Bug#19955: unsigned bigint used as signed with MOD function

Problem:  When we have a really large number (between 2^63 and 2^64)
as the left side of the mod operator, it gets improperly corerced
into a signed value.

Solution:  Added check to see if the "negative" number is really
positive, and if so, cast it.

mysql-test/r/bigint.result

@@ -341,3 +341,14 @@ select * from t1 where bigint_col='17666000000000000000';
 bigint_col
 17666000000000000000
 drop table t1;
+
+bug 19955 -- mod is signed with bigint
+select cast(10000002383263201056 as unsigned) mod 50 as result;
+result
+6
+create table t1 (c1 bigint unsigned);
+insert into t1 values (10000002383263201056);
+select c1 mod 50 as result from t1;
+result
+6
+drop table t1;

mysql-test/t/bigint.test

@@ -278,4 +278,13 @@ select * from t1 where bigint_col=17666000000000000000;
 select * from t1 where bigint_col='17666000000000000000';
 drop table t1;
 
+--echo
+--echo bug 19955 -- mod is signed with bigint
+
+select cast(10000002383263201056 as unsigned) mod 50 as result;
+
+create table t1 (c1 bigint unsigned);
+insert into t1 values (10000002383263201056);
+select c1 mod 50 as result from t1;
+drop table t1;
 

sql/item_func.cc

@@ -1326,6 +1326,10 @@ longlong Item_func_mod::int_op()
     signal_divide_by_null();
     return 0;
   }
+
+  if (args[0]->unsigned_flag)
+    return ((ulonglong) value) % val2;
+
   return value % val2;
 }