Fix segmentation fault of StringScanner#charpos when String#byteslice returns non string value [Bug #17756] (#20)

Author: kachick

ext/strscan/strscan.c

@@ -445,13 +445,10 @@ static VALUE
 strscan_get_charpos(VALUE self)
 {
  struct strscanner *p;
- VALUE substr;
 
  GET_SCANNER(self, p);
 
- substr = rb_funcall(p->str, id_byteslice, 2, INT2FIX(0), LONG2NUM(p->curr));
-
- return rb_str_length(substr);
+ return LONG2NUM(rb_enc_strlen(S_PBEG(p), CURPTR(p), rb_enc_get(p->str)));
 }
 
 /*

test/strscan/test_stringscanner.rb

@@ -206,6 +206,23 @@ def test_pos_unicode
  assert_equal 11, s.charpos
  end
 
+ def test_charpos_not_use_string_methods
+ string = +'abcädeföghi'
+ scanner = create_string_scanner(string)
+
+ class << string
+ EnvUtil.suppress_warning do
+ undef_method(*instance_methods)
+ end
+ end
+
+ assert_equal 0, scanner.charpos
+ assert_equal "abcä", scanner.scan_until(/ä/)
+ assert_equal 4, scanner.charpos
+ assert_equal "defö", scanner.scan_until(/ö/)
+ assert_equal 8, scanner.charpos
+ end
+
  def test_concat
  s = create_string_scanner('a'.dup)
  s.scan(/a/)