Make sure shared buffer is copied on mutation

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 @@
 
   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
@@ -22,7 +40,7 @@
     assert_kind_of StringIO, StringIO.new
     assert_kind_of StringIO, StringIO.new('str')
     assert_kind_of StringIO, StringIO.new('str', 'r+')
     assert_kind_of StringIO, StringIO.new(nil)
     assert_raise(ArgumentError) { StringIO.new('', 'x') }
     assert_raise(ArgumentError) { StringIO.new('', 'rx') }
     assert_raise(ArgumentError) { StringIO.new('', 'rbt') }
@@ -42,7 +60,7 @@
   end
 
   def test_null
     io = StringIO.new(nil)
     assert_nil io.gets
     io.puts "abc"
     assert_nil io.string
@@ -55,7 +73,7 @@
     io.puts "def"
     assert_equal("\0\0\0\0def\n", io.string, "[ruby-dev:24190]")
     assert_raise(Errno::EINVAL) { io.truncate(-1) }
     assert_equal(0, io.truncate(10))
     assert_equal("\0\0\0\0def\n\0\0", io.string)
   end
 
@@ -280,7 +298,7 @@
     assert_equal("foo\x83".b, f.gets)
 
     f = StringIO.new()
     f.set_encoding("ISO-8859-16:ISO-8859-1")
     assert_equal(Encoding::ISO_8859_16, f.external_encoding)
     assert_equal(Encoding::ISO_8859_16, f.string.encoding)
     assert_nil(f.internal_encoding)
@@ -587,7 +605,7 @@
     russian_part = io.read(40).force_encoding('UTF-8')
 
     assert_equal("你好", chinese_part)
     assert_equal("построить", russian_part)
   end
 
   def test_gets2
@@ -706,7 +724,7 @@
     s = ""
     assert_equal(Encoding::UTF_8, s.encoding, bug20418)
     f.read(4, s)
     assert_equal(Encoding::UTF_8, s.encoding, bug20418)
 
     f.rewind
     s = ""
@@ -779,7 +797,7 @@
     assert_raise(ArgumentError) { f.pread(-1, 0) }
     assert_raise(Errno::EINVAL) { f.pread(3, -1) }
 
     assert_equal "".b, StringIO.new("").pread(0, 0)
     assert_equal "".b, StringIO.new("").pread(0, -10)
 
     buf = "stale".b
@@ -883,7 +901,7 @@
     s = StringIO.new
     s.print 'a' * count
     s.ungetbyte('b' * (count * 5))
     assert_equal((count * 5), s.string.size)
     assert_match(/\Ab+\z/, s.string)
   end
 
@@ -962,12 +980,12 @@
   def test_encoding_write
     s = StringIO.new("", "w:utf-32be")
     s.print "abc"
     assert_equal("abc".encode("utf-32be"), s.string)
   end
 
   def test_encoding_read
     s = StringIO.new("abc".encode("utf-32be"), "r:utf-8")
     assert_equal("\0\0\0a\0\0\0b\0\0\0c", s.read)
   end
 
   %w/UTF-8 UTF-16BE UTF-16LE UTF-32BE UTF-32LE/.each do |name|