Fix for bug #41078: With CURSOR_TYPE_READ_ONLY mysql_stmt_fetch()

returns short string value. 
 
Multibyte character sets were not taken into account when 
calculating max_length in Item_param::convert_str_value(). As a 
result, string parameters of a prepared statement could be 
truncated later when calculating string length in characters by 
dividing length in bytes by the charset's mbmaxlen value (e.g. in 
Field_varstring::store()). 
 
Fixed by taking charset's mbmaxlen into account when calculating 
max_length in Item_param::convert_str_value().

sql/item.cc

@@ -3010,7 +3010,7 @@ bool Item_param::convert_str_value(THD *thd)
       str_value.set_charset(value.cs_info.final_character_set_of_str_value);
     /* Here str_value is guaranteed to be in final_character_set_of_str_value */
 
-    max_length= str_value.length();
+    max_length= str_value.numchars() * str_value.charset()->mbmaxlen;
     decimals= 0;
     /*
       str_value_ptr is returned from val_str(). It must be not alloced

tests/mysql_client_test.c

@@ -16410,6 +16410,69 @@ static void test_bug36326()
 
 #endif
 
+/**
+  Bug#41078: With CURSOR_TYPE_READ_ONLY mysql_stmt_fetch() returns short
+             string value.
+*/
+
+static void test_bug41078(void)
+{
+  uint         rc;
+  MYSQL_STMT   *stmt= 0;
+  MYSQL_BIND   param, result;
+  ulong        cursor_type= CURSOR_TYPE_READ_ONLY;
+  ulong        len;
+  char         str[64];
+  const char   param_str[]= "abcdefghijklmn";
+  my_bool      is_null, error;
+
+  DBUG_ENTER("test_bug41078");
+
+  rc= mysql_query(mysql, "SET NAMES UTF8");
+  myquery(rc);
+
+  stmt= mysql_simple_prepare(mysql, "SELECT ?");
+  check_stmt(stmt);
+  verify_param_count(stmt, 1);
+
+  rc= mysql_stmt_attr_set(stmt, STMT_ATTR_CURSOR_TYPE, &cursor_type);
+  check_execute(stmt, rc);
+  
+  bzero(&param, sizeof(param));
+  param.buffer_type= MYSQL_TYPE_STRING;
+  param.buffer= (void *) param_str;
+  len= sizeof(param_str) - 1;
+  param.length= &len;
+
+  rc= mysql_stmt_bind_param(stmt, &param);
+  check_execute(stmt, rc);
+
+  rc= mysql_stmt_execute(stmt);
+  check_execute(stmt, rc);
+
+  bzero(&result, sizeof(result));
+  result.buffer_type= MYSQL_TYPE_STRING;
+  result.buffer= str;
+  result.buffer_length= sizeof(str);
+  result.is_null= &is_null;
+  result.length= &len;
+  result.error=  &error;
+  
+  rc= mysql_stmt_bind_result(stmt, &result);
+  check_execute(stmt, rc);
+
+  rc= mysql_stmt_store_result(stmt);
+  check_execute(stmt, rc);
+
+  rc= mysql_stmt_fetch(stmt);
+  check_execute(stmt, rc);
+
+  DIE_UNLESS(len == sizeof(param_str) - 1 && !strcmp(str, param_str));
+
+  mysql_stmt_close(stmt);
+
+  DBUG_VOID_RETURN;
+}
 
 /*
   Read and parse arguments and MySQL options from my.cnf
@@ -16713,6 +16776,7 @@ static struct my_tests_st my_tests[]= {
 #ifdef HAVE_QUERY_CACHE
   { "test_bug36326", test_bug36326 },
 #endif
+  { "test_bug41078", test_bug41078 },
   { 0, 0 }
 };