read_frame python3 compatible for large payloads (#248)

read_frame is using str.join to concatenate the payload if the received
frame is bigger than SIGNED_INT_MAX.

That's fine with python2, however in python3 documentation is stated

str.joinReturn a string which is the concatenation of the strings in the
iterable iterable. A TypeError will be raised if there are any
non-string values in iterable, including bytes objects. The separator
between elements is the string providing this method.

So we have to use the byte object join() method

Signed-off-by: aojeagarcia <aojeagarcia@suse.com>

amqp/transport.py

@@ -255,7 +255,7 @@ def read_frame(self, unpack=unpack):
             if size > SIGNED_INT_MAX:
                 part1 = read(SIGNED_INT_MAX)
                 part2 = read(size - SIGNED_INT_MAX)
-                payload = ''.join([part1, part2])
+                payload = b''.join([part1, part2])
             else:
                 payload = read(size)
             read_frame_buffer += payload

t/unit/test_transport.py

@@ -11,6 +11,8 @@
 from amqp.platform import pack
 from amqp.transport import _AbstractTransport
 
+SIGNED_INT_MAX = 0x7FFFFFFF
+
 
 class DummyException(Exception):
     pass
@@ -319,6 +321,15 @@ def on_read1(size, *args):
         with pytest.raises(UnexpectedFrame):
             self.t.read_frame()
 
+    def test_read_frame__long(self):
+        self.t._read = Mock()
+        self.t._read.side_effect = [pack('>BHI', 1, 1, SIGNED_INT_MAX + 16),
+                                    b'read1', b'read2', b'\xce']
+        frame_type, channel, payload = self.t.read_frame()
+        assert frame_type == 1
+        assert channel == 1
+        assert payload == b'read1read2'
+
     def transport_read_EOF(self):
         for host, ssl in (('localhost:5672', False),
                           ('localhost:5671', True),):