MCS-2045 / wrong byte (0xFF/0x3F) returned instead of SQL NULL for CHAR/VARCHAR(width<=8) read via CrossEngineStep

Summary

A NULL value in a CHAR/VARCHAR column with width ≤ 8 read from InnoDB via CrossEngineStep is delivered to the client as the byte 0xFF (CHAR(1) → ? / 0x3F after charset conversion) instead of NULL.

Repro (standalone, no state)

CREATE TABLE t (a CHAR(1)) ENGINE=InnoDB;

INSERT INTO t VALUES (NULL),('x');

SET SESSION columnstore_innodb_queries_use_mcs=ON;

SELECT a, HEX(a), a IS NULL FROM t;

-- expected: NULL, NULL, 1   |   x, 78, 0

-- actual:   ?,    FF,   0   |   x, 78, 0

Root cause

dbcon/joblist/crossenginestep.cpp::convertValueNum() hardcodes the 8-byte NULL marker for any CHAR/VARCHAR width:

case CHAR: case VARCHAR:

  if (nullFlag) rv = joblist::CHAR8NULL;   // 0xFEFFFFFFFFFFFFFFULL

For colWidth=1, setIntField then truncates to the low byte = 0xFF (which is CHAR1EMPTYROW, not CHAR1NULL=0xFE). Per-width markers already exist in joblist/joblisttypes.h (CHAR1NULL, CHAR2NULL, CHAR4NULL, CHAR8NULL) — fix is a switch (ct.colWidth).

Trigger / scope

Bug is pre-existing. Exposed in CI by columnstore commit 09783833b (2026-04-17) which set innodb_queries_use_mcs=ON globally in build/run_mtr.sh. With OFF, MTR test passes (verified in rocky9 container).

Affected MTR test / artifacts

columnstore/basic.mcol_2000 — disabled in c4635ff6b under MCOL-6340. Artifacts: mcol_2000_artifacts.tgz.