Fix for bug #28125: ERROR 2013 when adding index.

Problem: we may break a multibyte char sequence using a key 
reduced to maximum allowed length for a storage engine
(that leads to failed assertion in the innodb code, 
see also #17530). 

Fix: align truncated key length to multibyte char boundary.

mysql-test/r/innodb_mysql.result

@@ -418,7 +418,7 @@ DROP TABLE t1,t2;
 create table t1(f1 varchar(800) binary not null, key(f1)) engine = innodb 
 character set utf8 collate utf8_general_ci;
 Warnings:
-Warning	1071	Specified key was too long; max key length is 765 bytes
+Warning	1071	Specified key was too long; max key length is 767 bytes
 insert into t1 values('aaa');
 drop table t1;
 CREATE TABLE t1 (a INT PRIMARY KEY, b INT, c FLOAT, KEY b(b)) ENGINE = INNODB;
@@ -735,4 +735,16 @@ COUNT(*)
 3072
 set @@sort_buffer_size=default;
 DROP TABLE t1,t2;
+create table t1(a text) engine=innodb default charset=utf8;
+insert into t1 values('aaa');
+alter table t1 add index(a(1024));
+Warnings:
+Warning	1071	Specified key was too long; max key length is 767 bytes
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` text,
+  KEY `a` (`a`(255))
+) ENGINE=InnoDB DEFAULT CHARSET=utf8
+drop table t1;
 End of 5.0 tests

mysql-test/t/innodb_mysql.test

@@ -741,4 +741,13 @@ set @@sort_buffer_size=default;
 
 DROP TABLE t1,t2;
 
+#
+# Bug #28125: ERROR 2013 when adding index.
+#
+create table t1(a text) engine=innodb default charset=utf8; 
+insert into t1 values('aaa');
+alter table t1 add index(a(1024));
+show create table t1;
+drop table t1;
+
 --echo End of 5.0 tests

sql/sql_table.cc

@@ -1357,6 +1357,8 @@ static int mysql_prepare_table(THD *thd, HA_CREATE_INFO *create_info,
 			  length);
 	      push_warning(thd, MYSQL_ERROR::WARN_LEVEL_WARN,
 			   ER_TOO_LONG_KEY, warn_buff);
+              /* Align key length to multibyte char boundary */
+              length-= length % sql_field->charset->mbmaxlen;
 	    }
 	    else
 	    {
@@ -1387,8 +1389,6 @@ static int mysql_prepare_table(THD *thd, HA_CREATE_INFO *create_info,
       if (length > file->max_key_part_length() && key->type != Key::FULLTEXT)
       {
         length= file->max_key_part_length();
-        /* Align key length to multibyte char boundary */
-        length-= length % sql_field->charset->mbmaxlen;
 	if (key->type == Key::MULTIPLE)
 	{
 	  /* not a critical problem */
@@ -1397,6 +1397,8 @@ static int mysql_prepare_table(THD *thd, HA_CREATE_INFO *create_info,
 		      length);
 	  push_warning(thd, MYSQL_ERROR::WARN_LEVEL_WARN,
 		       ER_TOO_LONG_KEY, warn_buff);
+          /* Align key length to multibyte char boundary */
+          length-= length % sql_field->charset->mbmaxlen;
 	}
 	else
 	{