Fix for bug #7515 "from_unixtime(0) now returns NULL instead of

the Epoch". (With after review fixes).

mysql-test/r/func_time.result

@@ -470,9 +470,12 @@ unix_timestamp(@a)
 select unix_timestamp('1969-12-01 19:00:01');
 unix_timestamp('1969-12-01 19:00:01')
 0
-select from_unixtime(0);
-from_unixtime(0)
+select from_unixtime(-1);
+from_unixtime(-1)
 NULL
 select from_unixtime(2145916800);
 from_unixtime(2145916800)
 NULL
+select from_unixtime(0);
+from_unixtime(0)
+1970-01-01 03:00:00

mysql-test/t/func_time.test

@@ -228,7 +228,11 @@ select unix_timestamp('1969-12-01 19:00:01');
 
 #
 # Test for bug #6439 "unix_timestamp() function returns wrong datetime 
-# values for too big argument". It should return error instead.
+# values for too big argument" and bug #7515 "from_unixtime(0) now
+# returns NULL instead of the epoch". unix_timestamp() should return error
+# for too big or negative argument. It should return Epoch value for zero
+# argument since it seems that many user's rely on this fact.
 #
-select from_unixtime(0);
+select from_unixtime(-1);
 select from_unixtime(2145916800);
+select from_unixtime(0);

sql/item_timefunc.cc

@@ -946,10 +946,12 @@ bool Item_func_from_unixtime::get_date(TIME *ltime,
 {
   struct tm tm_tmp;
   time_t tmp;
-  longlong arg= args[0]->val_int();
-  if ((null_value= (args[0]->null_value ||
-                    arg < TIMESTAMP_MIN_VALUE ||
-                    arg > TIMESTAMP_MAX_VALUE)))
+  ulonglong arg= (ulonglong)(args[0]->val_int());
+  /*
+    "arg > TIMESTAMP_MAX_VALUE" check also covers case of negative
+    from_unixtime() argument since arg is unsigned.
+  */
+  if ((null_value= (args[0]->null_value || arg > TIMESTAMP_MAX_VALUE)))
     return 1;
   tmp= arg;
   localtime_r(&tmp,&tm_tmp);

sql/item_timefunc.h

@@ -359,8 +359,7 @@ class Item_func_from_unixtime :public Item_date_func
   longlong val_int();
   String *val_str(String *str);
   const char *func_name() const { return "from_unixtime"; }
-  void fix_length_and_dec() { decimals=0; max_length=19; }
-//  enum Item_result result_type () const { return STRING_RESULT; }
+  void fix_length_and_dec() { decimals=0; max_length=19; maybe_null= 1; }
   bool get_date(TIME *res,bool fuzzy_date);
 };