Loop-variable references in properties the compiled plan is built from give wrong output #8749

What happened?

Inside a loop block, a loop-variable reference (#8635, #8639) in a property that the compiler builds an operator's output schema or partitioning from gives wrong output or a misleading error.

Root cause: the compiler builds the schema and the partitioning when the workflow compiles, from the reference's placeholder. The loop state writes the variable's value into the operator's setting only later, when the loop runs, so the value never reaches them.

Before:  Loop Start (a = "x") -> Projection (alias $a) -> Loop End
         output column named "$a", null in every row
After:   compile error naming the property: its value decides the plan before the loop runs

The same holds for a running workflow's operator that is modified while it runs: its rebuilt executor never has the loop variables written into it.

How to reproduce?

On the #8639 branch: build Text Input -> Loop Start (a = "x") -> Projection (alias of a column set to $a) -> Loop End and run it. Loop End's result has a column named $a whose values are all null.

Version/Branch

1.3.0-incubating-SNAPSHOT (main)

Commit Hash (Optional)

64c8160 (the #8639 branch)

What browsers are you seeing the problem on?

No response

Relevant log output

No response