Make sure shared buffer is copied on mutation (#117)

We need to ensure shared buffers are made independent on mutation.
Otherwise we could end up mutating unrelated string buffers.

---------

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>
Co-authored-by: Alan Wu <XrXr@users.noreply.github.com>
Co-authored-by: Aaron Patterson <aaron.patterson@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Author: 4 people

ext/stringio/stringio.c

@@ -180,6 +180,7 @@ check_modifiable(struct StringIO *ptr)
     else if (OBJ_FROZEN_RAW(ptr->string)) {
 	rb_raise(rb_eIOError, "not modifiable string");
     }
+    rb_str_modify(ptr->string);
 }
 
 static VALUE

test/stringio/test_stringio.rb

@@ -14,6 +14,24 @@ def open_file(content)
 
   include TestEOF::Seek
 
+  def test_do_not_mutate_shared_buffers
+    # Ensure we have two strings that are not embedded but have the same shared
+    # string reference.
+    #
+    # In this case, we must use eval because we need two strings literals that
+    # are long enough they cannot be embedded, but also contain the same bytes.
+
+    a = eval(("x" * 1024).dump)
+    b = eval(("x" * 1024).dump)
+
+    s = StringIO.new(b)
+    s.getc
+    s.ungetc '#'
+
+    # We mutated b, so a should not be mutated
+    assert_equal("x", a[0])
+  end
+
   def test_version
     assert_kind_of(String, StringIO::VERSION)
   end