MDEV-8208: Sporadic SEGFAULT on startup

Addendum: * Before calling THD::init_for_queries(), flip the current_thd to wsrep thread so that memory gets allocated for the right THD. * Use wsrep_creating_startup_threads instead of plugins_are_initialized as the condition for the execution of THD::init_for_queries() within start_wsrep_THD(), as use of latter could still leave some room for race.

Author: Nirbhay Choubey

mysys/my_alloc.c

@@ -105,6 +105,7 @@ void init_alloc_root(MEM_ROOT *mem_root, size_t block_size,
 void reset_root_defaults(MEM_ROOT *mem_root, size_t block_size,
  size_t pre_alloc_size __attribute__((unused)))
 {
+ DBUG_ENTER("reset_root_defaults");
  DBUG_ASSERT(alloc_root_inited(mem_root));
 
  mem_root->block_size= (((block_size - ALLOC_ROOT_MIN_BLOCK_SIZE) & ~1) |
@@ -127,7 +128,7 @@ void reset_root_defaults(MEM_ROOT *mem_root, size_t block_size,
  {
  /* We found a suitable block, no need to do anything else */
  mem_root->pre_alloc= mem;
- return;
+ DBUG_VOID_RETURN;
  }
  if (mem->left + ALIGN_SIZE(sizeof(USED_MEM)) == mem->size)
  {
@@ -157,6 +158,8 @@ void reset_root_defaults(MEM_ROOT *mem_root, size_t block_size,
  else
 #endif
  mem_root->pre_alloc= 0;
+
+ DBUG_VOID_RETURN;
 }
 
 

sql/mysqld.cc

@@ -5215,7 +5215,17 @@ static int init_server_components()
  {
  if (tmp->wsrep_applier == true)
  {
+ /*
+ Set THR_THD to temporally point to this THD to register all the
+ variables that allocates memory for this THD.
+ */
+ THD *current_thd_saved= current_thd;
+ set_current_thd(tmp);
+
  tmp->init_for_queries();
+
+ /* Restore current_thd. */
+ set_current_thd(current_thd_saved);
  }
  }
  mysql_mutex_unlock(&LOCK_thread_count);

sql/wsrep_mysqld.cc

@@ -1801,7 +1801,7 @@ pthread_handler_t start_wsrep_THD(void *arg)
  thd->proc_info= 0;
  thd->set_command(COM_SLEEP);
 
- if (plugins_are_initialized)
+ if (wsrep_creating_startup_threads == 0)
  {
  thd->init_for_queries();
  }