MDEV-10563 Crash during shutdown in Master_info_index::any_slave_sql_running

In well defined C code, the "this" pointer is never NULL. Currently, we
were potentially dereferencing a NULL pointer (master_info_index). GCC v6
removes any "if (!this)" conditions as it assumes this is always a
non-null pointer. In order to prevent undefined behaviour, check the
pointer before dereferencing and remove the check within member
functions.

sql/item_func.cc

@@ -3942,7 +3942,7 @@ longlong Item_master_pos_wait::val_int()
   longlong timeout = (arg_count>=3) ? args[2]->val_int() : 0 ;
   String connection_name_buff;
   LEX_STRING connection_name;
-  Master_info *mi;
+  Master_info *mi= NULL;
   if (arg_count >= 4)
   {
     String *con;
@@ -3962,6 +3962,7 @@ longlong Item_master_pos_wait::val_int()
     connection_name= thd->variables.default_master_connection;
 
   mysql_mutex_lock(&LOCK_active_mi);
+  if (master_info_index)  // master_info_index is set to NULL on shutdown.
     mi= master_info_index->get_master_info(&connection_name,
                                            Sql_condition::WARN_LEVEL_WARN);
   mysql_mutex_unlock(&LOCK_active_mi);

sql/mysqld.cc

@@ -7307,7 +7307,10 @@ static int show_slaves_running(THD *thd, SHOW_VAR *var, char *buff)
   var->value= buff;
   mysql_mutex_lock(&LOCK_active_mi);
 
+  if (master_info_index)
     *((longlong *)buff)= master_info_index->any_slave_sql_running();
+  else
+    *((longlong *)buff)= 0;
 
   mysql_mutex_unlock(&LOCK_active_mi);
   return 0;

sql/rpl_mi.cc

@@ -1095,8 +1095,6 @@ Master_info_index::get_master_info(LEX_STRING *connection_name,
               connection_name->str));
 
   mysql_mutex_assert_owner(&LOCK_active_mi);
-  if (!this) // master_info_index is set to NULL on server shutdown
-    DBUG_RETURN(NULL);
 
   /* Make name lower case for comparison */
   res= strmake(buff, connection_name->str, connection_name->length);
@@ -1250,8 +1248,6 @@ bool Master_info_index::give_error_if_slave_running()
 {
   DBUG_ENTER("give_error_if_slave_running");
   mysql_mutex_assert_owner(&LOCK_active_mi);
-  if (!this) // master_info_index is set to NULL on server shutdown
-    DBUG_RETURN(TRUE);
 
   for (uint i= 0; i< master_info_hash.records; ++i)
   {
@@ -1282,8 +1278,7 @@ uint Master_info_index::any_slave_sql_running()
 {
   uint count= 0;
   DBUG_ENTER("any_slave_sql_running");
-  if (!this) // master_info_index is set to NULL on server shutdown
-    DBUG_RETURN(count);
+  mysql_mutex_assert_owner(&LOCK_active_mi);
 
   for (uint i= 0; i< master_info_hash.records; ++i)
   {

sql/slave.cc

@@ -649,6 +649,7 @@ int terminate_slave_threads(Master_info* mi,int thread_mask,bool skip_lock)
     mysql_mutex_unlock(log_lock);
   }
   if (opt_slave_parallel_threads > 0 &&
+      master_info_index &&// master_info_index is set to NULL on server shutdown
       !master_info_index->any_slave_sql_running())
     rpl_parallel_inactivate_pool(&global_rpl_thread_pool);
   if (thread_mask & (SLAVE_IO|SLAVE_FORCE_ALL))

sql/sys_vars.cc

@@ -1538,7 +1538,8 @@ Sys_var_gtid_slave_pos::do_check(THD *thd, set_var *var)
   }
 
   mysql_mutex_lock(&LOCK_active_mi);
-  running= master_info_index->give_error_if_slave_running();
+  running= (!master_info_index ||
+            master_info_index->give_error_if_slave_running());
   mysql_mutex_unlock(&LOCK_active_mi);
   if (running)
     return true;
@@ -1578,7 +1579,7 @@ Sys_var_gtid_slave_pos::global_update(THD *thd, set_var *var)
 
