Bug#45152 crash with round() function on longtext column in a derived table

The crash happens due to wrong max_length value which is set on
Item_func_round::fix_length_and_dec() stage. The value is set to
args[0]->max_length which is too big in case of LONGTEXT(LONGBLOB) fields.
The fix is to set max_length using float_length() function.

mysql-test/r/func_math.result

@@ -390,4 +390,11 @@ a	ROUND(a)
 -1e+16	-10000000000000002
 1e+16	10000000000000002
 DROP TABLE t1;
+CREATE TABLE t1(f1 LONGTEXT) engine=myisam;
+INSERT INTO t1 VALUES ('a');
+SELECT 1 FROM (SELECT ROUND(f1) AS a FROM t1) AS s WHERE a LIKE 'a';
+1
+SELECT 1 FROM (SELECT ROUND(f1, f1) AS a FROM t1) AS s WHERE a LIKE 'a';
+1
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/func_math.test

@@ -250,4 +250,13 @@ SELECT a, ROUND(a) FROM t1;
 
 DROP TABLE t1;
 
+#
+# Bug#45152 crash with round() function on longtext column in a derived table
+#
+CREATE TABLE t1(f1 LONGTEXT) engine=myisam;
+INSERT INTO t1 VALUES ('a');
+SELECT 1 FROM (SELECT ROUND(f1) AS a FROM t1) AS s WHERE a LIKE 'a';
+SELECT 1 FROM (SELECT ROUND(f1, f1) AS a FROM t1) AS s WHERE a LIKE 'a';
+DROP TABLE t1;
+
 --echo End of 5.0 tests

sql/item_func.cc

@@ -1958,8 +1958,8 @@ void Item_func_round::fix_length_and_dec()
   unsigned_flag= args[0]->unsigned_flag;
   if (!args[1]->const_item())
   {
-    max_length= args[0]->max_length;
     decimals= args[0]->decimals;
+    max_length= float_length(decimals);
     if (args[0]->result_type() == DECIMAL_RESULT)
     {
       max_length++;
@@ -1979,8 +1979,8 @@ void Item_func_round::fix_length_and_dec()
 
   if (args[0]->decimals == NOT_FIXED_DEC)
   {
-    max_length= args[0]->max_length;
     decimals= min(decimals_to_set, NOT_FIXED_DEC);
+    max_length= float_length(decimals);
     hybrid_type= REAL_RESULT;
     return;
   }