MDEV-23032 FLOOR()/CEIL() incorrectly calculate the precision of a DECIMAL(M,D) column

The code in Item_func_int_val::fix_length_and_dec_int_or_decimal()
calculated badly the result data type for FLOOR()/CEIL(), so for example
the decimal(38,10) input created a decimal(28,0) result.
That was not correct, because one extra integer digit is needed.
   floor(-9.9) -> -10
   ceil(9.9)   ->  10

Rewritting the code in a more straightforward way.
Additional changes:
- FLOOR() now takes into account the presence of the UNSIGNED
flag of the argument: FLOOR(unsigned decimal) does not need an extra digits.
- FLOOR()/CEILING() now preserve the unsigned flag in the result
  data type is decimal.
These changes give nicer data types.

1 files changed, 6 insertions, 4 deletions

diff --git a/mysql-test/main/ctype_utf8.result b/mysql-test/main/ctype_utf8.result
index 9585931584b..bd35fd10f19 100644
--- a/mysql-test/main/ctype_utf8.result
+++ b/mysql-test/main/ctype_utf8.result
@@ -2817,21 +2817,23 @@ drop table t1;
 select hex(concat(ceiling(0.5)));
 hex(concat(ceiling(0.5)))
 31
-create table t1 as select concat(ceiling(0.5)) as c1;
+create table t1 as select ceiling(0.5) as c0, concat(ceiling(0.5)) as c1;
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `c1` varchar(4) CHARACTER SET utf8 DEFAULT NULL
+  `c0` int(3) NOT NULL,
+  `c1` varchar(3) CHARACTER SET utf8 DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 select hex(concat(floor(0.5)));
 hex(concat(floor(0.5)))
 30
-create table t1 as select concat(floor(0.5)) as c1;
+create table t1 as select floor(0.5) as c0, concat(floor(0.5)) as c1;
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `c1` varchar(4) CHARACTER SET utf8 DEFAULT NULL
+  `c0` int(3) NOT NULL,
+  `c1` varchar(3) CHARACTER SET utf8 DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
 select hex(concat(round(0.5)));