Fix handling of frozens strings.

io_buffer.c

@@ -279,7 +279,12 @@ rb_io_buffer_type_for(VALUE klass, VALUE string)
 
     rb_str_locktmp(string);
 
-    io_buffer_initialize(data, RSTRING_PTR(string), RSTRING_LEN(string), RB_IO_BUFFER_EXTERNAL, string);
+    enum rb_io_buffer_flags flags = RB_IO_BUFFER_EXTERNAL;
+
+    if (RB_OBJ_FROZEN(string))
+        flags |= RB_IO_BUFFER_IMMUTABLE;
+
+    io_buffer_initialize(data, RSTRING_PTR(string), RSTRING_LEN(string), flags, string);
 
     return instance;
 }

test/ruby/test_io_buffer.rb

@@ -83,6 +83,7 @@ class TestIOBuffer < Test::Unit::TestCase
   def test_string_mapped
     string = "Hello World"
     buffer = IO::Buffer.for(string)
+    refute buffer.immutable?
 
     # Cannot modify string as it's locked by the buffer:
     assert_raise RuntimeError do
@@ -99,6 +100,13 @@ class TestIOBuffer < Test::Unit::TestCase
     assert_equal "Hello World", string
   end
 
+  def test_string_mapped_frozen
+    string = "Hello World".freeze
+    buffer = IO::Buffer.for(string)
+
+    assert buffer.immutable?
+  end
+
   def test_non_string
     not_string = Object.new