A cleanup for MDEV-16939: avoid timeval initialization related problems in the...

A cleanup for MDEV-16939: avoid timeval initialization related problems in the future (compilation failures on Windows)

Adding a helper class Timeval, to initialize "struct timeval" in a safe way.
As a bonus, adding a  method Timeval::trunc(), so the caller now can have
one line instead of five lines (declaration, initializations of sec and usec,
truncation, passing to store_TIMEVAL()).

sql/field.cc

@@ -5059,12 +5059,7 @@ int Field_timestamp::store_TIME_with_warning(THD *thd, const Datetime *dt,
   }
 
   // Adjust and store the value
-  timeval tv;
-  tv.tv_sec= timestamp;
-  tv.tv_usec= l_time->second_part;
-
-  my_timeval_trunc(&tv, decimals());
-  store_TIMEVAL(tv);
+  store_TIMEVAL(Timeval(timestamp, l_time->second_part).trunc(decimals()));
 
   // Calculate return value and send warnings if needed
   if (unlikely(conversion_error)) // e.g. DATETIME in the DST gap

sql/field.h

@@ -2756,11 +2756,7 @@ class Field_timestamp :public Field_temporal {
   }
   void store_TIME(my_time_t timestamp, ulong sec_part)
   {
-    timeval tv;
-    tv.tv_sec= timestamp;
-    tv.tv_usec= sec_part;
-    my_timeval_trunc(&tv, decimals());
-    store_TIMEVAL(tv);
+    store_TIMEVAL(Timeval(timestamp, sec_part).trunc(decimals()));
   }
   bool get_date(MYSQL_TIME *ltime, ulonglong fuzzydate);
   uchar *pack(uchar *to, const uchar *from,

sql/structs.h

@@ -831,4 +831,20 @@ class Load_data_outvar
 };
 
 
+class Timeval: public timeval
+{
+public:
+  Timeval(my_time_t sec, ulong usec)
+  {
+    tv_sec= sec;
+    tv_usec= usec;
+  }
+  Timeval &trunc(uint dec)
+  {
+    my_timeval_trunc(this, dec);
+    return *this;
+  }
+};
+
+
 #endif /* STRUCTS_INCLUDED */