MDEV-27544 database() function should return 64 characters

Database names are 64 utf8 characters per the system tables
that refer to them.

The current database() function is returning 34 characters.

The result of limiting this function results to max length of 34
became apparent when used in a UNION ALL where the results are
truncated to 34 characters.

For (uninvestigated) reasons, SELECT DATABASE() on its own
would always return the right number of characters.

Thanks Alexander Barkov for the review.

Thanks dave for noticing the bug in the stackexchange post
https://dba.stackexchange.com/questions/306183/why-is-my-database-name-truncated

Author: grooverdan

mysql-test/r/func_system.result

@@ -46,7 +46,7 @@ create table t1 (version char(60)) select database(), user(), version() as 'vers
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `database()` varchar(34) CHARACTER SET utf8 DEFAULT NULL,
+  `database()` varchar(64) CHARACTER SET utf8 DEFAULT NULL,
   `user()` varchar(141) CHARACTER SET utf8 DEFAULT NULL,
   `version` char(60) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
@@ -95,3 +95,21 @@ select left(concat(a,version()),1) from t1;
 left(concat(a,version()),1)
 a
 drop table t1;
+#
+# Start of 10.2 tests
+#
+
+MDEV-27544 database() function under UNION ALL truncates results to 34 characters
+
+
+SET NAMES utf8;
+create database betäubungsmittelverschreibungsverordnung;
+use betäubungsmittelverschreibungsverordnung;
+select database() as "database" union all select database();
+database
+betäubungsmittelverschreibungsverordnung
+betäubungsmittelverschreibungsverordnung
+drop database betäubungsmittelverschreibungsverordnung;
+#
+# End of 10.2 tests
+#

mysql-test/t/func_system.test

@@ -55,3 +55,23 @@ select left(concat(a,version()),1) from t1;
 drop table t1;
 
 # End of 4.1 tests
+
+--echo #
+--echo # Start of 10.2 tests
+--echo #
+
+--echo
+--echo MDEV-27544 database() function under UNION ALL truncates results to 34 characters
+--echo
+--echo
+
+SET NAMES utf8;
+create database betäubungsmittelverschreibungsverordnung;
+use betäubungsmittelverschreibungsverordnung;
+select database() as "database" union all select database();
+drop database betäubungsmittelverschreibungsverordnung;
+
+
+--echo #
+--echo # End of 10.2 tests
+--echo #

sql/item_strfunc.h

@@ -717,7 +717,7 @@ class Item_func_database :public Item_func_sysconst
   String *val_str(String *);
   bool fix_length_and_dec()
   {
-    max_length= MAX_FIELD_NAME * system_charset_info->mbmaxlen;
+    max_length= NAME_CHAR_LEN * system_charset_info->mbmaxlen;
     maybe_null=1;
     return FALSE;
   }