MDEV-29644 a potential bug of null pointer dereference in spider_db_mbase::print_warnings()

The function spider_db_mbase::print_warnings() can potentially result
in a null pointer dereference.

Remove the null pointer dereference by cleaning up the function.

Some small changes to the original commit
422fb63a.
Co-Authored-By: Yuchen Pei <yuchen.pei@mariadb.com>

storage/spider/mysql-test/spider/bugfix/r/mdev_29644.result

+#
+# MDEV-29644 a potential bug of null pointer dereference in spider_db_mbase::print_warnings()
+#
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3
+connection child2_1;
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+CREATE TABLE tbl_a (
+a CHAR(5)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8;
+SET GLOBAL sql_mode='';
+connection master_1;
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+CREATE TABLE tbl_a (
+a CHAR(255)
+) ENGINE=Spider DEFAULT CHARSET=utf8 COMMENT='table "tbl_a", srv "s_2_1"';
+SET sql_mode='';
+INSERT INTO tbl_a VALUES ("this will be truncated");
+NOT FOUND /\[WARN SPIDER RESULT\].* Warning 1265 Data truncated for column 'a' at row 1.*/ in mysqld.1.1.err
+SET GLOBAL spider_log_result_errors=4;
+INSERT INTO tbl_a VALUES ("this will be truncated");
+FOUND 1 /\[WARN SPIDER RESULT\].* Warning 1265 Data truncated for column 'a' at row 1.*/ in mysqld.1.1.err
+connection master_1;
+SET GLOBAL spider_log_result_errors=DEFAULT;
+SET sql_mode=DEFAULT;
+DROP DATABASE IF EXISTS auto_test_local;
+connection child2_1;
+SET GLOBAL sql_mode=DEFAULT;
+DROP DATABASE IF EXISTS auto_test_remote;
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3

storage/spider/mysql-test/spider/bugfix/t/mdev_29644.cnf

+!include include/default_mysqld.cnf
+!include ../my_1_1.cnf
+!include ../my_2_1.cnf

storage/spider/mysql-test/spider/bugfix/t/mdev_29644.test

+--echo #
+--echo # MDEV-29644 a potential bug of null pointer dereference in spider_db_mbase::print_warnings()
+--echo #
+
+# The test case below does not cause the potential null pointer dereference.
+# It is just for checking spider_db_mbase::fetch_and_print_warnings() works.
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+--connection child2_1
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+eval CREATE TABLE tbl_a (
+    a CHAR(5)
+) $CHILD2_1_ENGINE $CHILD2_1_CHARSET;
+
+SET GLOBAL sql_mode='';
+
+--connection master_1
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+eval CREATE TABLE tbl_a (
+    a CHAR(255)
+) $MASTER_1_ENGINE $MASTER_1_CHARSET COMMENT='table "tbl_a", srv "s_2_1"';
+
+SET sql_mode='';
+
+let SEARCH_FILE= $MYSQLTEST_VARDIR/log/mysqld.1.1.err;
+let SEARCH_PATTERN= \[WARN SPIDER RESULT\].* Warning 1265 Data truncated for column 'a' at row 1.*;
+
+INSERT INTO tbl_a VALUES ("this will be truncated");
+--source include/search_pattern_in_file.inc # should not find
+
+SET GLOBAL spider_log_result_errors=4;
+
+INSERT INTO tbl_a VALUES ("this will be truncated");
+--source include/search_pattern_in_file.inc # should find
+
+--connection master_1
+SET GLOBAL spider_log_result_errors=DEFAULT;
+SET sql_mode=DEFAULT;
+DROP DATABASE IF EXISTS auto_test_local;
+
+--connection child2_1
+SET GLOBAL sql_mode=DEFAULT;
+DROP DATABASE IF EXISTS auto_test_remote;
+
+--disable_query_log
+--disable_result_log
+--source ../t/test_deinit.inc
+--enable_query_log
+--enable_result_log

storage/spider/spd_db_mysql.cc

@@ -2162,7 +2162,7 @@ int spider_db_mbase::exec_query(
         db_conn->affected_rows, db_conn->insert_id,
         db_conn->server_status, db_conn->warning_count);
       if (spider_param_log_result_errors() >= 3)
-        print_warnings(l_time);
+        fetch_and_print_warnings(l_time);
     } else if (log_result_errors >= 4)
     {
       time_t cur_time = (time_t) time((time_t*) 0);
@@ -2244,56 +2244,45 @@ bool spider_db_mbase::is_xa_nota_error(
   DBUG_RETURN(xa_nota);
 }
 
