Fix suffix-byte-range responses for zero-byte replicated objects.

Previously, given a GET request like "Range: bytes=-12345" for a
zero-byte object, Swift would return a 206 response with the header
"Content-Range: bytes 0--1/0". This is clearly incorrect. Now Swift
returns a 200 with the whole (zero-byte) object.

Note: this does not fix the bug for EC objects, only for replicated
ones. The fix for EC objects will follow in a separate commit.

Change-Id: If1edb665b0ae000da78c4efff6faddd94d75da6b
Partial-Bug: 1736840

swift/common/swob.py

@@ -1256,6 +1256,20 @@ class Response(object):
                 # body text from RESPONSE_REASONS.
                 body = None
                 app_iter = None
+            elif self.content_length == 0:
+                # If ranges_for_length found ranges but our content length
+                # is 0, then that means we got a suffix-byte-range request
+                # (e.g. "bytes=-512"). This is asking for *up to* the last N
+                # bytes of the file. If we had any bytes to send at all,
+                # we'd return a 206 with an apropriate Content-Range header,
+                # but we can't construct a Content-Range header because we
+                # have no byte indices because we have no bytes.
+                #
+                # The only reasonable thing to do is to return a 200 with
+                # the whole object (all zero bytes of it). This is also what
+                # Apache and Nginx do, so if we're wrong, at least we're in
+                # good company.
+                pass
             elif ranges:
                 range_size = len(ranges)
                 if range_size > 0:

test/unit/obj/test_server.py

@@ -2918,6 +2918,22 @@ class TestObjectController(unittest.TestCase):
         self.assertEqual(resp.headers['X-Backend-Timestamp'],
                          utils.Timestamp(timestamp).internal)
 
+    def test_GET_range_zero_byte_object(self):
+        timestamp = normalize_timestamp(time())
+        req = Request.blank('/sda1/p/a/c/zero-byte',
+                            environ={'REQUEST_METHOD': 'PUT'},
+                            headers={'X-Timestamp': timestamp,
+                                     'Content-Type': 'application/x-test'})
+        req.body = ''
+        resp = req.get_response(self.object_controller)
+        self.assertEqual(resp.status_int, 201)
+
+        req = Request.blank('/sda1/p/a/c/zero-byte',
+                            environ={'REQUEST_METHOD': 'GET'},
+                            headers={'Range': 'bytes=-10'})
+        resp = req.get_response(self.object_controller)
+        self.assertEqual(resp.status_int, 200)
+
     def test_GET_if_match(self):
         req = Request.blank('/sda1/p/a/c/o', environ={'REQUEST_METHOD': 'PUT'},
                             headers={