that LGTM.

[cloud-fan]

Yes this is the implementation I was expecting. Can we do a bit more encapsulation? e.g.

val labelCtx = new ScritpingLabelContext()
visitBeginEndCompoundBlockImpl...
...
class ScritpingLabelContext {
  private val seenLabels = Set[String]()
  
  def newLabel(label: String)...
  ...
}
``

[miland-db]

@cloud-fan done.

[davidm-db]

Just a tiny comment, otherwise LGTM.
I would maybe use SqlScriptingLabelContext since we used SqlScripting* I think everywhere, so just to keep it consistent.

[cloud-fan]

The linter failure is unrelated, thanks, merging to master!