fixed substring() length calculation in case of constant negative argument (BUG#10269)

mysql-test/r/func_str.result:
  Correct length reporting from substring()
mysql-test/t/func_str.test:
  Correct length reporting from substring()
sql/item_strfunc.cc:
  fixed substring() length calculation in case of constant negative argument

mysql-test/r/func_str.result

@@ -800,3 +800,18 @@ SELECT * FROM t1, t2 WHERE num=substring(str from 1 for 6);
 str	num
 notnumber	0
 DROP TABLE t1,t2;
+create table t1 (b varchar(5));
+insert t1 values ('ab'), ('abc'), ('abcd'), ('abcde');
+select *,substring(b,1),substring(b,-1),substring(b,-2),substring(b,-3),substring(b,-4),substring(b,-5) from t1;
+b	substring(b,1)	substring(b,-1)	substring(b,-2)	substring(b,-3)	substring(b,-4)	substring(b,-5)
+ab	ab	b	ab			
+abc	abc	c	bc	abc		
+abcd	abcd	d	cd	bcd	abcd	
+abcde	abcde	e	de	cde	bcde	abcde
+select * from (select *,substring(b,1),substring(b,-1),substring(b,-2),substring(b,-3),substring(b,-4),substring(b,-5) from t1) t;
+b	substring(b,1)	substring(b,-1)	substring(b,-2)	substring(b,-3)	substring(b,-4)	substring(b,-5)
+ab	ab	b	ab			
+abc	abc	c	bc	abc		
+abcd	abcd	d	cd	bcd	abcd	
+abcde	abcde	e	de	cde	bcde	abcde
+drop table t1;

mysql-test/t/func_str.test

@@ -541,3 +541,12 @@ SELECT * FROM t1, t2 WHERE num=str;
 SELECT * FROM t1, t2 WHERE num=substring(str from 1 for 6);
 
 DROP TABLE t1,t2;
+
+#
+# Correct length reporting from substring() (BUG#10269)
+#
+create table t1 (b varchar(5));
+insert t1 values ('ab'), ('abc'), ('abcd'), ('abcde');
+select *,substring(b,1),substring(b,-1),substring(b,-2),substring(b,-3),substring(b,-4),substring(b,-5) from t1;
+select * from (select *,substring(b,1),substring(b,-1),substring(b,-2),substring(b,-3),substring(b,-4),substring(b,-5) from t1) t;
+drop table t1;

sql/item_strfunc.cc

@@ -1065,7 +1065,8 @@ void Item_func_substr::fix_length_and_dec()
   collation.set(args[0]->collation);
   if (args[1]->const_item())
   {
-    int32 start=(int32) args[1]->val_int()-1;
+    int32 start= (int32) args[1]->val_int();
+    start= (int32)((start < 0) ? max_length + start : start - 1);
     if (start < 0 || start >= (int32) max_length)
       max_length=0; /* purecov: inspected */
     else