A fix (Bug #4878: Service crashes on query execution)

mysql-test/r/func_str.result

@@ -673,3 +673,6 @@ c1	c2
 2147483647	4294967295
 -2147483648	0
 drop table t1;
+select left(1234, 3) + 0;
+left(1234, 3) + 0
+123

mysql-test/t/func_str.test

@@ -402,3 +402,9 @@ insert into t1 values ('-21474836461','-21474836461');
 show warnings;
 select * from t1;
 drop table t1;
+
+#
+# Bug #4878: LEFT() in integer/float context
+#
+
+select left(1234, 3) + 0;

sql/item_strfunc.cc

@@ -984,6 +984,9 @@ String *Item_func_left::val_str(String *str)
     return &my_empty_string;
   if (res->length() <= (uint) length)
     return res;
+  if (&str_value == res)
+    str_value.length(res->charpos(length));
+  else
     str_value.set(*res, 0, res->charpos(length));
   return &str_value;
 }

sql/sql_string.h

@@ -98,6 +98,7 @@ public:
 
   void set(String &str,uint32 offset,uint32 arg_length)
   {
+    DBUG_ASSERT(&str != this);
     free();
     Ptr=(char*) str.ptr()+offset; str_length=arg_length; alloced=0;
     if (str.Alloced_length)