expr: apply target length/truncation to JSON-to-CHAR cast pushdown - #20041

Fixes #19883

cast_json_as_bytes captured only EvalContext and returned the full converted JSON string unconditionally, with no length or truncation handling. TiDB's builtinCastJSONAsStringSig applies ProduceStrWithSpecifiedTp using the return FieldType's flen, which can truncate the result or, under strict mode, abort the statement for an overlong value. Since this conversion backs predicates and DML pushed down to the coprocessor (e.g. CAST(json_col AS CHAR(n)) <> value), a query could select or mutate different rows than the same statement evaluated at the TiDB root -- in the reported case, a DELETE using an overlong cast value removed rows that TiDB's root evaluation would have aborted before touching.

Added extra: &RpnFnCallExtra to cast_json_as_bytes's capture list and routed the converted value through the existing cast_as_string_helper (already used by cast_string_as_string), which applies produce_str_with_specified_tp -- the same flen-based truncation/error logic TiDB uses. This is a pure wiring fix: ret_field_type was already transported via RpnFnCallExtra, just not consumed by this function.

Added a regression test using the exact reproduction value (1234.5 cast to CHAR(4)), confirming it now errors under strict mode rather than silently truncating.

What is changed and how it works?

Issue Number: Close #19883

What's Changed:

expr: apply target length/truncation to JSON-to-CHAR cast pushdown

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Release note

Fix a bug where casting a JSON value to `CHAR(n)` ignored the target length, when the cast was pushed down to TiKV's coprocessor. This could cause a query to select or modify different rows than the same statement evaluated at the TiDB root, since TiDB's root evaluation truncates or, under strict mode, aborts for an overlong value.

Summary by CodeRabbit

• Bug Fixes
  • Improved JSON-to-string casting to match TiDB-compatible length limits, truncation, and binary padding.
  • Corrected handling of overlong JSON values cast to fixed-length character types.
  • Added strict-mode validation for oversized casts.