Bug#35720 ucs2 + pad_char_to_full_length = failure

Problem: with @@sql_mode=pad_char_to_full_length
a CHAR column returned additional garbage
after trailing space characters due to
incorrect my_charpos() call.
Fix: call my_charpos() with correct arguments.

mysql-test/r/ctype_ucs.result

@@ -1098,6 +1098,17 @@ ERROR HY000: Illegal mix of collations (ascii_general_ci,IMPLICIT) and (ucs2_gen
 select * from t1 where a=if(b<10,_ucs2 0x0062,_ucs2 0x00C0);
 ERROR HY000: Illegal mix of collations (ascii_general_ci,IMPLICIT) and (ucs2_general_ci,COERCIBLE) for operation '='
 drop table t1;
+CREATE TABLE t1 (s1 CHAR(5) CHARACTER SET UCS2);
+INSERT INTO t1 VALUES ('a');
+SET @@sql_mode=pad_char_to_full_length;
+SELECT HEX(s1) FROM t1;
+HEX(s1)
+00610020002000200020
+SET @@sql_mode=default;
+SELECT HEX(s1) FROM t1;
+HEX(s1)
+0061
+DROP TABLE t1;
 set collation_connection=ucs2_general_ci;
 drop table if exists t1;
 create table t1 as

mysql-test/t/ctype_ucs.test

@@ -678,6 +678,17 @@ select * from t1 where a=if(b<10,_ucs2 0x00C0,_ucs2 0x0062);
 select * from t1 where a=if(b<10,_ucs2 0x0062,_ucs2 0x00C0);
 drop table t1;
 
+#
+# Bug#35720 ucs2 + pad_char_to_full_length = failure
+#
+CREATE TABLE t1 (s1 CHAR(5) CHARACTER SET UCS2);
+INSERT INTO t1 VALUES ('a');
+SET @@sql_mode=pad_char_to_full_length;
+SELECT HEX(s1) FROM t1;
+SET @@sql_mode=default;
+SELECT HEX(s1) FROM t1;
+DROP TABLE t1;
+
 set collation_connection=ucs2_general_ci;
 --source include/ctype_regex.inc
 set names latin1;

sql/field.cc

@@ -6610,7 +6610,8 @@ String *Field_string::val_str(String *val_buffer __attribute__((unused)),
   uint length;
   if (table->in_use->variables.sql_mode &
       MODE_PAD_CHAR_TO_FULL_LENGTH)
-    length= my_charpos(field_charset, ptr, ptr + field_length, field_length);
+    length= my_charpos(field_charset, ptr, ptr + field_length,
+                       field_length / field_charset->mbmaxlen);
   else
     length= field_charset->cset->lengthsp(field_charset, (const char*) ptr,
                                           field_length);