MDEV-30727 Check spider_hton_ptr in spider udfs

We have to #undef my_error and find it from udfs when spider is not
installed.

storage/spider/mysql-test/spider/bugfix/r/mdev_30727.result

+CREATE FUNCTION spider_direct_sql RETURNS INT SONAME 'ha_spider.so';
+SELECT spider_direct_sql ('SELECT * FROM s','a','srv "b"');
+ERROR HY000: Plugin 'SPIDER' is not loaded
+CREATE FUNCTION spider_bg_direct_sql RETURNS INT SONAME 'ha_spider.so';
+SELECT spider_bg_direct_sql ('SELECT * FROM s','a','srv "b"');
+ERROR HY000: Plugin 'SPIDER' is not loaded
+CREATE FUNCTION spider_copy_tables RETURNS INT SONAME 'ha_spider.so';
+SELECT spider_copy_tables ('t', '0', '0');
+ERROR HY000: Plugin 'SPIDER' is not loaded
+CREATE FUNCTION spider_flush_table_mon_cache RETURNS INT SONAME 'ha_spider.so';
+SELECT spider_flush_table_mon_cache ();
+spider_flush_table_mon_cache ()
+1
+install soname 'ha_spider';
+SELECT spider_direct_sql ('SELECT * FROM s','a','srv "b"');
+ERROR HY000: The foreign server name you are trying to reference does not exist. Data source error:  b
+call mtr.add_suppression(".*\\[Error\\] (mysqld|mariadbd): Can't find record in 'spider_tables'");
+SELECT spider_copy_tables ('t', '0', '0');
+ERROR HY000: Can't find record in 'spider_tables'
+SELECT spider_flush_table_mon_cache ();
+spider_flush_table_mon_cache ()
+1
+Warnings:
+Warning	1620	Plugin is busy and will be uninstalled on shutdown

storage/spider/mysql-test/spider/bugfix/t/mdev_30727.test

+CREATE FUNCTION spider_direct_sql RETURNS INT SONAME 'ha_spider.so';
+--error ER_PLUGIN_IS_NOT_LOADED
+SELECT spider_direct_sql ('SELECT * FROM s','a','srv "b"');
+
+CREATE FUNCTION spider_bg_direct_sql RETURNS INT SONAME 'ha_spider.so';
+--error ER_PLUGIN_IS_NOT_LOADED
+SELECT spider_bg_direct_sql ('SELECT * FROM s','a','srv "b"');
+
+CREATE FUNCTION spider_copy_tables RETURNS INT SONAME 'ha_spider.so';
+--error ER_PLUGIN_IS_NOT_LOADED
+SELECT spider_copy_tables ('t', '0', '0');
+
+# spider_flush_table_mon_cache does not require spider init to function
+CREATE FUNCTION spider_flush_table_mon_cache RETURNS INT SONAME 'ha_spider.so';
+SELECT spider_flush_table_mon_cache ();
+
+# The function functions properly after the plugin is installed
+install soname 'ha_spider';
+
+--error ER_FOREIGN_SERVER_DOESNT_EXIST
+SELECT spider_direct_sql ('SELECT * FROM s','a','srv "b"');
+
+call mtr.add_suppression(".*\\[Error\\] (mysqld|mariadbd): Can't find record in 'spider_tables'");
+--error ER_KEY_NOT_FOUND
+SELECT spider_copy_tables ('t', '0', '0');
+
+SELECT spider_flush_table_mon_cache ();
+
+--disable_query_log
+--source ../../include/clean_up_spider.inc

storage/spider/spd_copy_tables.cc

@@ -1154,12 +1154,20 @@ long long spider_copy_tables_body(
   DBUG_RETURN(0);
 }
 
+#undef my_error
+extern "C" void my_error(unsigned int nr, unsigned long MyFlags, ...);
+
 my_bool spider_copy_tables_init_body(
   UDF_INIT *initid,
   UDF_ARGS *args,
   char *message
 ) {
   DBUG_ENTER("spider_copy_tables_init_body");
+  if (!spider_hton_ptr)
+  {
+    my_error(ER_PLUGIN_IS_NOT_LOADED, MYF(0), "SPIDER");
+    goto error;
+  }
   if (args->arg_count != 3 && args->arg_count != 4)
   {
     strcpy(message, "spider_copy_tables() requires 3 or 4 arguments");

storage/spider/spd_direct_sql.cc

@@ -1789,6 +1789,9 @@ long long spider_direct_sql_body(
   DBUG_RETURN(0);
 }
 
+#undef my_error
+extern "C" void my_error(unsigned int nr, unsigned long MyFlags, ...);
+
 my_bool spider_direct_sql_init_body(
   UDF_INIT *initid,
   UDF_ARGS *args,
@@ -1797,6 +1800,11 @@ my_bool spider_direct_sql_init_body(
 ) {
   SPIDER_BG_DIRECT_SQL *bg_direct_sql;
   DBUG_ENTER("spider_direct_sql_init_body");
+  if (!spider_hton_ptr)
+  {
+    my_error(ER_PLUGIN_IS_NOT_LOADED, MYF(0), "SPIDER");
+    goto error;
+  }
   if (args->arg_count != 3)
   {
     strcpy(message, "spider_(bg)_direct_sql() requires 3 arguments");