MDEV-6616 Server crashes in my_hash_first if shutdown is performed when FLUSH LOGS is running

master_info_index becomes zero during shutdown.
check that it's valid (under a mutex) before dereferencing.

sql/item_func.cc

@@ -3961,9 +3961,13 @@ longlong Item_master_pos_wait::val_int()
   else
     connection_name= thd->variables.default_master_connection;
 
-  if (!(mi= master_info_index->get_master_info(&connection_name,
-                                               Sql_condition::WARN_LEVEL_WARN)))
+  mysql_mutex_lock(&LOCK_active_mi);
+  mi= master_info_index->get_master_info(&connection_name,
+                                         Sql_condition::WARN_LEVEL_WARN);
+  mysql_mutex_unlock(&LOCK_active_mi);
+  if (!mi)
     goto err;
+
   if ((event_count = mi->rli.wait_for_pos(thd, log_name, pos, timeout)) == -2)
   {
     null_value = 1;

sql/rpl_mi.cc

@@ -891,6 +891,9 @@ bool Master_info_index::init_all_master_info()
   File index_file_nr;
   DBUG_ENTER("init_all_master_info");
 
+  mysql_mutex_assert_owner(&LOCK_active_mi);
+  DBUG_ASSERT(master_info_index);
+
   if ((index_file_nr= my_open(index_file_name,
                               O_RDWR | O_CREAT | O_BINARY ,
                               MYF(MY_WME | ME_NOREFRESH))) < 0 ||
@@ -1090,6 +1093,10 @@ Master_info_index::get_master_info(LEX_STRING *connection_name,
              ("connection_name: '%.*s'", (int) connection_name->length,
               connection_name->str));
 
+  mysql_mutex_assert_owner(&LOCK_active_mi);
+  if (!this) // master_info_index is set to NULL on server shutdown
+    return NULL;
+
   /* Make name lower case for comparison */
   res= strmake(buff, connection_name->str, connection_name->length);
   my_casedn_str(system_charset_info, buff); 
@@ -1117,6 +1124,9 @@ bool Master_info_index::check_duplicate_master_info(LEX_STRING *name_arg,
   Master_info *mi;
   DBUG_ENTER("check_duplicate_master_info");
 
+  mysql_mutex_assert_owner(&LOCK_active_mi);
+  DBUG_ASSERT(master_info_index);
+
   /* Get full host and port name */
   if ((mi= master_info_index->get_master_info(name_arg,
                                               Sql_condition::WARN_LEVEL_NOTE)))
@@ -1239,6 +1249,8 @@ bool Master_info_index::give_error_if_slave_running()
 {
   DBUG_ENTER("warn_if_slave_running");
   mysql_mutex_assert_owner(&LOCK_active_mi);
+  if (!this) // master_info_index is set to NULL on server shutdown
+    return TRUE;
 
   for (uint i= 0; i< master_info_hash.records; ++i)
   {

sql/slave.cc

@@ -2858,7 +2858,8 @@ bool show_all_master_info(THD* thd)
   if (send_show_master_info_header(thd, 1, gtid_pos.length()))
     DBUG_RETURN(TRUE);
 
-  if (!(elements= master_info_index->master_info_hash.records))
+  if (!master_info_index ||
+      !(elements= master_info_index->master_info_hash.records))
     goto end;
 
   /*

sql/sql_parse.cc

@@ -2693,6 +2693,9 @@ case SQLCOM_PREPARE:
       goto error;
     mysql_mutex_lock(&LOCK_active_mi);
 
+    if (!master_info_index)
+      goto error;
+
     mi= master_info_index->get_master_info(&lex_mi->connection_name,
                                            Sql_condition::WARN_LEVEL_NOTE);
 
