Dir.chdir can now be called without a block, and another error state

in chdir is taken care of. Specifically, where chdir into a
non-existent directory could put the filesystem into a broken state.

lib/fakefs.rb

@@ -224,14 +224,15 @@ module FakeFS
     end
 
     def chdir(dir, &blk)
-      raise "you must pass in a block" unless blk
-
       new_dir = find(dir)
-      raise Errno::ENOENT unless new_dir
-      dir_levels.push dir
-      blk.call
+      dir_levels.push dir if blk
+
+      raise Errno::ENOENT, dir unless new_dir
+
+      dir_levels.push dir if !blk
+      blk.call if blk
     ensure
-      dir_levels.pop
+      dir_levels.pop if blk
     end
 
     def path_parts(path)
@@ -246,6 +247,10 @@ module FakeFS
         File.expand_path(File.join(*parts))
       end
     end
+
+    def current_dir
+      find(normalize_path('.'))
+    end
   end
 
   class MockFile

test/fakefs_test.rb

@@ -165,10 +165,6 @@ class FakeFSTest < Test::Unit::TestCase
     end
 
     assert_equal 'foo', c
-
-    assert_raise(RuntimeError, 'no block chdir is not yet supported') do
-      Dir.chdir('/path')
-    end
   end
 
   def test_chdir_shouldnt_keep_us_from_absolute_paths
@@ -226,9 +222,34 @@ class FakeFSTest < Test::Unit::TestCase
     rescue Exception # hardcore
     end
 
+    Dir.chdir('/path') do
+      begin
+        Dir.chdir('nope'){ }
+      rescue Errno::ENOENT
+      end
+
+      assert_equal ['/path'], FileSystem.dir_levels
+    end
+
     assert_equal(['foo', 'foobar'], FileSystem.fs['path'].keys.sort)
   end
 
+  def test_chdir_with_no_block_is_awesome
+    FileUtils.mkdir_p '/path'
+    Dir.chdir('/path')
+    FileUtils.mkdir_p 'subdir'
+    assert_equal ['subdir'], FileSystem.current_dir.keys
+    Dir.chdir('subdir')
+    File.open('foo', 'w'){|f| f.write 'foo'}
+    assert_equal ['foo'], FileSystem.current_dir.keys
+
+    assert_raises(Errno::ENOENT) do
+      Dir.chdir('subsubdir')
+    end
+
+    assert_equal ['foo'], FileSystem.current_dir.keys
+  end
+
   def test_file_open_defaults_to_read
     File.open('foo','w'){|f| f.write 'bar' }
     assert_equal 'bar', File.open('foo'){|f| f.read }