Don't log defect warnings on multipart/* responses

httplib expects the response body to be available to the HTTP header parser but that's not the case for `parse_headers()` so the following defects are always added to multipart/* responses. We simply ignore these defects at this stage as they are a known issue.

src/urllib3/util/response.py

@@ -1,4 +1,5 @@
 from __future__ import absolute_import
+from email.errors import StartBoundaryNotFoundDefect, MultipartInvariantViolationDefect
 from ..packages.six.moves import http_client as httplib
 
 from ..exceptions import HeaderParsingError
@@ -66,6 +67,25 @@ def assert_header_parsing(headers):
 
             if isinstance(payload, (bytes, str)):
                 unparsed_data = payload
+    if defects:
+        # httplib is assuming a response body is available
+        # when parsing headers even when httplib only sends
+        # header data to parse_headers() This results in
+        # defects on multipart responses in particular.
+        # See: https://github.com/urllib3/urllib3/issues/800
+
+        # So we ignore the following defects:
+        # - StartBoundaryNotFoundDefect:
+        #     The claimed start boundary was never found.
+        # - MultipartInvariantViolationDefect:
+        #     A message claimed to be a multipart but no subparts were found.
+        defects = [
+            defect
+            for defect in defects
+            if not isinstance(
+                defect, (StartBoundaryNotFoundDefect, MultipartInvariantViolationDefect)
+            )
+        ]
 
     if defects or unparsed_data:
         raise HeaderParsingError(defects=defects, unparsed_data=unparsed_data)

test/test_util.py

@@ -748,6 +748,20 @@ def test_assert_header_parsing_throws_typeerror_with_non_headers(self, headers):
         with pytest.raises(TypeError):
             assert_header_parsing(headers)
 
+    @onlyPy3
+    def test_assert_header_parsing_no_error_on_multipart(self):
+        from http import client
+
+        header_msg = io.BytesIO()
+        header_msg.write(
+            b'Content-Type: multipart/encrypted;protocol="application/'
+            b'HTTP-SPNEGO-session-encrypted";boundary="Encrypted Boundary"'
+            b"\nServer: Microsoft-HTTPAPI/2.0\nDate: Fri, 16 Aug 2019 19:28:01 GMT"
+            b"\nContent-Length: 1895\n\n\n"
+        )
+        header_msg.seek(0)
+        assert_header_parsing(client.parse_headers(header_msg))
+
 
 class TestUtilSSL(object):
     """Test utils that use an SSL backend."""

test/with_dummyserver/test_socketlevel.py

@@ -1808,3 +1808,40 @@ def socket_handler(listener):
         ) as pool:
             pool.urlopen("GET", "/not_found", preload_content=False)
             assert pool.num_connections == 1
+
+
+class TestMultipartResponse(SocketDummyServerTestCase):
+    def test_multipart_assert_header_parsing_no_defects(self):
+        def socket_handler(listener):
+            for _ in range(2):
+                sock = listener.accept()[0]
+                while not sock.recv(65536).endswith(b"\r\n\r\n"):
+                    pass
+
+                sock.sendall(
+                    b"HTTP/1.1 404 Not Found\r\n"
+                    b"Server: example.com\r\n"
+                    b"Content-Type: multipart/mixed; boundary=36eeb8c4e26d842a\r\n"
+                    b"Content-Length: 73\r\n"
+                    b"\r\n"
+                    b"--36eeb8c4e26d842a\r\n"
+                    b"Content-Type: text/plain\r\n"
+                    b"\r\n"
+                    b"1\r\n"
+                    b"--36eeb8c4e26d842a--\r\n",
+                )
+                sock.close()
+
+        self._start_server(socket_handler)
+        from urllib3.connectionpool import log
+
+        with mock.patch.object(log, "warning") as log_warning:
+            with HTTPConnectionPool(self.host, self.port, timeout=3) as pool:
+                resp = pool.urlopen("GET", "/")
+                assert resp.status == 404
+                assert (
+                    resp.headers["content-type"]
+                    == "multipart/mixed; boundary=36eeb8c4e26d842a"
+                )
+                assert len(resp.data) == 73
+                log_warning.assert_not_called()