MDEV-7254: Assigned expression is evaluated twice when updating

column TIMESTAMP NOT NULL
      
Analysis: Problem was that value->is_null() function is called
even when user had explicitly set the value for timestamp
field. Calling this function had the side effect that
expression was evaluated twice.
      
Fix: (by Sergei Golubchik) check instead value->null_value.

mysql-test/r/type_timestamp.result

@@ -645,3 +645,47 @@ MAX(dt) = '2011-01-06 12:34:30'
 1
 DROP TABLE t1;
 End of 5.5 tests
+#
+# MDEV-7254: Assigned expression is evaluated twice when updating column TIMESTAMP NOT NULL
+#
+create table t1(value timestamp not null);
+set @a:=0;
+create function f1 () returns timestamp
+begin
+set @a = @a + 1;
+return NULL;
+end//
+set timestamp=12340;
+insert t1 values (f1());
+select @a, value from t1;
+@a	value
+1	1970-01-01 05:25:40
+set timestamp=12350;
+update t1 set value = f1();
+select @a, value from t1;
+@a	value
+2	1970-01-01 05:25:50
+drop table t1;
+drop function f1;
+set timestamp=0;
+create table t1(value timestamp null);
+set @a:=0;
+create function f1 () returns timestamp
+begin
+set @a = @a + 1;
+return NULL;
+end//
+set timestamp=12340;
+insert t1 values (f1());
+select @a, value from t1;
+@a	value
+1	NULL
+set timestamp=12350;
+update t1 set value = f1();
+select @a, value from t1;
+@a	value
+2	NULL
+drop table t1;
+drop function f1;
+set timestamp=0;
+End of 10.0 tests

mysql-test/t/type_timestamp.test

@@ -446,3 +446,48 @@ SELECT MAX(dt) = '2011-01-06 12:34:30' FROM t1;
 DROP TABLE t1;
 
 --echo End of 5.5 tests
+
+--echo #
+--echo # MDEV-7254: Assigned expression is evaluated twice when updating column TIMESTAMP NOT NULL
+--echo #
+
+create table t1(value timestamp not null);
+set @a:=0;
+delimiter //;
+create function f1 () returns timestamp
+begin
+  set @a = @a + 1;
+  return NULL;
+end//
+delimiter ;//
+set timestamp=12340;
+insert t1 values (f1());
+select @a, value from t1;
+set timestamp=12350;
+update t1 set value = f1();
+select @a, value from t1;
+drop table t1;
+drop function f1;
+set timestamp=0;
+
+# Verify no regressions to TIMESTAMP NULL
+create table t1(value timestamp null);
+set @a:=0;
+delimiter //;
+create function f1 () returns timestamp
+begin
+  set @a = @a + 1;
+  return NULL;
+end//
+delimiter ;//
+set timestamp=12340;
+insert t1 values (f1());
+select @a, value from t1;
+set timestamp=12350;
+update t1 set value = f1();
+select @a, value from t1;
+drop table t1;
+drop function f1;
+set timestamp=0;
+
+--echo End of 10.0 tests

sql/field.cc

@@ -4899,7 +4899,7 @@ void Field_timestamp::set_explicit_default(Item *value)
 {
   if (((value->type() == Item::DEFAULT_VALUE_ITEM &&
         !((Item_default_value*)value)->arg) ||
-       (!maybe_null() && value->is_null())))
+       (!maybe_null() && value->null_value)))
     return;
   set_has_explicit_value();
 }