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.

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();
   }