MDEV-13868 cannot insert 1288481126 in a timestamp column in Europe/Moscow

make insert NULL into a timestamp mark the field as having an
explicit value. So that the field won't be assigned the value
again in TABLE::update_default_field()

make Item_func_now_local::save_in_field(timestamp_field) not to go
through MYSQL_TIME - this conversion is lossy around DST change times.
This fixes inserting a default value into a timestamp field.

mysql-test/r/old-mode.result

@@ -132,21 +132,29 @@ set global mysql56_temporal_format=false;
 create table t1 (a timestamp);
 set timestamp=1288477526;
 insert t1 values (null);
+insert t1 values ();
 set timestamp=1288481126;
 insert t1 values (null);
+insert t1 values ();
 select a, unix_timestamp(a) from t1;
 a	unix_timestamp(a)
 2010-10-31 02:25:26	1288477526
+2010-10-31 02:25:26	1288477526
+2010-10-31 02:25:26	1288481126
 2010-10-31 02:25:26	1288481126
 set global mysql56_temporal_format=true;
 select a, unix_timestamp(a) from t1;
 a	unix_timestamp(a)
 2010-10-31 02:25:26	1288477526
+2010-10-31 02:25:26	1288477526
+2010-10-31 02:25:26	1288481126
 2010-10-31 02:25:26	1288481126
 alter table t1 modify a timestamp;
 select a, unix_timestamp(a) from t1;
 a	unix_timestamp(a)
 2010-10-31 02:25:26	1288477526
+2010-10-31 02:25:26	1288477526
+2010-10-31 02:25:26	1288481126
 2010-10-31 02:25:26	1288481126
 drop table t1;
 set global mysql56_temporal_format=false;

mysql-test/t/old-mode.test

@@ -93,8 +93,10 @@ set global mysql56_temporal_format=false;
 create table t1 (a timestamp);
 set timestamp=1288477526;
 insert t1 values (null);
+insert t1 values ();
 set timestamp=1288481126;
 insert t1 values (null);
+insert t1 values ();
 select a, unix_timestamp(a) from t1;
 set global mysql56_temporal_format=true;
 select a, unix_timestamp(a) from t1;

sql/field.cc

@@ -5260,36 +5260,6 @@ int Field_timestamp::set_time()
   return 0;
 }
 
-/**
-  Mark the field as having an explicit default value.
-
-  @param value  if available, the value that the field is being set to
-  @returns whether the explicit default bit was set
-
-  @note
-    Fields that have an explicit default value should not be updated
-    automatically via the DEFAULT or ON UPDATE functions. The functions
-    that deal with data change functionality (INSERT/UPDATE/LOAD),
-    determine if there is an explicit value for each field before performing
-    the data change, and call this method to mark the field.
-
-    For timestamp columns, the only case where a column is not marked
-    as been given a value are:
-    - It's explicitly assigned with DEFAULT
-    - We assign NULL to a timestamp field that is defined as NOT NULL.
-      This is how MySQL has worked since it's start.
-*/
-
-bool Field_timestamp::set_explicit_default(Item *value)
-{
-  if (((value->type() == Item::DEFAULT_VALUE_ITEM &&
-        !((Item_default_value*)value)->arg) ||
-       (!maybe_null() && value->null_value)))
-    return false;
-  set_has_explicit_value();
-  return true;
-}
-
 #ifdef NOT_USED
 static void store_native(ulonglong num, uchar *to, uint bytes)
 {

sql/field.h

@@ -975,7 +975,7 @@ public:
   {
     return bitmap_is_set(&table->has_value_set, field_index);
   }
-  virtual bool set_explicit_default(Item *value);
+  bool set_explicit_default(Item *value);
 
   /**
      Evaluates the @c UPDATE default function, if one exists, and stores the
@@ -2403,7 +2403,6 @@ public:
   void sql_type(String &str) const;
   bool zero_pack() const { return 0; }
   int set_time();
-  bool set_explicit_default(Item *value);
   int evaluate_update_default_function()
   {
     int res= 0;

sql/item_timefunc.cc

@@ -1734,6 +1734,25 @@ void Item_func_now::print(String *str, enum_query_type query_type)
   str->append(')');
 }
 
+
+int Item_func_now_local::save_in_field(Field *field, bool no_conversions)
+{
+  if (field->type() == MYSQL_TYPE_TIMESTAMP)
+  {
+    THD *thd= field->get_thd();
+    my_time_t ts= thd->query_start();
+    uint dec= MY_MIN(decimals, field->decimals());
+    ulong sec_part= dec ? thd->query_start_sec_part() : 0;
+    sec_part-= my_time_fraction_remainder(sec_part, dec);
+    field->set_notnull();
+    ((Field_timestamp*)field)->store_TIME(ts, sec_part);
+    return 0;
+  }
+  else
+    return Item_temporal_func::save_in_field(field, no_conversions);
+}
+
+
 /**
     Converts current time in my_time_t to MYSQL_TIME represenatation for local
     time zone. Defines time zone (local) used for whole NOW function.

sql/item_timefunc.h

@@ -748,6 +748,7 @@ class Item_func_now_local :public Item_func_now
 public:
   Item_func_now_local(THD *thd, uint dec): Item_func_now(thd, dec) {}
   const char *func_name() const { return "current_timestamp"; }
+  int save_in_field(Field *field, bool no_conversions);
   virtual void store_now_in_TIME(THD *thd, MYSQL_TIME *now_time);
   virtual enum Functype functype() const { return NOW_FUNC; }
   Item *get_copy(THD *thd, MEM_ROOT *mem_root)