MDEV-26: Global transaction ID

Fix lock order violation. Make sure we temporarily release
LOCK_global_system_variables before taking LOCK_active_mi.
Otherwise we get a lock dependency cycle when we use
open_tables() under LOCK_active_mi.

sql/mysqld.cc

@@ -4005,6 +4005,7 @@ static int init_thread_environment()
   mysql_mutex_init(key_LOCK_active_mi, &LOCK_active_mi, MY_MUTEX_INIT_FAST);
   mysql_mutex_init(key_LOCK_global_system_variables,
                    &LOCK_global_system_variables, MY_MUTEX_INIT_FAST);
+  mysql_mutex_record_order(&LOCK_active_mi, &LOCK_global_system_variables);
   mysql_rwlock_init(key_rwlock_LOCK_system_variables_hash,
                     &LOCK_system_variables_hash);
   mysql_mutex_init(key_LOCK_prepared_stmt_count,

sql/sql_base.cc

@@ -379,6 +379,7 @@ bool table_def_init(void)
   init_tdc_psi_keys();
 #endif
   mysql_mutex_init(key_LOCK_open, &LOCK_open, MY_MUTEX_INIT_FAST);
+  mysql_mutex_record_order(&LOCK_active_mi, &LOCK_open);
   oldest_unused_share= &end_of_unused_share;
   end_of_unused_share.prev= &oldest_unused_share;
 

sql/sys_vars.cc

@@ -3400,6 +3400,7 @@ get_master_info_uint_value(THD *thd, ptrdiff_t offset)
 {
   Master_info *mi;
   uint res= 0;                                  // Default value
+  mysql_mutex_unlock(&LOCK_global_system_variables);
   mysql_mutex_lock(&LOCK_active_mi);
   mi= master_info_index->
     get_master_info(&thd->variables.default_master_connection,
@@ -3411,6 +3412,7 @@ get_master_info_uint_value(THD *thd, ptrdiff_t offset)
     mysql_mutex_unlock(&mi->rli.data_lock);
   }
   mysql_mutex_unlock(&LOCK_active_mi);    
+  mysql_mutex_lock(&LOCK_global_system_variables);
   return res;
 }
   
@@ -3422,6 +3424,7 @@ bool update_multi_source_variable(sys_var *self_var, THD *thd,
   bool result= true;
   Master_info *mi;
 
+  mysql_mutex_unlock(&LOCK_global_system_variables);
   mysql_mutex_lock(&LOCK_active_mi);
   mi= master_info_index->
     get_master_info(&thd->variables.default_master_connection,
@@ -3435,6 +3438,7 @@ bool update_multi_source_variable(sys_var *self_var, THD *thd,
     mysql_mutex_unlock(&mi->rli.run_lock);
   }
   mysql_mutex_unlock(&LOCK_active_mi);
+  mysql_mutex_lock(&LOCK_global_system_variables);
   return result;
 }