MDEV-21619 Server crash or assertion failures in my_datetime_to_str

Item_cache_datetime::decimals was always copied from example->decimals
without limiting to 6 (maximum possible fractional digits), so
val_str() later crashed on asserts inside my_time_to_str() and
my_datetime_to_str().

mysql-test/r/type_date.result

@@ -914,3 +914,21 @@ DROP TABLE t1;
 #
 # End of 10.1 tests
 #
+#
+# Start of 10.2 tests
+#
+#
+# MDEV-21619 Server crash or assertion failures in my_datetime_to_str
+#
+CREATE  TABLE t1 (f DATE, KEY(f));
+INSERT INTO t1 VALUES ('2020-01-01'),('2020-01-02');
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE 1995.0000000 BETWEEN f AND '2012-12-12';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	index	f	f	4	NULL	2	50.00	Using where; Using index
+Warnings:
+Warning	1292	Incorrect datetime value: '1995.0000000'
+Note	1003	select `test`.`t1`.`f` AS `f` from `test`.`t1` where '0000-00-00' between `test`.`t1`.`f` and <cache>('2012-12-12')
+DROP TABLE t1;
+#
+# End of 10.2 tests
+#

mysql-test/r/type_datetime.result

@@ -1330,5 +1330,17 @@ a
 2000-10-00 00:00:00
 DROP TABLE t1;
 #
+# MDEV-21619 Server crash or assertion failures in my_datetime_to_str
+#
+CREATE  TABLE t1 (f DATETIME, KEY(f));
+INSERT INTO t1 VALUES ('2020-01-01 00:00:00'),('2020-01-02 00:00:00');
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE 1995.0000000 BETWEEN f AND '2012-12-12';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	index	f	f	6	NULL	2	50.00	Using where; Using index
+Warnings:
+Warning	1292	Incorrect datetime value: '1995.0000000'
+Note	1003	select `test`.`t1`.`f` AS `f` from `test`.`t1` where '0000-00-00 00:00:00.000000' between `test`.`t1`.`f` and <cache>('2012-12-12')
+DROP TABLE t1;
+#
 # End of 10.2 tests
 #

mysql-test/r/type_time.result

@@ -1356,5 +1356,17 @@ CAST(f1() AS TIME)
 00:00:00
 DROP FUNCTION  f1;
 #
+# MDEV-21619 Server crash or assertion failures in my_datetime_to_str
+#
+CREATE TABLE t1 (f TIME, KEY(f));
+INSERT INTO t1 VALUES ('10:10:10'),('20:20:20');
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE 1995.0000000 BETWEEN f AND '23:59:59';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	index	f	f	4	NULL	2	100.00	Using where; Using index
+Warnings:
+Warning	1292	Incorrect datetime value: '1995.0000000'
+Note	1003	select `test`.`t1`.`f` AS `f` from `test`.`t1` where '00:00:00.000000' between `test`.`t1`.`f` and <cache>('23:59:59')
+DROP TABLE t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/type_date.test

@@ -627,3 +627,21 @@ DROP TABLE t1;
 --echo #
 --echo # End of 10.1 tests
 --echo #
+
+--echo #
+--echo # Start of 10.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-21619 Server crash or assertion failures in my_datetime_to_str
+--echo #
+
+CREATE  TABLE t1 (f DATE, KEY(f));
+INSERT INTO t1 VALUES ('2020-01-01'),('2020-01-02');
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE 1995.0000000 BETWEEN f AND '2012-12-12';
+DROP TABLE t1;
+
+
+--echo #
+--echo # End of 10.2 tests
+--echo #

mysql-test/t/type_datetime.test

@@ -837,6 +837,15 @@ ALTER TABLE t1 MODIFY a DATETIME;
 SELECT * FROM t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-21619 Server crash or assertion failures in my_datetime_to_str
+--echo #
+
+CREATE  TABLE t1 (f DATETIME, KEY(f));
+INSERT INTO t1 VALUES ('2020-01-01 00:00:00'),('2020-01-02 00:00:00');
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE 1995.0000000 BETWEEN f AND '2012-12-12';
+DROP TABLE t1;
+
 
 --echo #
 --echo # End of 10.2 tests

mysql-test/t/type_time.test

@@ -814,6 +814,15 @@ SELECT CAST(f1() AS TIME);
 DROP FUNCTION  f1;
 
 
+--echo #
+--echo # MDEV-21619 Server crash or assertion failures in my_datetime_to_str
+--echo #
+
+CREATE TABLE t1 (f TIME, KEY(f));
+INSERT INTO t1 VALUES ('10:10:10'),('20:20:20');
+EXPLAIN EXTENDED SELECT * FROM t1 WHERE 1995.0000000 BETWEEN f AND '23:59:59';
+DROP TABLE t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/item.h

@@ -5803,6 +5803,13 @@ class Item_cache_temporal: public Item_cache_int
   bool cache_value();
   bool get_date(MYSQL_TIME *ltime, ulonglong fuzzydate);
   int save_in_field(Field *field, bool no_conversions);
+  bool setup(THD *thd, Item *item)
+  {
+    if (Item_cache_int::setup(thd, item))
+      return true;
+    set_if_smaller(decimals, TIME_SECOND_PART_DIGITS);
+    return false;
+  }
   Item_result cmp_type() const { return TIME_RESULT; }
   void store_packed(longlong val_arg, Item *example);
   /*