BUG#14485479: INSTALL AUDIT PLUGIN HANGS IF WE TRY TO

              DISABLE AND ENABLED DURING DDL OPERATION

PROBLEM: Same thread trying to acquire the same mutex
         second time leads to hang/server crash.
         While [un]installing audit_log plugin
         a thread acquires the LOCK_plugin mutex
         and after successful initialization tries
         to write in mysql.plugin table. It holds
         this mutex for a long time. If some how
         plugin table is corrupted then a write to 
         plugin table will throw an error, thread try
         to log this error in the audit_log plugin,
         doing so it tries to acquire the mutex
         again and results is server hang/crash.

SOLUTION: Releasing the LOCK_plugin mutex before
          writing in mysql.plugin table. We dont
          need to hold this mutex as thread already
          acquired a TL_WRITE lock on mysql.plugin
          table.

mysql-test/include/have_null_audit_plugin.inc

+disable_query_log;
+#
+# Check if server has support for loading plugins
+#
+if (`SELECT @@have_dynamic_loading != 'YES'`) {
+  --skip Null audit plugin requires dynamic loading
+}
+
+#
+# Check if the variable AUDIT_NULL is set
+#
+if (!$AUDIT_NULL) {
+  --skip Audit_null plugin requires the environment variable \$AUDIT_NULL to be set (normally done by mtr)
+}
+
+#
+# Check if --plugin-dir was setup for null_audit db
+#
+if (`SELECT CONCAT('--plugin-dir=', REPLACE(@@plugin_dir, '\\\\', '/')) != '$AUDIT_NULL_OPT/'`) {
+  --skip null audit plugin requires that --plugin-dir is set to the null audit plugin dir (either the .opt file does not contain \$AUDIT_NULL_OPT or another plugin is in use)
+}
+enable_query_log;

mysql-test/include/plugin.defs

@@ -40,3 +40,4 @@ ha_blackhole       storage/blackhole  BLACKHOLE_PLUGIN
 ha_federated       storage/federated  FEDERATED_PLUGIN
 mypluglib          plugin/fulltext    SIMPLE_PARSER
 libdaemon_example  plugin/daemon_example DAEMONEXAMPLE
+adt_null           plugin/audit_null  AUDIT_NULL

sql/sql_plugin.cc

@@ -1820,6 +1820,7 @@ bool mysql_install_plugin(THD *thd, const LEX_STRING *name, const LEX_STRING *dl
   {
     if (plugin_initialize(tmp))
     {
+      mysql_mutex_unlock(&LOCK_plugin);
       my_error(ER_CANT_INITIALIZE_UDF, MYF(0), name->str,
                "Plugin initialization function failed.");
       goto deinit;
@@ -1831,6 +1832,7 @@ bool mysql_install_plugin(THD *thd, const LEX_STRING *name, const LEX_STRING *dl
     of the insert into the plugin table, so that it is not replicated in
     row based mode.
   */
+  mysql_mutex_unlock(&LOCK_plugin);
   tmp_disable_binlog(thd);
   table->use_all_columns();
   restore_record(table, s->default_values);
@@ -1843,10 +1845,9 @@ bool mysql_install_plugin(THD *thd, const LEX_STRING *name, const LEX_STRING *dl
     table->file->print_error(error, MYF(0));
     goto deinit;
   }
-
-  mysql_mutex_unlock(&LOCK_plugin);
   DBUG_RETURN(FALSE);
 deinit:
+  mysql_mutex_lock(&LOCK_plugin);
   tmp->state= PLUGIN_IS_DELETED;
   reap_needed= true;
   reap_plugins();