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From "Ernie Allen" <eal...@redhat.com>
Subject Re: Review Request 34560: [python] receiver.fetch raises KeyError after network glitch
Date Thu, 04 Jun 2015 18:10:58 GMT

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(Updated June 4, 2015, 6:10 p.m.)

Review request for qpid, Alan Conway and Kenneth Giusti.


Until SessionManager::attach supports the force flag, we can't reconnect using the existing
session name. So in this special case (temporary network glitch after session is opened),
rename the sessions. 
This of course results in stranded sessions on the broker. Is there a way to close those when
we reconnect?

Repository: qpid


Calling receiver.fetch(timeout=10) in a loop, when network drops packages for a while causes
uncaught exception KeyError in python-qpid-0.22. It causes on semi-infinite recursion on python-qpid-0.30.

The recursion problem was solved independently.

The attached patch does two things:
1) session.close() checks to see if the session is already closed. If so, it just returns.
This prevents an exception from being displayed when the session is already closed.
2) In driver.py, if we get a do_session_detached() event, check to see if the channel is in
our list of sessions before using it. If it isn't, close the session.

Here is my estimation on what is happening when the network drops:
- The driver detects the socket error, closes the engine and goes into its retry loop.
- Once the network comes back, the engine is restarted and all the sessions on the connection
are re-attached.
- However, the broker sees the attempt to attach using a channel that it thinks is already
- The broker logs the following: 2015-05-21 14:51:35 [Broker] error Channel exception: session-busy:
Session already attached: anonymous.5c6f079c-571e-46f8-8ce6-72997da200a3:0 (/home/eallen/workspace/32/rh-qpid/qpid/cpp/src/qpid/broker/SessionManager.cpp:55)
2015-05-21 14:51:35 [Broker] error Channel exception: not-attached: Channel 0 is not attached
- This results in a do_session_detached() event in the engine.
- However, since the engine was closed when the socket error occurred and reopened when it
cleared, it doesn't know about the old session.

If I test to see if the channel number being detached is associated with a session, and just
return, then the client is hung. So.. when I see an event to detach an unknown session, I'm
closing the engine and raising a ConnectionError back to the client.

Ideally the driver/engine would recover, but I don't see how we can get the broker and client
back into agreement.

Diffs (updated)

  trunk/qpid/python/qpid/messaging/driver.py 1680941 
  trunk/qpid/python/qpid/messaging/endpoints.py 1680941 

Diff: https://reviews.apache.org/r/34560/diff/


1. Run this script against a qpidd broker:
#!/usr/bin/env python
from qpid.messaging import *
import datetime

conn = Connection("localhost:5672", reconnect=10)

  sess = conn.session()

  recv = sess.receiver("testQueue;{create:always}")
  while (1):
    print "%s: before fetch, timeout=%s" %(datetime.datetime.now(), timeout)
    msg = Message()
      msg = recv.fetch(timeout=timeout)
    except ReceiverError, e:
      print e
    except ConnectError, e:
      print "ConnectError", str(e)
    print "%s: after fetch, msg=%s"  (datetime.datetime.now(), msg)

  print "about to close session"

except ReceiverError, e:
  print e
except KeyboardInterrupt:

print "about to close connection"

2. Simulate network outage:
iptables -A OUTPUT -p tcp --dport 5672 -j REJECT; date

3. Once python script writes "No handlers could be found for logger "qpid.messaging"", flush
iptables (iptables -F)

4. Wait up to 10 seconds

The ConnectError is received by the client and the loop can be exited.


Ernie Allen

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