Bug#27492122 SHUTDOWN HANG WHEN FIREWALL HAS MORE THAN 250K RULES

Problem
=======

Deletion of 250k elements from the hash takes very long time.

Analysis
========
Elements are removed from the hash using its synchronization feature, where this
is not required. All elements are removed in two scenarios:
- Firewall shutdown
- set_firewall_mode UDF is called. Exclusive access is guaranteed by the rwlock:
  WR_lock lock(&fw_users_lock).

Fix
===

Native destructor hash feature is used, which clears all elements on hash destroy
(lf_hash_destroy). Are elements are destroyed within a second.

Reviewed-by
===========
Kristofer Alvring <[email protected]>
Georgi Kodinov <[email protected]>
Marc Alff <[email protected]>

Author: maras007

include/lf.h

@@ -1,4 +1,4 @@
-/* Copyright (c) 2007, 2016, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2007, 2018, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -149,7 +149,7 @@ typedef void lf_hash_init_func(uchar *dst, const uchar* src);
 #define LF_HASH_UNIQUE 1
 
 /* lf_hash overhead per element (that is, sizeof(LF_SLIST) */
-extern const int LF_HASH_OVERHEAD;
+extern MYSQL_PLUGIN_IMPORT const int LF_HASH_OVERHEAD;
 
 typedef struct st_lf_hash {
   LF_DYNARRAY array;                    /* hash itself */