MDEV-18661 loading the audit plugin causes performance regression.

Plugin fixed to not lock the LOCK_operations when not active.
Server fixed to lock the LOCK_plugin less - do it once per
thread and then only if a plugin was installed/uninstalled.

sql/sql_audit.cc

@@ -212,6 +212,7 @@ void mysql_audit_acquire_plugins(THD *thd, ulong *event_class_mask)
   {
     plugin_foreach(thd, acquire_plugins, MYSQL_AUDIT_PLUGIN, event_class_mask);
     add_audit_mask(thd->audit_class_mask, event_class_mask);
+    thd->audit_plugin_version= global_plugin_version;
   }
   DBUG_VOID_RETURN;
 }
@@ -241,6 +242,20 @@ void mysql_audit_notify(THD *thd, uint event_class, uint event_subtype, ...)
 }
 
 
+/**
+  Check if there were changes in the state of plugins
+  so we need to do the mysql_audit_release asap.
+
+  @param[in] thd
+
+*/
+
+my_bool mysql_audit_release_required(THD *thd)
+{
+  return thd && (thd->audit_plugin_version != global_plugin_version);
+}
+
+
 /**
   Release any resources associated with the current thd.
   
@@ -276,6 +291,7 @@ void mysql_audit_release(THD *thd)
   /* Reset the state of thread values */
   reset_dynamic(&thd->audit_class_plugins);
   bzero(thd->audit_class_mask, sizeof(thd->audit_class_mask));
+  thd->audit_plugin_version= -1;
 }
 
 

sql/sql_audit.h

@@ -60,6 +60,7 @@ static inline void mysql_audit_notify(THD *thd, uint event_class,
 #define mysql_audit_connection_enabled() 0
 #define mysql_audit_table_enabled() 0
 #endif
+extern my_bool mysql_audit_release_required(THD *thd);
 extern void mysql_audit_release(THD *thd);
 
 #define MAX_USER_HOST_SIZE 512

sql/sql_class.cc

@@ -776,6 +776,9 @@ THD::THD(bool is_wsrep_applier)
    waiting_on_group_commit(FALSE), has_waiter(FALSE),
    spcont(NULL),
    m_parser_state(NULL),
+#ifndef EMBEDDED_LIBRARY
+   audit_plugin_version(-1),
+#endif
 #if defined(ENABLED_DEBUG_SYNC)
    debug_sync_control(0),
 #endif /* defined(ENABLED_DEBUG_SYNC) */
@@ -1562,7 +1565,6 @@ THD::~THD()
 
   mdl_context.destroy();
   ha_close_connection(this);
-  mysql_audit_release(this);
   plugin_thdvar_cleanup(this);
 
   main_security_ctx.destroy();

sql/sql_class.h

@@ -2978,6 +2978,7 @@ class THD :public Statement,
     added to the list of audit plugins which are currently in use.
   */
   unsigned long audit_class_mask[MYSQL_AUDIT_CLASS_MASK_SIZE];
+  int audit_plugin_version;
 #endif
 
 #if defined(ENABLED_DEBUG_SYNC)

sql/sql_connect.cc

@@ -1326,6 +1326,7 @@ void do_handle_one_connection(THD *thd_arg)
 
     while (thd_is_connection_alive(thd))
     {
+      if (mysql_audit_release_required(thd))
         mysql_audit_release(thd);
       if (do_command(thd))
 	break;

sql/sql_plugin.cc

@@ -228,6 +228,7 @@ static DYNAMIC_ARRAY plugin_array;
 static HASH plugin_hash[MYSQL_MAX_PLUGIN_TYPE_NUM];
 static MEM_ROOT plugin_mem_root;
 static bool reap_needed= false;
+volatile int global_plugin_version= 1;
 
 static bool initialized= 0;
 ulong dlopen_count;
@@ -2181,6 +2182,7 @@ bool mysql_install_plugin(THD *thd, const LEX_STRING *name,
     reap_plugins();
   }
 err:
+  global_plugin_version++;
   mysql_mutex_unlock(&LOCK_plugin);
   if (argv)
     free_defaults(argv);
@@ -2327,6 +2329,7 @@ bool mysql_uninstall_plugin(THD *thd, const LEX_STRING *name,
   }
   reap_plugins();
 
+  global_plugin_version++;
   mysql_mutex_unlock(&LOCK_plugin);
   DBUG_RETURN(error);
 

sql/sql_plugin.h

@@ -37,6 +37,7 @@ enum enum_plugin_load_option { PLUGIN_OFF, PLUGIN_ON, PLUGIN_FORCE,
   PLUGIN_FORCE_PLUS_PERMANENT };
 extern const char *global_plugin_typelib_names[];
 
+extern volatile int global_plugin_version;
 extern ulong dlopen_count;
 
 #include <my_sys.h>

sql/threadpool_common.cc

@@ -266,6 +266,7 @@ int threadpool_process_request(THD *thd)
   {
     Vio *vio;
     thd->net.reading_or_writing= 0;
+    if (mysql_audit_release_required(thd))
       mysql_audit_release(thd);
 
     if ((retval= do_command(thd)) != 0)