   mysql_mutex_unlock(&LOCK_global_system_variables);
   mysql_mutex_lock(&LOCK_active_mi);
-  if (master_info_index->give_error_if_slave_running())
+  if (!master_info_index || master_info_index->give_error_if_slave_running())
     err= true;
   else
     err= rpl_gtid_pos_update(thd, var->save_result.string_value.str,
@@ -1767,7 +1768,8 @@ check_slave_parallel_threads(sys_var *self, THD *thd, set_var *var)
   bool running;
 
   mysql_mutex_lock(&LOCK_active_mi);
-  running= master_info_index->give_error_if_slave_running();
+  running= (!master_info_index ||
+            master_info_index->give_error_if_slave_running());
   mysql_mutex_unlock(&LOCK_active_mi);
   if (running)
     return true;
@@ -1782,7 +1784,8 @@ fix_slave_parallel_threads(sys_var *self, THD *thd, enum_var_type type)
 
   mysql_mutex_unlock(&LOCK_global_system_variables);
   mysql_mutex_lock(&LOCK_active_mi);
-  err= master_info_index->give_error_if_slave_running();
+  err= (!master_info_index ||
+        master_info_index->give_error_if_slave_running());
   mysql_mutex_unlock(&LOCK_active_mi);
   mysql_mutex_lock(&LOCK_global_system_variables);
 
@@ -1809,7 +1812,8 @@ check_slave_domain_parallel_threads(sys_var *self, THD *thd, set_var *var)
   bool running;
 
   mysql_mutex_lock(&LOCK_active_mi);
-  running= master_info_index->give_error_if_slave_running();
+  running= (!master_info_index ||
+            master_info_index->give_error_if_slave_running());
   mysql_mutex_unlock(&LOCK_active_mi);
   if (running)
     return true;
@@ -1824,7 +1828,8 @@ fix_slave_domain_parallel_threads(sys_var *self, THD *thd, enum_var_type type)
 
   mysql_mutex_unlock(&LOCK_global_system_variables);
   mysql_mutex_lock(&LOCK_active_mi);
-  running= master_info_index->give_error_if_slave_running();
+  running= (!master_info_index ||
+            master_info_index->give_error_if_slave_running());
   mysql_mutex_unlock(&LOCK_active_mi);
   mysql_mutex_lock(&LOCK_global_system_variables);
 
@@ -1862,7 +1867,8 @@ check_gtid_ignore_duplicates(sys_var *self, THD *thd, set_var *var)
   bool running;
 
   mysql_mutex_lock(&LOCK_active_mi);
-  running= master_info_index->give_error_if_slave_running();
+  running= (!master_info_index ||
+            master_info_index->give_error_if_slave_running());
   mysql_mutex_unlock(&LOCK_active_mi);
   if (running)
     return true;
@@ -1877,7 +1883,8 @@ fix_gtid_ignore_duplicates(sys_var *self, THD *thd, enum_var_type type)
 
   mysql_mutex_unlock(&LOCK_global_system_variables);
   mysql_mutex_lock(&LOCK_active_mi);
-  running= master_info_index->give_error_if_slave_running();
+  running= (!master_info_index ||
+            master_info_index->give_error_if_slave_running());
   mysql_mutex_unlock(&LOCK_active_mi);
   mysql_mutex_lock(&LOCK_global_system_variables);
 
@@ -2830,7 +2837,7 @@ Sys_var_replicate_events_marked_for_skip::global_update(THD *thd, set_var *var)
 
   mysql_mutex_unlock(&LOCK_global_system_variables);
   mysql_mutex_lock(&LOCK_active_mi);
-  if (!master_info_index->give_error_if_slave_running())
+  if (master_info_index && !master_info_index->give_error_if_slave_running())
     result= Sys_var_enum::global_update(thd, var);
   mysql_mutex_unlock(&LOCK_active_mi);
   mysql_mutex_lock(&LOCK_global_system_variables);