Narrow the scope of ensure

The ensure in postorder_traverse was added for [Bug #6756].
The intention was to try to delete the parent directory if it failed to
get the children. (It may be possible to delete the directory if it is
empty.)

However, the ensure region rescue'ed not only "failure to get children"
but also "failure to delete each child". Thus, the following raised
Errno::ENOTEMPTY, but we expect it to raise Errno::EACCES.

```
$ mkdir foo
$ touch foo/bar
$ chmod 555 foo
$ ruby -rfileutils -e 'FileUtils.rm_rf("foo")'
```

This changeset narrows the ensure region so that it rescues only
"failure to get children".

Author: mame

lib/fileutils.rb

@@ -2328,13 +2328,21 @@ def preorder_traverse
 
     def postorder_traverse
       if directory?
-        entries().each do |ent|
+        begin
+          children = entries()
+        rescue Errno::EACCES
+          # Failed to get the list of children.
+          # Assuming there is no children, try to process the parent directory.
+          yield self
+          return
+        end
+
+        children.each do |ent|
           ent.postorder_traverse do |e|
             yield e
           end
         end
       end
-    ensure
       yield self
     end
 

test/fileutils/test_fileutils.rb

@@ -750,6 +750,24 @@ def test_rm_r_pathname
     assert_file_not_exist 'tmp/tmpdir3'
   end
 
+  def test_rm_r_no_permissions
+    check_singleton :rm_rf
+
+    return if /mswin|mingw/ =~ RUBY_PLATFORM
+
+    mkdir 'tmpdatadir'
+    touch 'tmpdatadir/tmpdata'
+    chmod "-x", 'tmpdatadir'
+
+    begin
+      assert_raise Errno::EACCES do
+        rm_r 'tmpdatadir'
+      end
+    ensure
+      chmod "+x", 'tmpdatadir'
+    end
+  end
+
   def test_remove_entry_cjk_path
     dir = "tmpdir\u3042"
     my_rm_rf dir