python gevent websocket client getting error "DecodeError: Truncated message" for echoed back protocol buffer message

Question

I have simple websocket server, which echoes all the message back to the client.

import gevent
from geventwebsocket.resource import WebSocketApplication
from geventwebsocket.server import WebSocketServer
from geventwebsocket.resource import Resource
import ams_pb2

class AMSWebSocketServer(WebSocketApplication):
    def __init__(self, ws):
        super(AMSWebSocketServer, self).__init__(ws)
        pass

    def on_open(self):
        pass

    def on_message(self, message):
        print 'received message'
        print message
        if message is None:
            print 'message none'
            return

        print 'echo message back'
        self.ws.send(message)

    def on_close(self, reason):
        print "connection closed"
        gevent.sleep(0)


resource = Resource({'/': AMSWebSocketServer})

The server is spawned using gunicorn command

gunicorn -k "geventwebsocket.gunicorn.workers.GeventWebSocketWorker" -b 127.0.0.1:9000 gunicorn_test:resource

I have a test client, which sends the websocket message to be echoed back

from ws4py.client.threadedclient import WebSocketClient
import ams_pb2


class DummySwitch(WebSocketClient):
    def closed(self, code, reason=None):
        pass

    def received_message(self, msg):
        if msg is None:
            print 'none'
            return
        print 'received message'
        ams_message = ams_pb2.AMSConfig()
        ams_message.ParseFromString(msg)
        print ams_message
        print msg


if __name__ == '__main__':
    end_point = 'ws://127.0.0.1:9000'

    client = DummySwitch(
        end_point,
        headers=[
        ]
    )
    client.connect()

    print 'sending message'

    AMSConfig = ams_pb2.AMSConfig()
    AMSConfig.CliConfig = True
    print AMSConfig
    msg = AMSConfig.SerializeToString()

    #msg = 'Hello'
    print msg

    client.send(msg)

    client.run_forever()

My protobuff file is : package ams;

message AMSConfig {
  optional bool CliConfig = 1;
}

Whenever my client send a protobuff message to the server, i am able to see it getting parsed in the server, but when the server echoes back the same message to the client, the client fails due to:

File "client_test.py", line 15, in received_message ams_message.ParseFromString(msg) File "/usr/lib/python2.6/site-packages/google/protobuf/message.py", line 186, in ParseFromString self.MergeFromString(serialized) File "/usr/lib/python2.6/site-packages/google/protobuf/internal/python_message.py", line 847, in MergeFromString raise message_mod.DecodeError('Truncated message.') DecodeError: Truncated message.

So, i modified the code to send a simple string and i see that the 'Hello' string being sent to the server is being echoed back and client is able to print the message. But, the client fails to parse protobuff message echoed back.

I am unable to understand why the echo back for a string works but for a protocol buffer it doesn't work in my example.

Thanks for help.

The message content is getting corrupted somehow. Perhaps you could print out the base64 of the message right before you send it, and also the base64 of the message you get back, and compare? It may be clearer then what is going wrong. One possibility is that the content is being interpreted as text/unicode somewhere, which would lead to corruption since it's actually binary data. — Kenton Varda
the issue is on the client side, i need to convert the object received to str. I was missing isinstance(msg, str) before using msg, which lead to the issue of client code not able to parse the code. — Anjanish

lulyon lulyon · Accepted Answer · 2016-02-19T05:20:17

I encounter same problem on client side, but this might be solved by enlarge receiving buffer size in client side code. For example:

maxpkglen = 1024 * 1024 // original maxpkglen = 1024
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.sendto(req, (server, port))
rsp, server = s.recvfrom(maxpkglen)
s.close()

python gevent websocket client getting error "DecodeError: Truncated message" for echoed back protocol buffer message

3 Answers