Backport fix for MDEV-29352 to 10.3-10.5

The fix for MDEV-29352 was pushed to 10.6+ but the code causing the
bug is old and the bug is unlikely to be a recent regression in 10.6.
So, we apply the fix also to older versions, 10.3-10.5.

The original commit message:

MDEV-29352 SIGSEGV's in strlen and unknown location on optimized builds at SHUTDOWN

When the UDF creation frails to write the newly created UDF into
the related system table, the UDF is still created in memory.

However, as it is now, the related DLL is unloaded in this case right
in the mysql_create_function. And failure happens when the UDF handle
is freed and tries to unload the respective DLL which is still unloaded.

sql/sql_udf.cc

@@ -606,7 +606,7 @@ int mysql_create_function(THD *thd,udf_func *udf)
 
   /* Allow creation of functions even if we can't open func table */
   if (unlikely(!table))
-    goto err;
+    goto err_open_func_table;
   table->use_all_columns();
   restore_record(table, s->default_values);	// Default values for fields
   table->field[0]->store(u_d->name.str, u_d->name.length, system_charset_info);
@@ -620,7 +620,7 @@ int mysql_create_function(THD *thd,udf_func *udf)
   {
     my_error(ER_ERROR_ON_WRITE, MYF(0), "mysql.func", error);
     del_udf(u_d);
-    goto err;
+    goto err_open_func_table;
   }
 
 done:
@@ -635,6 +635,7 @@ int mysql_create_function(THD *thd,udf_func *udf)
 err:
   if (new_dl)
     dlclose(dl);
+err_open_func_table:
   mysql_rwlock_unlock(&THR_LOCK_udf);
   DBUG_RETURN(1);
 }

storage/spider/mysql-test/spider/bugfix/include/restart_spider.inc

+--let $_expect_file_name= $MYSQLTEST_VARDIR/tmp/mysqld.1.1.expect
+
+--exec echo "wait" > $_expect_file_name
+--shutdown_server
+--source include/wait_until_disconnected.inc
+--exec echo "restart" > $_expect_file_name
+--enable_reconnect
+--source include/wait_until_connected_again.inc

storage/spider/mysql-test/spider/bugfix/r/mdev_29352.result

+CREATE TABLE t (c INT);
+SHOW CREATE TABLE t;
+Table	Create Table
+t	CREATE TABLE `t` (
+  `c` int(11) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
+FLUSH TABLES WITH READ LOCK;
+CREATE FUNCTION spider_bg_direct_sql RETURNS INT SONAME 'ha_spider.so';
+ERROR HY000: Can't execute the query because you have a conflicting read lock
+SELECT * FROM t;
+c
+DROP TABLE t;

storage/spider/mysql-test/spider/bugfix/t/mdev_29352.test

+CREATE TABLE t (c INT);
+SHOW CREATE TABLE t;
+FLUSH TABLES WITH READ LOCK;
+
+--error ER_CANT_UPDATE_WITH_READLOCK
+CREATE FUNCTION spider_bg_direct_sql RETURNS INT SONAME 'ha_spider.so';
+SELECT * FROM t;
+
+--source include/restart_spider.inc
+
+DROP TABLE t;