MDEV-23118 FORMAT(d1,dec) where dec=0/38 and d1 is DECIMAL(38,38) gives incorrect results

FORMAT() can print more integer digits (than the argument has)
if rounding happens:

  FORMAT(9.9,0) -> '10'

The old code did not take this into account.

Fix:

1. One extra digit is needed in case of rounding

- If args[1] is a not-NULL constant, then reserve space for one extra integer
  digit if the requested number of decimals is less than args[0]->decimals.

- Otherwise, reserve space for one extra integer digit if
  args[0]->decimals is not 0, because rounding can potentially happen
  (depending on the exact data in arguments).

2. One extra digit is also needed if the argument has no integer digits,
   e.g. in a data type like DECIMAL(38,38).

The conditions 1 and 2 are ORed.

3. Fixing FORMAT_MAX_DECIMALS from 30 to 38. This was forgotten in 10.2.1
   (when the limit for the number of fractional digits in DECIMAL was extended).

mysql-test/main/func_math.result

@@ -2295,7 +2295,7 @@ FORMAT(-1e308,2)
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `FORMAT(-1e308,2)` varchar(416) DEFAULT NULL
+  `FORMAT(-1e308,2)` varchar(417) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1;
 CREATE OR REPLACE TABLE t1 AS SELECT FORMAT('-1e308',2);
@@ -2305,7 +2305,7 @@ FORMAT('-1e308',2)
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
-  `FORMAT('-1e308',2)` varchar(416) DEFAULT NULL
+  `FORMAT('-1e308',2)` varchar(417) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1;
 CREATE OR REPLACE TABLE t1 AS SELECT FORMAT(DATE'20191231',0),FORMAT(TIME'99:05:00',0),FORMAT(TIMESTAMP'2019-12-31 23:59:59.123456',0);
@@ -2317,7 +2317,7 @@ Table	Create Table
 t1	CREATE TABLE `t1` (
   `FORMAT(DATE'20191231',0)` varchar(11) DEFAULT NULL,
   `FORMAT(TIME'99:05:00',0)` varchar(10) DEFAULT NULL,
-  `FORMAT(TIMESTAMP'2019-12-31 23:59:59.123456',0)` varchar(19) DEFAULT NULL
+  `FORMAT(TIMESTAMP'2019-12-31 23:59:59.123456',0)` varchar(21) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1;
 CREATE OR REPLACE TABLE t1 (y YEAR);
@@ -3549,7 +3549,7 @@ FORMAT(f,0)
 SHOW CREATE TABLE t2;
 Table	Create Table
 t2	CREATE TABLE `t2` (
-  `FORMAT(f,0)` varchar(53) DEFAULT NULL
+  `FORMAT(f,0)` varchar(54) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1,t2;
 #

mysql-test/main/func_str.result

@@ -2708,7 +2708,7 @@ create table t1(a float);
 insert into t1 values (1.33);
 select format(a, 2) from t1;
 Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
-def					format(a, 2)	253	56	4	Y	0	39	8
+def					format(a, 2)	253	57	4	Y	0	39	8
 format(a, 2)
 1.33
 drop table t1;

