Fix crash during shutdown in audit plugin code path

Summary:
On server shutdown, there exists a race condition that can trigger a segfault if an audit plugin is installed. The root cause because the `THD::audit_class_plugins` array is accessed concurrently without locks.

This is the sequence of actions that can trigger a crash:
1. The shutdown thread calls close_connections on all connected clients.
2. The connection thread will call into the audit plugin because it will receive a network error from `close_connections`, and will log the error into the audit plugin. The thread then adds the plugin to `THD::audit_class_plugins`, but it has not updated `audit_class_mask` yet.
3. The shutdown thread sees that `audit_class_mask` is not set, and also adds the plugin into `THD::audit_class_plugins`, but it has only extended the array, without setting the value of the 2nd element of the array.
4. The connection thread then goes ahead and updates `audit_class_mask` and finishes `acquire_plugins`. It then calls `mysql_audit_notify` and sees the array of size 2, and when it loops over the 2nd element, it segfaults because the value is uninitalized.

The fix is to add a lock per THD that protects these data structures. They should be relatively uncontended since the shutdown thread is the only thread that calls `acquire_plugins` on another THD.

The corresponding code in 5.7/8.0 has been largely refactored, but I suspect this will still be an issue since it doesn't seem like the corresponding data structures are synchronized.

Reviewed By: asandryh

Differential Revision: D6103777

fbshipit-source-id: 17763c6

sql/mysqld.cc

@@ -12055,7 +12055,7 @@ PSI_mutex_key
   key_LOCK_server_started, key_LOCK_status,
   key_LOCK_system_variables_hash, key_LOCK_table_share, key_LOCK_thd_data,
   key_LOCK_thd_db_read_only_hash,
-  key_LOCK_db_metadata,
+  key_LOCK_db_metadata, key_LOCK_thd_audit_data,
   key_LOCK_user_conn, key_LOCK_uuid_generator, key_LOG_LOCK_log,
   key_master_info_data_lock, key_master_info_run_lock,
   key_master_info_sleep_lock, key_master_info_thd_lock,

sql/mysqld.h

@@ -1061,7 +1061,7 @@ extern PSI_mutex_key
   key_LOCK_server_started, key_LOCK_status,
   key_LOCK_table_share, key_LOCK_thd_data,
   key_LOCK_thd_db_read_only_hash,
-  key_LOCK_db_metadata,
+  key_LOCK_db_metadata, key_LOCK_thd_audit_data,
   key_LOCK_user_conn, key_LOCK_uuid_generator, key_LOG_LOCK_log,
   key_master_info_data_lock, key_master_info_run_lock,
   key_master_info_sleep_lock, key_master_info_thd_lock,

sql/sql_audit.cc

@@ -153,10 +153,11 @@ static my_bool acquire_plugins(THD *thd, plugin_ref plugin, void *arg)
   st_mysql_audit *data= plugin_data(plugin, struct st_mysql_audit *);
 
   set_audit_mask(event_class_mask, event_class);
+  mysql_mutex_lock(&thd->LOCK_thd_audit_data);
 
   /* Check if this plugin is interested in the event */
   if (check_audit_mask(data->class_mask, event_class_mask))
-    return 0;
+    goto exit;
 
   /*
     Check if this plugin may already be registered. This will fail to
@@ -165,7 +166,7 @@ static my_bool acquire_plugins(THD *thd, plugin_ref plugin, void *arg)
     are an event class of which the audit plugin has interest.
   */
   if (!check_audit_mask(data->class_mask, thd->audit_class_mask))
-    return 0;
+    goto exit;
 
   /* Check if we need to initialize the array of acquired plugins */
   if (unlikely(!thd->audit_class_plugins.buffer))
@@ -179,6 +180,8 @@ static my_bool acquire_plugins(THD *thd, plugin_ref plugin, void *arg)
   plugin= my_plugin_lock(NULL, &plugin);
   insert_dynamic(&thd->audit_class_plugins, &plugin);
 
+exit:
+  mysql_mutex_unlock(&thd->LOCK_thd_audit_data);
   return 0;
 }
 
@@ -238,10 +241,14 @@ void mysql_audit_notify(THD *thd, uint event_class, uint event_subtype, ...)
 
 void mysql_audit_release(THD *thd)
 {
+  mysql_mutex_lock(&thd->LOCK_thd_audit_data);
   plugin_ref *plugins, *plugins_last;
 
   if (!thd || !(thd->audit_class_plugins.elements))
+  {
+    mysql_mutex_unlock(&thd->LOCK_thd_audit_data);
     return;
+  }
 
   plugins= (plugin_ref*) thd->audit_class_plugins.buffer;
   plugins_last= plugins + thd->audit_class_plugins.elements;
@@ -264,6 +271,7 @@ void mysql_audit_release(THD *thd)
   /* Reset the state of thread values */
   reset_dynamic(&thd->audit_class_plugins);
   memset(thd->audit_class_mask, 0, sizeof(thd->audit_class_mask));
+  mysql_mutex_unlock(&thd->LOCK_thd_audit_data);
 }
 
 
@@ -276,8 +284,10 @@ void mysql_audit_release(THD *thd)
 
 void mysql_audit_init_thd(THD *thd)
 {
+  mysql_mutex_lock(&thd->LOCK_thd_audit_data);
   memset(&thd->audit_class_plugins, 0, sizeof(thd->audit_class_plugins));
   memset(thd->audit_class_mask, 0, sizeof(thd->audit_class_mask));
+  mysql_mutex_unlock(&thd->LOCK_thd_audit_data);
 }
 
 
@@ -294,8 +304,10 @@ void mysql_audit_init_thd(THD *thd)
 void mysql_audit_free_thd(THD *thd)
 {
   mysql_audit_release(thd);
+  mysql_mutex_lock(&thd->LOCK_thd_audit_data);
   DBUG_ASSERT(thd->audit_class_plugins.elements == 0);
   delete_dynamic(&thd->audit_class_plugins);
+  mysql_mutex_unlock(&thd->LOCK_thd_audit_data);
 }
 
 #ifdef HAVE_PSI_INTERFACE
@@ -491,6 +503,7 @@ static void event_class_dispatch(THD *thd, unsigned int event_class,
   }
   else
   {
+    mysql_mutex_lock(&thd->LOCK_thd_audit_data);
     plugin_ref *plugins, *plugins_last;
 
     /* Use the cached set of audit plugins */
@@ -499,6 +512,7 @@ static void event_class_dispatch(THD *thd, unsigned int event_class,
 
     for (; plugins < plugins_last; plugins++)
       plugins_dispatch(thd, *plugins, &event_generic);
+    mysql_mutex_unlock(&thd->LOCK_thd_audit_data);
   }
 }
 

sql/sql_class.cc

@@ -1050,6 +1050,8 @@ THD::THD(bool enable_plugins)
   dbug_sentry=THD_SENTRY_MAGIC;
 #endif
 #ifndef EMBEDDED_LIBRARY
+  mysql_mutex_init(key_LOCK_thd_audit_data, &LOCK_thd_audit_data,
+                   MY_MUTEX_INIT_FAST);
   mysql_audit_init_thd(this);
   net.vio=0;
 #endif

sql/sql_class.h

@@ -2429,6 +2429,7 @@ class THD :public MDL_context_owner,
   mysql_mutex_t LOCK_thd_data;
   mysql_mutex_t LOCK_thd_db_read_only_hash;
   mysql_mutex_t LOCK_db_metadata;
+  mysql_mutex_t LOCK_thd_audit_data;
 
   /* all prepared statements and cursors of this connection */
   Statement_map stmt_map;