@@ -3150,7 +3153,7 @@ end_with_restore_list:
   case SQLCOM_SLAVE_ALL_START:
   {
     mysql_mutex_lock(&LOCK_active_mi);
-    if (!master_info_index->start_all_slaves(thd))
+    if (master_info_index && !master_info_index->start_all_slaves(thd))
       my_ok(thd);
     mysql_mutex_unlock(&LOCK_active_mi);
     break;
@@ -3166,7 +3169,7 @@ end_with_restore_list:
       goto error;
     }
     mysql_mutex_lock(&LOCK_active_mi);
-    if (!master_info_index->stop_all_slaves(thd))
+    if (master_info_index && !master_info_index->stop_all_slaves(thd))
       my_ok(thd);      
     mysql_mutex_unlock(&LOCK_active_mi);
     break;

sql/sql_reload.cc

@@ -174,18 +174,21 @@ bool reload_acl_and_cache(THD *thd, unsigned long long options,
     */
     tmp_write_to_binlog= 0;
     mysql_mutex_lock(&LOCK_active_mi);
-    if (!(mi= (master_info_index->
-               get_master_info(&connection_name,
-                               Sql_condition::WARN_LEVEL_ERROR))))
+    if (master_info_index)
     {
-      result= 1;
-    }
-    else
-    {
-      mysql_mutex_lock(&mi->data_lock);
-      if (rotate_relay_log(mi))
-        *write_to_binlog= -1;
-      mysql_mutex_unlock(&mi->data_lock);
+      if (!(mi= (master_info_index->
+                 get_master_info(&connection_name,
+                                 Sql_condition::WARN_LEVEL_ERROR))))
+      {
+        result= 1;
+      }
+      else
+      {
+        mysql_mutex_lock(&mi->data_lock);
+        if (rotate_relay_log(mi))
+          *write_to_binlog= -1;
+        mysql_mutex_unlock(&mi->data_lock);
+      }
     }
     mysql_mutex_unlock(&LOCK_active_mi);
 #endif
@@ -346,22 +349,24 @@ bool reload_acl_and_cache(THD *thd, unsigned long long options,
    Master_info *mi;
    tmp_write_to_binlog= 0;
    mysql_mutex_lock(&LOCK_active_mi);
-
-   if (!(mi= (master_info_index->
-              get_master_info(&lex_mi->connection_name,
-                              Sql_condition::WARN_LEVEL_ERROR))))
+   if (master_info_index)
    {
-     result= 1;
-   }
-   else if (reset_slave(thd, mi))
-   {
-     /* NOTE: my_error() has been already called by reset_slave(). */
-     result= 1;
-   }
-   else if (mi->connection_name.length && thd->lex->reset_slave_info.all)
-   {
-     /* If not default connection and 'all' is used */
-     master_info_index->remove_master_info(&mi->connection_name);
+     if (!(mi= (master_info_index->
+                get_master_info(&lex_mi->connection_name,
+                                Sql_condition::WARN_LEVEL_ERROR))))
+     {
+       result= 1;
+     }
+     else if (reset_slave(thd, mi))
+     {
+       /* NOTE: my_error() has been already called by reset_slave(). */
+       result= 1;
+     }
+     else if (mi->connection_name.length && thd->lex->reset_slave_info.all)
+     {
+       /* If not default connection and 'all' is used */
+       master_info_index->remove_master_info(&mi->connection_name);
+     }
    }
    mysql_mutex_unlock(&LOCK_active_mi);
  }

sql/sql_repl.cc

@@ -3226,6 +3226,9 @@ bool change_master(THD* thd, Master_info* mi, bool *master_info_added)
   LEX_MASTER_INFO* lex_mi= &thd->lex->mi;
   DBUG_ENTER("change_master");
 
+  mysql_mutex_assert_owner(&LOCK_active_mi);
+  DBUG_ASSERT(master_info_index);
+
   *master_info_added= false;
   /* 
     We need to check if there is an empty master_host. Otherwise
@@ -3632,7 +3635,8 @@ bool mysql_show_binlog_events(THD* thd)
   else  /* showing relay log contents */
   {
     mysql_mutex_lock(&LOCK_active_mi);
-    if (!(mi= master_info_index->
+    if (!master_info_index ||
+        !(mi= master_info_index->
           get_master_info(&thd->variables.default_master_connection,
                           Sql_condition::WARN_LEVEL_ERROR)))
     {