MDEV-5225 Server crashes on CREATE USER|ROLE CURRENT_ROLE or DROP ROLE CURRENT_ROLE

mysql-test/suite/roles/create_and_drop_current.result

+grant create user on *.* to foo@localhost;
+create user current_user;
+ERROR HY000: Operation CREATE USER failed for CURRENT_USER
+create user current_role;
+ERROR HY000: Operation CREATE USER failed for CURRENT_ROLE
+create role current_user;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'current_user' at line 1
+create role current_role;
+ERROR HY000: Operation CREATE ROLE failed for CURRENT_ROLE
+drop user current_user;
+drop user current_role;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'current_role' at line 1
+drop role current_user;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'current_user' at line 1
+drop role current_role;
+ERROR HY000: Operation DROP ROLE failed for CURRENT_ROLE
+show warnings;
+Level	Code	Message
+Error	1446	Invalid definer
+Error	1396	Operation DROP ROLE failed for CURRENT_ROLE
+create role r1;
+grant r1 to current_user;
+set role r1;
+select current_role();
+current_role()
+r1
+create user current_role;
+ERROR HY000: Operation CREATE USER failed for CURRENT_ROLE
+create role current_role;
+ERROR HY000: Operation CREATE ROLE failed for CURRENT_ROLE
+drop user current_role;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'current_role' at line 1
+drop role current_role;
+select user,host,is_role from mysql.user;
+user	host	is_role
+root	localhost	N
+root	meddwl	N
+root	127.0.0.1	N
+root	::1	N

mysql-test/suite/roles/create_and_drop_current.test

+#
+# MDEV-5225 Server crashes on CREATE USER|ROLE CURRENT_ROLE or DROP ROLE CURRENT_ROLE
+#
+
+# Where CURRENT_USER/CURRENT_ROLE is explicitly allowed by the grammar
+# the error (if any) should be ER_CANNOT_USER
+#
+# Where it's not explicitly allowed, the error is ER_PARSE_ERROR,
+# because CURRENT_USER/CURRENT_ROLE are reserved words and cannot be
+# accepted as an identifier.
+#
+
+--source include/not_embedded.inc
+
+grant create user on *.* to foo@localhost;
+--change_user foo
+
+--error ER_CANNOT_USER
+create user current_user;
+--error ER_CANNOT_USER
+create user current_role;
+--error ER_PARSE_ERROR
+create role current_user;
+--error ER_CANNOT_USER
+create role current_role;
+# this works
+drop user current_user;
+--error ER_PARSE_ERROR
+drop user current_role;
+--error ER_PARSE_ERROR
+drop role current_user;
+--error ER_CANNOT_USER
+drop role current_role;
+show warnings;
+
+--change_user root
+
+create role r1;
+grant r1 to current_user;
+set role r1;
+select current_role();
+
+--error ER_CANNOT_USER
+create user current_role;
+--error ER_CANNOT_USER
+create role current_role;
+--error ER_PARSE_ERROR
+drop user current_role;
+drop role current_role;
+
+select user,host,is_role from mysql.user;
+

sql/sql_acl.cc

@@ -9043,6 +9043,13 @@ static void append_user(String *str, LEX_USER *user)
   str->append('\'');
 }
 
+static void append_str(String *str, const char *s, size_t l)
+{
+  if (str->length())
+    str->append(',');
+  str->append(s, l);
+}
+
 /*
   Create a list of users.
 
@@ -9080,6 +9087,20 @@ bool mysql_create_user(THD *thd, List <LEX_USER> &list, bool handle_as_role)
 
   while ((user_name= user_list++))
   {
+    if (user_name->user.str == current_user.str)
+    {
+      append_str(&wrong_users, STRING_WITH_LEN("CURRENT_USER"));
+      result= TRUE;
+      continue;
+    }
+
+    if (user_name->user.str == current_role.str)
+    {
+      append_str(&wrong_users, STRING_WITH_LEN("CURRENT_ROLE"));
+      result= TRUE;
+      continue;
+    }
+
     if (handle_as_role && is_invalid_role_name(user_name->user.str))
     {
       append_user(&wrong_users, user_name);
@@ -9189,7 +9210,15 @@ bool mysql_drop_user(THD *thd, List <LEX_USER> &list, bool handle_as_role)
   while ((tmp_user_name= user_list++))
   {
     user_name= get_current_user(thd, tmp_user_name, false);
-    if (!user_name || handle_as_role != user_name->is_role())
+    if (!user_name)
+    {
+      thd->clear_error();
+      append_str(&wrong_users, STRING_WITH_LEN("CURRENT_ROLE"));
+      result= TRUE;
+      continue;
+    }
+
+    if (handle_as_role != user_name->is_role())
     {
       append_user(&wrong_users, tmp_user_name);
       result= TRUE;