LogDog: Properly emit Butler stream magic.

client/libs/logdog/tests/stream_test.py

 #!/usr/bin/env python
 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import json
 import os
 import sys
 import unittest
 import StringIO
@@ skipping 66 matching lines @@
     def write(self, v):
       self._assert_not_closed()
       self.buffer.write(v)
 
     def close(self):
       self._assert_not_closed()
       self.closed = True
 
     def interpret(self):
       data = StringIO.StringIO(self.buffer.getvalue())
+      magic = data.read(len(stream.BUTLER_MAGIC))
+      if magic != stream.BUTLER_MAGIC:
+        raise ValueError('Invalid magic value ([%s] != [%s])' % (
+            magic, stream.BUTLER_MAGIC))
       length, _ = varint.read_uvarint(data)
       header = data.read(length)
       return json.loads(header), data.read()
 
   class _TestStreamClient(stream.StreamClient):
     def __init__(self, value):
       super(StreamClientTestCase._TestStreamClient, self).__init__()
       self.value = value
       self.last_conn = None
 
@@ skipping 98 matching lines @@
     conn = client.last_conn
     self.assertTrue(conn.closed)
 
     header, data = conn.interpret()
     self.assertEqual(header, {'name': 'mystream', 'type': 'text'})
     self.assertEqual(data, 'Using a text stream.')
 
 
 if __name__ == '__main__':
   unittest.main()