Bug#29873343 WHEN WRITING EMPTY PRIVILEGES INTO GLOBAL_GRANTS TABLE

This patch improve stability of the global_grants table
when unexpected values has been inserted in the system table.

RB: 23168

Author: Kristofer Älvring

mysql-test/r/grant_dynamic.result

@@ -1131,3 +1131,22 @@ binlog.000001	#	Query	#	#	use `test`; GRANT GRANT OPTION ON *.* TO 'u1'@'%' WITH
 DROP USER u1;
 RESET MASTER;
 RESET SLAVE ALL;
+#
+# WRITING EMPTY PRIVILEGES INTO GLOBAL_GRANTS TABLE
+#
+INSERT INTO mysql.global_grants (user, host, priv) values ('', '%', ' ');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = '' AND host = '%' AND priv = ' ';
+INSERT INTO mysql.global_grants (user, host, priv) values ('', '', '');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = '' AND host = '' AND priv = '';
+INSERT INTO mysql.global_grants (user, host, priv) values (' ', '', '');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = '' AND host = ' ' AND priv = '';
+INSERT INTO mysql.global_grants (user, host, priv) values (' ', ' ', '');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = ' ' AND host = ' ' AND priv = '';
+INSERT INTO mysql.global_grants (user, host, priv) values ('', '', ' ');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = '' AND host = '' AND priv = ' ';
+FLUSH PRIVILEGES;

mysql-test/t/grant_dynamic.test

@@ -614,3 +614,24 @@ GRANT GRANT OPTION ON *.* TO u1;
 DROP USER u1;
 RESET MASTER;
 RESET SLAVE ALL;
+
+--echo #
+--echo # WRITING EMPTY PRIVILEGES INTO GLOBAL_GRANTS TABLE
+--echo #
+INSERT INTO mysql.global_grants (user, host, priv) values ('', '%', ' ');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = '' AND host = '%' AND priv = ' ';
+INSERT INTO mysql.global_grants (user, host, priv) values ('', '', '');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = '' AND host = '' AND priv = '';
+INSERT INTO mysql.global_grants (user, host, priv) values (' ', '', '');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = '' AND host = ' ' AND priv = '';
+INSERT INTO mysql.global_grants (user, host, priv) values (' ', ' ', '');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = ' ' AND host = ' ' AND priv = '';
+INSERT INTO mysql.global_grants (user, host, priv) values ('', '', ' ');
+FLUSH PRIVILEGES;
+DELETE FROM mysql.global_grants WHERE user = '' AND host = '' AND priv = ' ';
+FLUSH PRIVILEGES;
+

sql/auth/dynamic_privilege_table.cc

@@ -107,6 +107,7 @@ bool populate_dynamic_privilege_caches(THD *thd, TABLE_LIST *tablelst) {
   int read_rec_errcode;
   MEM_ROOT tmp_mem;
   char percentile_character[2] = {'%', '\0'};
+  char empty_str = '\0';
   /*
     We need the the dynamic privilege register in order to register any unknown
     privilege identifiers.
@@ -123,14 +124,14 @@ bool populate_dynamic_privilege_caches(THD *thd, TABLE_LIST *tablelst) {
       char *host =
           get_field(&tmp_mem, table->field[MYSQL_DYNAMIC_PRIV_FIELD_HOST]);
       if (host == 0) host = &percentile_character[0];
-      const char *user =
+      char *user =
           get_field(&tmp_mem, table->field[MYSQL_DYNAMIC_PRIV_FIELD_USER]);
-      if (user == nullptr) user = "\0";
+      if (user == nullptr) user = &empty_str;
       char *priv =
           get_field(&tmp_mem, table->field[MYSQL_DYNAMIC_PRIV_FIELD_PRIV]);
       char *with_grant_option = get_field(
           &tmp_mem, table->field[MYSQL_DYNAMIC_PRIV_FIELD_WITH_GRANT_OPTION]);
-
+      if (priv == nullptr) priv = &empty_str;
       my_caseup_str(system_charset_info, priv);
       LEX_CSTRING str_priv = {priv, strlen(priv)};
       LEX_CSTRING str_user = {user, strlen(user)};