When iterating over a range of a file, always close it

This is needed on Pythons without reference counting garbage collectors (e.g.
PyPy).

Change-Id: I1d06eb8fe08ee6eeb45caa47b653d6af0bb18267

swift/obj/diskfile.py

@@ -274,6 +274,7 @@ class DiskFile(object):
             length = stop - start
         else:
             length = None
+        try:
             for chunk in self:
                 if length is not None:
                     length -= len(chunk)
@@ -282,6 +283,9 @@ class DiskFile(object):
                         yield chunk[:length]
                         break
                 yield chunk
+        finally:
+            if not self.suppress_file_closing:
+                self.close()
 
     def app_iter_ranges(self, ranges, content_type, boundary, size):
         """Returns an iterator over the data file for a set of ranges"""

test/unit/obj/test_diskfile.py

@@ -71,6 +71,12 @@ class TestDiskFile(unittest.TestCase):
                                     FakeLogger(), keep_data_fp=True)
         self.assertEqual(''.join(df.app_iter_range(5, None)), '67890')
 
+    def test_disk_file_app_iter_partial_closes(self):
+        df = self._create_test_file('1234567890')
+        it = df.app_iter_range(0, 5)
+        self.assertEqual(''.join(it), '12345')
+        self.assertEqual(df.fp, None)
+
     def test_disk_file_app_iter_ranges(self):
         df = self._create_test_file('012345678911234567892123456789')
         it = df.app_iter_ranges([(0, 10), (10, 20), (20, 30)], 'plain/text',