Remove taint support

Ruby 2.7 deprecates taint and it no longer has an effect. The lack of taint support should not cause a problem in previous Ruby versions.

Author: jeremyevans

ext/stringio/stringio.c

@@ -623,7 +623,6 @@ strio_copy(VALUE copy, VALUE orig)
 strio_free(DATA_PTR(copy));
  }
  DATA_PTR(copy) = ptr;
- OBJ_INFECT(copy, orig);
  RBASIC(copy)->flags &= ~STRIO_READWRITE;
  RBASIC(copy)->flags |= RBASIC(orig)->flags & STRIO_READWRITE;
  ++ptr->count;
@@ -1436,7 +1435,6 @@ strio_write(VALUE self, VALUE str)
  if (ptr->pos == olen) {
 if (enc == ascii8bit || enc2 == ascii8bit) {
  rb_enc_str_buf_cat(ptr->string, RSTRING_PTR(str), len, enc);
- OBJ_INFECT(ptr->string, str);
 }
 else {
  rb_str_buf_append(ptr->string, str);
@@ -1445,9 +1443,7 @@ strio_write(VALUE self, VALUE str)
  else {
 strio_extend(ptr, ptr->pos, len);
 memmove(RSTRING_PTR(ptr->string)+ptr->pos, RSTRING_PTR(str), len);
-OBJ_INFECT(ptr->string, str);
  }
- OBJ_INFECT(ptr->string, self);
  RB_GC_GUARD(str);
  ptr->pos += len;
  return len;

test/stringio/test_stringio.rb

@@ -180,17 +180,6 @@ def o.to_s; "baz"; end
  f.close unless f.closed?
  end
 
- def test_write_infection
- bug9769 = '[ruby-dev:48118] [Bug #9769]'
- s = "".untaint
- f = StringIO.new(s, "w")
- f.print("bar".taint)
- f.close
- assert_predicate(s, :tainted?, bug9769)
- ensure
- f.close unless f.closed?
- end
-
  def test_write_encoding
  s = "".force_encoding(Encoding::UTF_8)
  f = StringIO.new(s)