MDEV-26765 UNIX_TIMESTAMP(CURRENT_TIME()) return null ?!?

Problem:

UNIX_TIMESTAMP() called for a expression of the TIME data type
returned NULL.

Inside Type_handler_timestamp_common::Item_val_native_with_conversion
the call for item->get_date() did not convert TIME to DATETIME
automatically (because it does not have to, by design).
As a result, Type_handler_timestamp_common::TIME_to_native() received
a MYSQL_TIME value with zero date 0000-00-00 and therefore returned "true"
(indicating SQL NULL value).

Fix:

Removing the call for item->get_date().
Instantiating Datetime(item) instead.
This forces automatic TIME to DATETIME conversion
(unless @@old_mode is zero_date_time_cast).

mysql-test/main/old-mode.result

@@ -221,3 +221,39 @@ a	UNIX_TIMESTAMP(t1.a)	a	UNIX_TIMESTAMP(t2.a)
 DROP TABLE t1;
 SET time_zone=DEFAULT;
 SET global mysql56_temporal_format=true;
+#
+# MDEV-26765 UNIX_TIMESTAMP(CURRENT_TIME()) return null ?!?
+#
+SET old_mode=zero_date_time_cast;
+SET @@time_zone='+00:00';
+SET timestamp=1234567;
+SELECT CURRENT_TIMESTAMP;
+CURRENT_TIMESTAMP
+1970-01-15 06:56:07
+SELECT UNIX_TIMESTAMP(CURRENT_TIME());
+UNIX_TIMESTAMP(CURRENT_TIME())
+NULL
+Warnings:
+Warning	1292	Truncated incorrect datetime value: '06:56:07'
+SELECT UNIX_TIMESTAMP(TIME'06:56:07');
+UNIX_TIMESTAMP(TIME'06:56:07')
+NULL
+Warnings:
+Warning	1292	Truncated incorrect datetime value: '06:56:07'
+SELECT UNIX_TIMESTAMP(TIME'10:20:30');
+UNIX_TIMESTAMP(TIME'10:20:30')
+NULL
+Warnings:
+Warning	1292	Truncated incorrect datetime value: '10:20:30'
+CREATE OR REPLACE TABLE t1 (a TIME);
+INSERT INTO t1 VALUES (TIME'06:56:07'),('10:20:30');
+SELECT UNIX_TIMESTAMP(a) FROM t1 ORDER BY a;
+UNIX_TIMESTAMP(a)
+NULL
+NULL
+Warnings:
+Warning	1264	Out of range value for column 'a' at row 1
+Warning	1264	Out of range value for column 'a' at row 2
+DROP TABLE t1;
+SET @@time_zone=DEFAULT;
+SET TIMESTAMP=DEFAULT;

mysql-test/main/old-mode.test

@@ -149,3 +149,23 @@ SELECT t1.a, UNIX_TIMESTAMP(t1.a), t2.a, UNIX_TIMESTAMP(t2.a) FROM t1 t1, t1 t2
 DROP TABLE t1;
 SET time_zone=DEFAULT;
 SET global mysql56_temporal_format=true;
+
+
+--echo #
+--echo # MDEV-26765 UNIX_TIMESTAMP(CURRENT_TIME()) return null ?!?
+--echo #
+
+SET old_mode=zero_date_time_cast;
+SET @@time_zone='+00:00';
+SET timestamp=1234567;
+SELECT CURRENT_TIMESTAMP;
+SELECT UNIX_TIMESTAMP(CURRENT_TIME());
+SELECT UNIX_TIMESTAMP(TIME'06:56:07');
+SELECT UNIX_TIMESTAMP(TIME'10:20:30');
+CREATE OR REPLACE TABLE t1 (a TIME);
+INSERT INTO t1 VALUES (TIME'06:56:07'),('10:20:30');
+SELECT UNIX_TIMESTAMP(a) FROM t1 ORDER BY a;
+DROP TABLE t1;
+
+SET @@time_zone=DEFAULT;
+SET TIMESTAMP=DEFAULT;

mysql-test/main/type_time.result

@@ -2420,5 +2420,31 @@ SET @@global.mysql56_temporal_format=default;
 DROP PROCEDURE p1;
 SET timestamp=DEFAULT;
 #
+# MDEV-26765 UNIX_TIMESTAMP(CURRENT_TIME()) return null ?!?
+#
+SET @@time_zone='+00:00';
+SET timestamp=1234567;
+SELECT CURRENT_TIMESTAMP;
+CURRENT_TIMESTAMP
+1970-01-15 06:56:07
+SELECT UNIX_TIMESTAMP(CURRENT_TIME());
+UNIX_TIMESTAMP(CURRENT_TIME())
+1234567
+SELECT UNIX_TIMESTAMP(TIME'06:56:07');
+UNIX_TIMESTAMP(TIME'06:56:07')
+1234567
+SELECT UNIX_TIMESTAMP(TIME'10:20:30');
+UNIX_TIMESTAMP(TIME'10:20:30')
+1246830
+CREATE OR REPLACE TABLE t1 (a TIME);
+INSERT INTO t1 VALUES (TIME'06:56:07'),('10:20:30');
+SELECT UNIX_TIMESTAMP(a) FROM t1 ORDER BY a;
+UNIX_TIMESTAMP(a)
+1234567
+1246830
+DROP TABLE t1;
+SET @@time_zone=DEFAULT;
+SET TIMESTAMP=DEFAULT;
+#
 # End of 10.4 tests
 #

mysql-test/main/type_time.test

@@ -1566,6 +1566,25 @@ SET @@global.mysql56_temporal_format=default;
 DROP PROCEDURE p1;
 SET timestamp=DEFAULT;
 
+--echo #
+--echo # MDEV-26765 UNIX_TIMESTAMP(CURRENT_TIME()) return null ?!?
+--echo #
+
+SET @@time_zone='+00:00';
+SET timestamp=1234567;
+SELECT CURRENT_TIMESTAMP;
+SELECT UNIX_TIMESTAMP(CURRENT_TIME());
+SELECT UNIX_TIMESTAMP(TIME'06:56:07');
+SELECT UNIX_TIMESTAMP(TIME'10:20:30');
+CREATE OR REPLACE TABLE t1 (a TIME);
+INSERT INTO t1 VALUES (TIME'06:56:07'),('10:20:30');
+SELECT UNIX_TIMESTAMP(a) FROM t1 ORDER BY a;
+DROP TABLE t1;
+
+SET @@time_zone=DEFAULT;
+SET TIMESTAMP=DEFAULT;
+
+
 --echo #
 --echo # End of 10.4 tests
 --echo #

sql/sql_type.cc

@@ -8619,13 +8619,13 @@ Type_handler_timestamp_common::Item_val_native_with_conversion(THD *thd,
                                                                Item *item,
                                                                Native *to) const
 {
-  MYSQL_TIME ltime;
   if (item->type_handler()->type_handler_for_native_format() ==
       &type_handler_timestamp2)
     return item->val_native(thd, to);
+  Datetime dt(thd, item, Datetime::Options(TIME_NO_ZERO_IN_DATE, thd));
   return
-    item->get_date(thd, &ltime, Datetime::Options(TIME_NO_ZERO_IN_DATE, thd)) ||
+    !dt.is_valid_datetime() ||
-    TIME_to_native(thd, &ltime, to, item->datetime_precision(thd));
+    TIME_to_native(thd, dt.get_mysql_time(), to, item->datetime_precision(thd));
 }
 
 bool Type_handler_null::union_element_finalize(Item_type_holder *item) const