MDEV-34069 Zero datetime reinterprets as '1970-01-01 00:00:00' on field_datetime=field_timestamp

The code in Field_timestamp::save_in_field() did not catch
zero datetime and stored it to the other field like a usual value
using store_timestamp_dec(), which knows nothing about zero date and
treats {tv_sec=0, tv_usec=0} as a normal timeval value corresponding to
'1970-01-01 00:00:00 +00:00'.

Fixing the code to catch the special combination (ts==0 && sec_pat==0) and
store it using store_time_dec() with a zero datetime passed as an argument.

mysql-test/main/type_timestamp.result

@@ -1368,5 +1368,35 @@ t1	CREATE TABLE `t1` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
 drop table t1;
 #
+# MDEV-34069 Zero datetime reinterprets as '1970-01-01 00:00:00' on field_datetime=field_timestamp
+#
+SET sql_mode='';
+SET time_zone='+00:00';
+CREATE TABLE t1 (a TIMESTAMP);
+INSERT INTO t1 VALUES ('0000-00-00 00:00:00');
+SELECT * FROM t1;
+a
+0000-00-00 00:00:00
+CREATE TABLE t2 (a DATETIME);
+INSERT INTO t2 SELECT a FROM t1;
+SELECT * FROM t2;
+a
+0000-00-00 00:00:00
+SET sql_mode='NO_ZERO_DATE,NO_ZERO_IN_DATE';
+INSERT INTO t2 VALUES ('0000-00-00 00:00:00');
+Warnings:
+Warning	1264	Out of range value for column 'a' at row 1
+INSERT INTO t2 SELECT a FROM t1;
+Warnings:
+Warning	1264	Out of range value for column 'a' at row 1
+SELECT * FROM t2;
+a
+0000-00-00 00:00:00
+0000-00-00 00:00:00
+0000-00-00 00:00:00
+DROP TABLE t2, t1;
+SET time_zone=DEFAULT;
+SET sql_mode=DEFAULT;
+#
 # End of 10.5 tests
 #

mysql-test/main/type_timestamp.test

@@ -920,6 +920,26 @@ create table t1 (f1 timestamp, f2 timestamp);
 show create table t1;
 drop table t1;
 
+--echo #
+--echo # MDEV-34069 Zero datetime reinterprets as '1970-01-01 00:00:00' on field_datetime=field_timestamp
+--echo #
+
+SET sql_mode='';
+SET time_zone='+00:00';
+CREATE TABLE t1 (a TIMESTAMP);
+INSERT INTO t1 VALUES ('0000-00-00 00:00:00');
+SELECT * FROM t1;
+CREATE TABLE t2 (a DATETIME);
+INSERT INTO t2 SELECT a FROM t1;
+SELECT * FROM t2;
+SET sql_mode='NO_ZERO_DATE,NO_ZERO_IN_DATE';
+INSERT INTO t2 VALUES ('0000-00-00 00:00:00');
+INSERT INTO t2 SELECT a FROM t1;
+SELECT * FROM t2;
+DROP TABLE t2, t1;
+SET time_zone=DEFAULT;
+SET sql_mode=DEFAULT;
+
 --echo #
 --echo # End of 10.5 tests
 --echo #

sql/field.cc

@@ -5184,6 +5184,8 @@ int Field_timestamp::save_in_field(Field *to)
 {
   ulong sec_part;
   my_time_t ts= get_timestamp(&sec_part);
+  if (!ts && !sec_part)
+    return to->store_time_dec(Datetime::zero().get_mysql_time(), decimals());
   return to->store_timestamp_dec(Timeval(ts, sec_part), decimals());
 }