MDEV-29447 MDEV-26285 MDEV-31338 Refactor spider_db_mbase_util::open_item_func

spider_db_mbase_util::open_item_func() is a monster function.
It is difficult to maintain while it is expected that we need to
modify it when a new SQL function or a new func_type is added.

We split the function into two distinct functions: one handles the
case of str != NULL and the other handles the case of str == NULL.

This refactoring was done in a conservative way because we do not
have comprehensive tests on the function.

It also fixes MDEV-29447 and MDEV-31338 where field items that are
arguments of a func item may be used before created / initialised.

Note this commit is adapted from a patch by Nayuta for MDEV-26285.

storage/spider/mysql-test/spider/bugfix/r/mdev_29447.result

@@ -0,0 +1,33 @@
+#
+# MDEV-29447 SIGSEGV in spider_db_open_item_field and SIGSEGV spider_db_print_item_type, on SELECT
+#
+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 INT
+) ENGINE=InnoDB DEFAULT CHARSET=utf8;
+connection master_1;
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+CREATE TABLE tbl_a (
+a INT
+) ENGINE=Spider DEFAULT CHARSET=utf8 COMMENT='table "tbl_a", srv "s_2_1"';
+SELECT TRIM(LEADING 'c' FROM a) FROM tbl_a;
+TRIM(LEADING 'c' FROM a)
+connection child2_1;
+DROP DATABASE auto_test_remote;
+connection master_1;
+DROP DATABASE auto_test_local;
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3

storage/spider/mysql-test/spider/bugfix/r/mdev_31338.result

@@ -0,0 +1,15 @@
+#
+# MDEV-31338 UBSAN: runtime error: member access within null pointer of type 'struct SPIDER_FIELD_CHAIN' and SIGSEGV in spider_db_open_item_ident on SELECT
+#
+for master_1
+for child2
+for child3
+CREATE SERVER srv FOREIGN DATA WRAPPER MYSQL OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+CREATE TABLE t (c BLOB) ENGINE=InnoDB;
+CREATE TABLE ts (c BLOB) ENGINE=Spider COMMENT='WRAPPER "mysql",srv "srv",TABLE "t"';
+SELECT TRIM(BOTH ' ' FROM c) FROM ts ORDER BY c;
+TRIM(BOTH ' ' FROM c)
+drop table t, ts;
+for master_1
+for child2
+for child3

storage/spider/mysql-test/spider/bugfix/t/mdev_29447.cnf

@@ -0,0 +1,3 @@
+!include include/default_mysqld.cnf
+!include ../my_1_1.cnf
+!include ../my_2_1.cnf

storage/spider/mysql-test/spider/bugfix/t/mdev_29447.test

@@ -0,0 +1,37 @@
+--echo #
+--echo # MDEV-29447 SIGSEGV in spider_db_open_item_field and SIGSEGV spider_db_print_item_type, on SELECT
+--echo #
+ 
+--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 INT
+) $CHILD2_1_ENGINE $CHILD2_1_CHARSET;
+ 
+--connection master_1
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+eval CREATE TABLE tbl_a (
+    a INT
+) $MASTER_1_ENGINE $MASTER_1_CHARSET COMMENT='table "tbl_a", srv "s_2_1"';
+ 
+SELECT TRIM(LEADING 'c' FROM a) FROM tbl_a;
+ 
+--connection child2_1
+DROP DATABASE auto_test_remote;
+ 
+--connection master_1
+DROP DATABASE auto_test_local;
+ 
+--disable_query_log
+--disable_result_log
+--source ../t/test_deinit.inc
+--enable_query_log
+--enable_result_log

storage/spider/mysql-test/spider/bugfix/t/mdev_31338.test

@@ -0,0 +1,22 @@
+--echo #
+--echo # MDEV-31338 UBSAN: runtime error: member access within null pointer of type 'struct SPIDER_FIELD_CHAIN' and SIGSEGV in spider_db_open_item_ident on SELECT
+--echo #
+--source include/have_innodb.inc
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+evalp CREATE SERVER srv FOREIGN DATA WRAPPER MYSQL OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+CREATE TABLE t (c BLOB) ENGINE=InnoDB;
+CREATE TABLE ts (c BLOB) ENGINE=Spider COMMENT='WRAPPER "mysql",srv "srv",TABLE "t"';
+SELECT TRIM(BOTH ' ' FROM c) FROM ts ORDER BY c;
+drop table t, ts;
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log

storage/spider/spd_db_mysql.h

@@ -122,6 +122,25 @@ public:
     bool use_fields,
     spider_fields *fields
   ) override;
+protected:
+  int check_item_func(
+    Item_func *item_func,
+    ha_spider *spider,
+    const char *alias,
+    uint alias_length,
+    bool use_fields,
+    spider_fields *fields
+  );
+  int print_item_func(
+    Item_func *item_func,
+    ha_spider *spider,
+    spider_string *str,
+    const char *alias,
+    uint alias_length,
+    bool use_fields,
+    spider_fields *fields
+  );
+public:
 #ifdef HANDLER_HAS_DIRECT_AGGREGATE
   int open_item_sum_func(
     Item_sum *item_sum,