-int spider_db_mbase::print_warnings(
-  struct tm *l_time
-) {
+int spider_db_mbase::fetch_and_print_warnings(struct tm *l_time)
+{
   int error_num = 0;
-  DBUG_ENTER("spider_db_mbase::print_warnings");
+  DBUG_ENTER("spider_db_mbase::fetch_and_print_warnings");
   DBUG_PRINT("info",("spider this=%p", this));
-  if (db_conn->status == MYSQL_STATUS_READY)
-  {
-    if (
-      !(db_conn->server_status & SERVER_MORE_RESULTS_EXISTS) &&
-      db_conn->warning_count
-    ) {
-      if (
-        spider_param_dry_access() ||
-        !mysql_real_query(db_conn, SPIDER_SQL_SHOW_WARNINGS_STR,
-          SPIDER_SQL_SHOW_WARNINGS_LEN)
-      ) {
-        MYSQL_RES *res = NULL;
-        MYSQL_ROW row = NULL;
-        uint num_fields;
-        if (
-          spider_param_dry_access() ||
-          !(res = mysql_store_result(db_conn)) ||
-          !(row = mysql_fetch_row(res))
-        ) {
-          if (mysql_errno(db_conn))
-          {
-            if (res)
-              mysql_free_result(res);
+
+  if (spider_param_dry_access() || db_conn->status != MYSQL_STATUS_READY ||
+      db_conn->server_status & SERVER_MORE_RESULTS_EXISTS ||
+      !db_conn->warning_count)
     DBUG_RETURN(0);
-          }
-          /* no record is ok */
-        }
-        num_fields = mysql_num_fields(res);
+
+  if (mysql_real_query(db_conn, SPIDER_SQL_SHOW_WARNINGS_STR,
+                       SPIDER_SQL_SHOW_WARNINGS_LEN))
+    DBUG_RETURN(0);
+
+  MYSQL_RES *res= mysql_store_result(db_conn);
+  if (!res)
+    DBUG_RETURN(0);
+
+  uint num_fields= mysql_num_fields(res);
   if (num_fields != 3)
   {
     mysql_free_result(res);
     DBUG_RETURN(0);
   }
+
+  MYSQL_ROW row= mysql_fetch_row(res);
   if (l_time)
   {
     while (row)
     {
-            fprintf(stderr, "%04d%02d%02d %02d:%02d:%02d [WARN SPIDER RESULT] "
-              "from [%s] %ld to %ld: %s %s %s\n",
+      fprintf(stderr,
+              "%04d%02d%02d %02d:%02d:%02d [WARN SPIDER RESULT] from [%s] %ld "
+              "to %ld: %s %s %s\n",
               l_time->tm_year + 1900, l_time->tm_mon + 1, l_time->tm_mday,
-              l_time->tm_hour, l_time->tm_min, l_time->tm_sec,
-              conn->tgt_host, (ulong) db_conn->thread_id,
-              (ulong) current_thd->thread_id, row[0], row[1], row[2]);
-            row = mysql_fetch_row(res);
+              l_time->tm_hour, l_time->tm_min, l_time->tm_sec, conn->tgt_host,
+              (ulong) db_conn->thread_id, (ulong) current_thd->thread_id, row[0],
+              row[1], row[2]);
+      row= mysql_fetch_row(res);
     }
   } else {
     while (row)
@@ -2309,11 +2298,9 @@ int spider_db_mbase::print_warnings(
       row = mysql_fetch_row(res);
     }
   }
-        if (res)
+    
   mysql_free_result(res);
-      }
-    }
-  }
+
   DBUG_RETURN(error_num);
 }
 
@@ -14209,7 +14196,7 @@ int spider_mbase_handler::show_table_status(
       DBUG_RETURN(error_num);
     }
   }
-  if ((error_num = ((spider_db_mbase *) conn->db_conn)->print_warnings(NULL)))
+  if ((error_num = ((spider_db_mbase *) conn->db_conn)->fetch_and_print_warnings(NULL)))
   {
     DBUG_RETURN(error_num);
   }

storage/spider/spd_db_mysql.h

@@ -440,7 +440,7 @@ class spider_db_mbase: public spider_db_conn
   bool is_xa_nota_error(
     int error_num
   );
-  int print_warnings(
+  int fetch_and_print_warnings(
     struct tm *l_time
   );
   spider_db_result *store_result(