revert a suggested "optimization" that introduced a bug

compilation error in mysys/my_getsystime.c fixed
some redundant code removed
sec_to_time, time_to_sec, from_unixtime, unix_timestamp, @@timestamp now
  use decimal, not double for numbers with a fractional part.
purge_master_logs_before_date() fixed
many bugs in corner cases fixed

mysys/my_getsystime.c:
  compilation failure fixed
sql/sql_parse.cc:
  don't cut corners. it backfires.

1 files changed, 7 insertions, 3 deletions

diff --git a/sql/set_var.cc b/sql/set_var.cc
index 376364bab60..d66a72fcf2b 100644
--- a/sql/set_var.cc
+++ b/sql/set_var.cc
@@ -2715,15 +2715,19 @@ int set_var_collation_client::update(THD *thd)
 
 bool sys_var_timestamp::check(THD *thd, set_var *var)
 {
-  double val= var->value->val_real();
-  if (val < 0 || val > MY_TIME_T_MAX)
+  ulonglong sec;
+  ulong sec_part;
+  if (var->value->get_seconds(&sec, &sec_part))
+    return TRUE;
+
+  if (sec > TIMESTAMP_MAX_VALUE)
   {
     my_message(ER_UNKNOWN_ERROR, 
                "This version of MySQL doesn't support dates later than 2038",
                MYF(0));
     return TRUE;
   }
-  var->save_result.ulonglong_value= hrtime_from_time(var->value->val_real());
+  var->save_result.ulonglong_value= hrtime_from_time(sec)+sec_part;
   return FALSE;
 }