mysql-test/main/type_newdecimal.result

@@ -2700,3 +2700,34 @@ ca	1
 LENGTH(FLOOR(a))	1
 LENGTH(CEILING(a))	1
 DROP PROCEDURE p1;
+#
+# MDEV-23118 FORMAT(d1,dec) where dec=0/38 and d1 is DECIMAL(38,38) gives incorrect results
+#
+CREATE OR REPLACE TABLE t1 (a DECIMAL(38,38));
+INSERT INTO t1 VALUES (-0.9999999999999999999999999999999999999), (0.9999999999999999999999999999999999999);
+SELECT a, FORMAT(a,0), FORMAT(a,38) FROM t1;
+a	-0.99999999999999999999999999999999999990
+FORMAT(a,0)	-1
+FORMAT(a,38)	-0.99999999999999999999999999999999999990
+a	0.99999999999999999999999999999999999990
+FORMAT(a,0)	1
+FORMAT(a,38)	0.99999999999999999999999999999999999990
+CREATE OR REPLACE TABLE t2 AS SELECT a, FORMAT(a,0), FORMAT(a,38) FROM t1;
+SELECT * FROM t2;
+a	-0.99999999999999999999999999999999999990
+FORMAT(a,0)	-1
+FORMAT(a,38)	-0.99999999999999999999999999999999999990
+a	0.99999999999999999999999999999999999990
+FORMAT(a,0)	1
+FORMAT(a,38)	0.99999999999999999999999999999999999990
+SHOW CREATE TABLE t2;
+Table	t2
+Create Table	CREATE TABLE `t2` (
+  `a` decimal(38,38) DEFAULT NULL,
+  `FORMAT(a,0)` varchar(2) DEFAULT NULL,
+  `FORMAT(a,38)` varchar(41) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t2,t1;
+#
+# End of 10.4 tests
+#

mysql-test/main/type_newdecimal.test

@@ -1919,3 +1919,22 @@ CALL p1(10,10,'  UNSIGNED');
 --horizontal_results
 
 DROP PROCEDURE p1;
+
+
+--echo #
+--echo # MDEV-23118 FORMAT(d1,dec) where dec=0/38 and d1 is DECIMAL(38,38) gives incorrect results
+--echo #
+
+--vertical_results
+CREATE OR REPLACE TABLE t1 (a DECIMAL(38,38));
+INSERT INTO t1 VALUES (-0.9999999999999999999999999999999999999), (0.9999999999999999999999999999999999999);
+SELECT a, FORMAT(a,0), FORMAT(a,38) FROM t1;
+CREATE OR REPLACE TABLE t2 AS SELECT a, FORMAT(a,0), FORMAT(a,38) FROM t1;
+SELECT * FROM t2;
+SHOW CREATE TABLE t2;
+DROP TABLE t2,t1;
+--horizontal_results
+
+--echo #
+--echo # End of 10.4 tests
+--echo #

sql/item_strfunc.cc

@@ -2661,18 +2661,42 @@ String *Item_func_soundex::val_str(String *str)
   This should be 'internationalized' sometimes.
 */
 
-const int FORMAT_MAX_DECIMALS= 30;
+/*
+  The maximum supported decimal scale:
+  38 - starting from 10.2.1
+  30 - before 10.2.1
+*/
+const int FORMAT_MAX_DECIMALS= 38;
 
 
 bool Item_func_format::fix_length_and_dec()
 {
   uint32 char_length= args[0]->type_handler()->Item_decimal_notation_int_digits(args[0]);
   uint dec= FORMAT_MAX_DECIMALS;
-  if (args[1]->const_item() && !args[1]->is_expensive() && !args[1]->null_value)
+  /*
+    Format can require one more integer digit if rounding happens:
+      FORMAT(9.9,0) -> '10'
+    Set need_extra_digit_for_rounding to true by default
+    if args[0] has some decimals: if args[1] is not
+    a constant, then format can potentially reduce
+    the number of decimals and round to the next integer.
+  */
+  bool need_extra_digit_for_rounding= args[0]->decimals > 0;
+  if (args[1]->const_item() && !args[1]->is_expensive())
   {
     Longlong_hybrid tmp= args[1]->to_longlong_hybrid();
-    dec= tmp.to_uint(FORMAT_MAX_DECIMALS);
+    if (!args[1]->null_value)
+    {
+      dec= tmp.to_uint(FORMAT_MAX_DECIMALS);
+      need_extra_digit_for_rounding= (dec < args[0]->decimals);
+    }
   }
+  /*
+    In case of a data type with zero integer digits, e.g. DECIMAL(4,4),
+    we'll print at least one integer digit.
+  */
+  if (need_extra_digit_for_rounding || !char_length)
+    char_length++;
   uint32 max_sep_count= (char_length / 3) + (dec ? 1 : 0) + /*sign*/1;
   collation.set(default_charset());
   fix_char_length(char_length + max_sep_count + dec);