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xmlrpclib hangs execution

P: n/a
Perhaps I should be a bit more specific. When using this code to connect
to a remote XML-RPC server (C++, xmlrpc++0.7 library):

import xmlrpclib
server = xmlrpclib.Server("http://10.10.101.62:29500")
print server.Connection_Request("roberto")

the Python command line 'hangs' until I kill the server. Then, the
correct output is suddenly displayed:

{'state': 0, 'id_session': '2Z3EUSLJFA13', 'port': 29501,
'str': 'Connection accepted. Session attached.'}

Yet a slightly simpler call works flawlessly:

import xmlrpclib
server = xmlrpclib.Server("http://10.10.101.62:29500")
print server.Info_RT('Master')

{'state': 0, 'str': 'Info_RT'}

Looking at the TCP packages (using Wireshark), the first
Connection_Request flow as follows (c=python client,s= xml-rpcserver):

1 c>s SYN
2 s>c SYN,ACK
3 c>s ACK
4 c>s PSH,ACK (setting content-type: text/xml)
5 s>c ACK
6 c>s PSH,ACK (xml-rpc request)
7 s>c ACK
8 s>c PSH,ACK (xml-rpc results, correct)
9 c>s ACK

At this point, Python 'hangs' until I kill (-9) the server (28 seconds
later), and then these last packages are send:

10 s>c FIN,ACK
11 c>s FIN,ACk
12 s>c ACK

After which Python continues and prints the xml-rpc results on the
screen. Strangely, before Python hangs, it has already received the
package with the results, but doesn't do anything with it.

For comparison, this is the package flow for the Info_RT method, which
returns within a second with the correct result on the Python command line:

1 c>s SYN
2 s>c SYN,ACK
3 c>s ACK
4 c>s PSH,ACK (setting content-type: text/xml)
5 s>c ACK
6 c>s PSH,ACK (xml-rpc request)
7 s>c ACK
8 s>c PSH,ACK (xml-rpc results, correct)
9 s>c FIN,ACK
10 c>s ACK
11 c>s FIN,ACk
12 s>c ACK

Notice the reverse order of packages 9 and 10. Could this be it? Is
Python waiting until the server sends a FIN package? But if so, why?

Looking at the Python trace, this might be it. A diff between the trace
of the killed Connection_Request and the Info_RT resulted only in
expected differences (due to different length of parameters). But when
diffing the traces of the killed and non-killed Connection_Request,
something else is visible. The killed Connection_Request ends with:

[...]
string:318: return sep.join(words)
xmlrpclib:866: self._type = "params"
xmlrpclib:769: try:
xmlrpclib:770: f = self.dispatch[tag]
xmlrpclib:771: except KeyError:
xmlrpclib:772: pass # unknown tag ?
xmlrpclib:1273: if sock:
xmlrpclib:1274: response = sock.recv(1024)
xmlrpclib:1277: if not response:
xmlrpclib:1278: break
xmlrpclib:1283: file.close()
socket:218: try:
socket:219: if self._sock:
socket:220: self.flush()
socket:232: if self._wbuf:
socket:222: self._sock = None
xmlrpclib:1284: p.close()
xmlrpclib:530: self._parser.Parse("", 1) # end of data
xmlrpclib:531: del self._target, self._parser # get rid of
circular references
xmlrpclib:1286: return u.close()
xmlrpclib:741: if self._type is None or self._marks:
xmlrpclib:743: if self._type == "fault":
xmlrpclib:745: return tuple(self._stack)
socket:225: try:
socket:226: self.close()
socket:218: try:
socket:219: if self._sock:
socket:222: self._sock = None
xmlrpclib:1386: if len(response) == 1:
xmlrpclib:1387: response = response[0]
xmlrpclib:1389: return response
__main__:26: print result
{'state': 0, 'id_session': 'QDLY3GP9FBM5', 'port': 29501, 'str':
'Connection accepted. Session attached.'}

But the non-killed Connection_Request looks like:

[...]
return sep.join(words)
xmlrpclib:866: self._type = "params"
xmlrpclib:769: try:
xmlrpclib:770: f = self.dispatch[tag]
xmlrpclib:771: except KeyError:
xmlrpclib:772: pass # unknown tag ?
xmlrpclib:1273: if sock:
xmlrpclib:1274: response = sock.recv(1024)
socket:225: try:
socket:226: self.close()
socket:218: try:
socket:219: if self._sock:
socket:220: self.flush()
socket:232: if self._wbuf:
socket:222: self._sock = None

(In the diff there is a gap between the xmlrpclib:1274 and socket:225
lines, as compared to the killed Connection_Request; perhaps Ctrl+C
still executes the "socket try close" lines?)

