string.c: ensure String#freeze resizes internal buffer

rb_str_freeze may resize oversized buffers to save memory, so favor
it over rb_obj_freeze.  This is useful because IO methods do not
prematurely shrink buffers, as they are likely to be overwritten
with full data.

* string.c (Init_String): use rb_str_freeze for String#freeze

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51170 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_optimization.rb

@@ -1,4 +1,5 @@
 require 'test/unit'
+require 'objspace'
 
 class TestRubyOptimization < Test::Unit::TestCase
 
@@ -122,6 +123,24 @@ class TestRubyOptimization < Test::Unit::TestCase
     assert_redefine_method('String', 'freeze', 'assert_nil "foo".freeze')
   end
 
+  def test_string_freeze_saves_memory
+    n = 16384
+    data = '.'.freeze
+    r, w = IO.pipe
+    w.write data
+
+    s = r.readpartial(n, '')
+    assert_operator ObjectSpace.memsize_of(s), :>=, n,
+      'IO buffer NOT resized prematurely because will likely be reused'
+
+    s.freeze
+    assert_equal ObjectSpace.memsize_of(data), ObjectSpace.memsize_of(s),
+      'buffer resized on freeze since it cannot be written to again'
+  ensure
+    r.close if r
+    w.close if w
+  end
+
   def test_string_eq_neq
     %w(== !=).each do |m|
       assert_redefine_method('String', m, <<-end)