Bulk execution loses warnings of earlier parameter sets when the statement is reprepared

In a per-row bulk operation (COM_STMT_BULK_EXECUTE that executes the statement once per parameter set: UPDATE, DELETE, INSERT with a subquery depending on a parameter after MDEV-23864) a reprepare in a later execution calls Prepared_statement::reprepare(), which calls thd->get_stmt_da()->clear_warning_info(). The warnings of the earlier parameter sets are dropped, but the warning count of the bulk operation still counts them.

How to repeat

SET sql_mode='';

CREATE TABLE t2 (a INT PRIMARY KEY, b TINYINT);

INSERT INTO t2 VALUES (1,0),(2,0),(3,0);

-- bulk, parameter sets (1000,1), (1,2), (1,3):

UPDATE t2 SET b = ? WHERE a = ?;

-- connection 2, between parameter sets 1 and 2:

ALTER TABLE t2 FORCE;

SHOW WARNINGS;

The client runs the bulk statement on connection 1 with SET debug_sync='before_execute_sql_command SIGNAL s1 WAIT_FOR go EXECUTE 2', and a second connection runs the DDL while the second parameter set waits (debug build). Without debug_sync the same happens when the DDL is concurrent and lands between two parameter sets.

With the MDEV-23864 patch (10.11, debug) the OK packet reports 1 warning, but SHOW WARNINGS is empty. Without the reprepare SHOW WARNINGS shows

Warning 1264 Out of range value for column 'b' at row 1

Without the patch (11.4.13 debug, source revision 4813c2f0a8a79e4e4c000cc8c9447096fafe7033) the server fails the assertion `! is_set() || m_can_overwrite_status' in Diagnostics_area::set_error_status() instead, see MDEV-41511.