Bug #20537246 SERVER CRASH WHILE CONNECTING WITH CLEARTEXT-PLUGIN USER

              WITH BLANK PWD
When user is created with blank password, NULL value gets stored in
authentication_string column of mysql.user. When we do flush privileges
this NULL value gets loaded into acl_user.auth_string. Thus during
authentication there is a strcmp for external plugins (like cleartext
password, authentication_pam) for auth_string which causes segmentation
fault.
Fix is to set the auth_string to empty string when there is a NULL value
in authentication_string column of mysql.user table.

(cherry picked from commit 7fe41b58ef86059b737f42d1c1960aa5958d8f01)

Author: Bharathy Satish

mysql-test/r/plugin_auth.result

@@ -567,3 +567,14 @@ employee@localhost
 user()
 empl_external@localhost
 DROP USER 'empl_external'@'localhost', 'employee'@'localhost';
+#
+# Bug #20537246: SERVER CRASH WHILE CONNECTING WITH CLEARTEXT-PLUGIN
+#                USER WITH BLANK PWD
+#
+CREATE USER bug20537246@localhost
+IDENTIFIED WITH 'cleartext_plugin_server' AS '';
+## test connection
+select USER(),CURRENT_USER();
+USER()	CURRENT_USER()
+bug20537246@localhost	bug20537246@localhost
+DROP USER bug20537246@localhost;

mysql-test/t/plugin_auth.test

@@ -647,3 +647,20 @@ ALTER USER employee@localhost PASSWORD EXPIRE;
 DROP USER 'empl_external'@'localhost', 'employee'@'localhost';
 
 --source include/mysql_upgrade_cleanup.inc
+
+--echo #
+--echo # Bug #20537246: SERVER CRASH WHILE CONNECTING WITH CLEARTEXT-PLUGIN
+--echo #                USER WITH BLANK PWD
+--echo #
+
+CREATE USER bug20537246@localhost
+    IDENTIFIED WITH 'cleartext_plugin_server' AS '';
+
+--echo ## test connection
+connect(cleartext_con,localhost,bug20537246,,,,,CLEARTEXT);
+select USER(),CURRENT_USER();
+
+connection default;
+disconnect cleartext_con;
+DROP USER bug20537246@localhost;
+

sql/auth/sql_auth_cache.cc

@@ -1410,6 +1410,8 @@ static my_bool acl_load(THD *thd, TABLE_LIST *tables)
     }
     if(user.auth_string.str)
       user.auth_string.length= strlen(user.auth_string.str);
+    else
+      user.auth_string= EMPTY_STR;
 
     {
       uint next_field;