Finish download on seeing 416 response with zero byte range. (#86)

Closes [google-cloud python #6572](googleapis/google-cloud-python#6572).

google/resumable_media/_download.py

@@ -28,6 +28,7 @@
     flags=re.IGNORECASE)
 _ACCEPTABLE_STATUS_CODES = (http_client.OK, http_client.PARTIAL_CONTENT)
 _GET = u'GET'
+_ZERO_CONTENT_RANGE_HEADER = u'bytes */0'
 
 
 class DownloadBase(object):
@@ -340,6 +341,11 @@ def _process_response(self, response):
         .. _sans-I/O: https://sans-io.readthedocs.io/
         """
         # Verify the response before updating the current instance.
+        if _check_for_zero_content_range(response, self._get_status_code,
+                                         self._get_headers):
+            self._finished = True
+            return
+
         _helpers.require_status_code(
             response, _ACCEPTABLE_STATUS_CODES,
             self._get_status_code, callback=self._make_invalid)
@@ -478,3 +484,28 @@ def get_range_info(response, get_headers, callback=_helpers.do_nothing):
         int(match.group(u'end_byte')),
         int(match.group(u'total_bytes'))
     )
+
+
+def _check_for_zero_content_range(response, get_status_code, get_headers):
+    """ Validate if response status code is 416 and content range is zero.
+
+    This is the special case for handling zero bytes files.
+
+    Args:
+        response (object): An HTTP response object.
+        get_status_code (Callable[Any, int]): Helper to get a status code
+            from a response.
+        get_headers (Callable[Any, Mapping[str, str]]): Helper to get headers
+            from an HTTP response.
+
+    Returns:
+        bool: True if content range total bytes is zero, false otherwise.
+    """
+    if get_status_code(response) == http_client. \
+            REQUESTED_RANGE_NOT_SATISFIABLE:
+        content_range = _helpers.header_required(
+                response, _helpers.CONTENT_RANGE_HEADER,
+                get_headers, callback=_helpers.do_nothing)
+        if content_range == _ZERO_CONTENT_RANGE_HEADER:
+            return True
+    return False

tests/unit/test__download.py

@@ -559,6 +559,25 @@ def test__process_response_when_reaching_end(self):
         assert download.total_bytes == 8 * chunk_size
         assert stream.getvalue() == data
 
+    def test__process_response_when_content_range_is_zero(self):
+        chunk_size = 10
+        stream = mock.Mock(spec=[u'write'])
+        download = _download.ChunkedDownload(
+            EXAMPLE_URL, chunk_size, stream)
+        _fix_up_virtual(download)
+
+        content_range = _download._ZERO_CONTENT_RANGE_HEADER
+        headers = {u'content-range': content_range}
+        status_code = http_client.REQUESTED_RANGE_NOT_SATISFIABLE
+        response = mock.Mock(headers=headers,
+                             status_code=status_code,
+                             spec=[u'headers', 'status_code'])
+        download._process_response(response)
+        stream.write.assert_not_called()
+        assert download.finished
+        assert download.bytes_downloaded == 0
+        assert download.total_bytes is None
+
     def test_consume_next_chunk(self):
         download = _download.ChunkedDownload(EXAMPLE_URL, 256, None)
         with pytest.raises(NotImplementedError) as exc_info:
@@ -662,6 +681,37 @@ def test_missing_header_with_callback(self):
         callback.assert_called_once_with()
 
 
+class Test__check_for_zero_content_range(object):
+
+    @staticmethod
+    def _make_response(content_range, status_code):
+        headers = {u'content-range': content_range}
+        return mock.Mock(headers=headers,
+                         status_code=status_code,
+                         spec=[u'headers', 'status_code'])
+
+    def test_status_code_416_and_test_content_range_zero_both(self):
+        content_range = _download._ZERO_CONTENT_RANGE_HEADER
+        status_code = http_client.REQUESTED_RANGE_NOT_SATISFIABLE
+        response = self._make_response(content_range, status_code)
+        assert _download._check_for_zero_content_range(
+            response, _get_status_code, _get_headers)
+
+    def test_status_code_416_only(self):
+        content_range = u'bytes 2-5/3'
+        status_code = http_client.REQUESTED_RANGE_NOT_SATISFIABLE
+        response = self._make_response(content_range, status_code)
+        assert not _download._check_for_zero_content_range(
+            response, _get_status_code, _get_headers)
+
+    def test_content_range_zero_only(self):
+        content_range = _download._ZERO_CONTENT_RANGE_HEADER
+        status_code = http_client.OK
+        response = self._make_response(content_range, status_code)
+        assert not _download._check_for_zero_content_range(
+            response, _get_status_code, _get_headers)
+
+
 def _get_status_code(response):
     return response.status_code