mo-tester writes placeholder column for empty show node list result #199

Background

MatrixOne issue: matrixorigin/matrixone#17598

show node list is internally planned by MatrixOne as select 1 where 0. The MySQL client and JDBC execution do not return a data row for this statement, but mo-tester genrs wrote the ResultSet metadata header into .result, which appeared as an unexpected placeholder 1.

Reproduction

Use a minimal case:

set save_query_result = on;
/* save_result */show node list;
select * from result_scan(last_query_id()) as u;
set save_query_result = off;
/* save_result */show node list;
select * from result_scan(last_query_id()) as u;

Before the fix, mo-tester generated output like:

/* save_result */show node list;
➤ 1[-5,64,0]

This is not a real show node list row; it is the placeholder column metadata from the internal select 1 where 0 rewrite.

Expected behavior

mo-tester should not emit the internal placeholder 1 for an empty show node list ResultSet. It should keep normal empty ResultSet behavior for ordinary SQL such as select 1 where 0.

Fix

PR: #198

The fix normalizes this specific empty show node list placeholder ResultSet to no result and avoids writing null result text in genrs mode.

Verification

• JAVA_HOME=$(/usr/libexec/java_home -v 20) PATH="$JAVA_HOME/bin:$PATH" mvn -q -DskipTests jar:jar
• Local minimal case: before fix mo-tester emitted ➤ 1[-5,64,0]; after fix it no longer emits the placeholder 1.
• Control case select 1 where 0 still preserves its header, so generic empty ResultSet behavior is not changed.