If you want to do your own 'diff' on the Python traces, or look at the
TCP packages in more details with Wireshark, here are the files for the
correctly working Info_RT and faulty Connection_Request methods:

http://ctw-bw008.ctw.utwente.nl/~arn...fort-python.ws
http://ctw-bw008.ctw.utwente.nl/~arn...-python-ws.txt
http://ctw-bw008.ctw.utwente.nl/~arn...fort-trace.txt

http://ctw-bw008.ctw.utwente.nl/~arn...nreq-python.ws
http://ctw-bw008.ctw.utwente.nl/~arn...-python-ws.txt
http://ctw-bw008.ctw.utwente.nl/~arn...nreq-trace.txt
http://ctw-bw008.ctw.utwente.nl/~arn...-hardbreak.txt

(The first three ConReq files all include killing the server after ~28
seconds, thus resulting in the correct output. In the fourth files,
tracing is stopped before the server is killed. It is therefor the trace
up to the 'hanging point'. Note that the first two Wireshark packages
are not TCP, but ARP, thus the file has a total of 14 packages, not 12
as above.)

Really, any help here is appreciated!

Kind regards,
Arno.

Arno Stienen wrote:
Hello all,

It has been a while, but I finally found the time to further investigate
my problems with connecting Python to a RTAI-XML server. As I cannot
tell if the problems I'm facing are caused by RTAI-XML (or more
precisely, the xmlrpc++0.7 library) or Python (xmlrpclib), I'm posting
this message on both the RTAI as the Python mailing list.

Just to recap, here are the symptoms I described earlier.

I am using two computers. One is running the RTAI-patched real-time
linux kernel, and this computer controls my robotic hardware. On this
RTAI computer I installed the RTAI-XML server, which uses incoming
XML-RPC calls to control real-time processes on the server. RTAI-XML is
programmed in C++, using the xmlrpc++0.7 library.

The second is a desktop machine (Windows XP or Ubuntu), from which a
XML-RPC connection can be made to the RTAI computer. Although in theory
it should make no difference if I use C, C++, Java or Python to make the
connection, in practise I can get the Java connection running, but have
problems using Python. And I really like to use Python to quickly create
interfaces!

When connecting with Python on the desktop computer to the RTAI-XML
server running on my RTAI machine, I used the following code.

import xmlrpclib
server = xmlrpclib.Server("http://10.10.101.62:29500")
print server.Info_RT('Master')

This results in the correct output:

{'state': 0, 'str': 'Info_RT'}

Yet, this is only an information call. The really interesting part
should comes when I ask the server to open a connection:

import xmlrpclib
server = xmlrpclib.Server("http://10.10.101.62:29500")
print server.Connection_Request("roberto")

But then nothing happens. Python just hangs and is unresponsive. Until,
I kill the RTAI-XML server on the RTAI computer! And then suddenly the
correct output appears on the desktop command line:

{'state': 0, 'id_session': '2Z3EUSLJFA13', 'port': 29501,
'str': 'Connection accepted. Session attached.'}

(Using an alternative Python XML-RPC library '#import pyxmlrpclib as
xmlrpclib' makes no difference to the result.)

So what happens? I did some experiments. I did some package sniffing
with Wireshark and I did Python tracing. I stored the sniffing results
in both the Wireshark as plain text format. The files for the first
Info_RT examples are here:

http://ctw-bw008.ctw.utwente.nl/~arn...fort-python.ws
http://ctw-bw008.ctw.utwente.nl/~arn...-python-ws.txt
http://ctw-bw008.ctw.utwente.nl/~arn...fort-trace.txt

The files for the second, not correctly working example are here:

http://ctw-bw008.ctw.utwente.nl/~arn...nreq-python.ws
http://ctw-bw008.ctw.utwente.nl/~arn...-python-ws.txt
http://ctw-bw008.ctw.utwente.nl/~arn...nreq-trace.txt
http://ctw-bw008.ctw.utwente.nl/~arn...-hardbreak.txt

The first three files all include killing the server after ~30 seconds,
thus resulting in the correct output. In the fourth files, tracing is
stopped before the server is killed. It is therefor the trace up to the
'hanging point'.

Any, really any, suggestions are appreciated!

Kind regards,
Arno Stienen.
May 28 '07 #1
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