Error when using twisted and greenlets

Question

I'm trying to use twisted with greenlets, so I can write synchronous looking code in twisted without using inlineCallbacks.

Here is my code:

import time, functools
from twisted.internet import reactor, threads
from twisted.internet.defer import Deferred
from functools import wraps
import greenlet

def make_async(func):
    @wraps(func)
    def wrapper(*pos, **kwds):
        d = Deferred()

        def greenlet_func():
            try:
                rc = func(*pos, **kwds)
                d.callback(rc)
            except Exception, ex:
                print ex
                d.errback(ex)

        g = greenlet.greenlet(greenlet_func)
        g.switch()

        return d
    return wrapper

def sleep(t):
    print "sleep(): greenelet:", greenlet.getcurrent()
    g = greenlet.getcurrent()
    reactor.callLater(t, g.switch)
    g.parent.switch()

def wait_one(d):
    print "wait_one(): greenelet:", greenlet.getcurrent()
    g = greenlet.getcurrent()
    active = True

    def callback(result):
        if not active:
            g.switch(result)
        else:
            reactor.callLater(0, g.switch, result)

    def errback(failure):
        if not active:
            g.throw(failure)
        else:
            reactor.callLater(0, g.throw, failure)

    d.addCallback(callback)
    d.addErrback(errback)

    active = False
    rc = g.parent.switch()
    return rc

@make_async
def inner():
    print "inner(): greenelet:", greenlet.getcurrent()

    import random, time
    interval = random.random()

    print "Sleeping for %s seconds..." % interval
    sleep(interval)
    print "done"

    return interval

@make_async
def outer():
    print "outer(): greenelet:", greenlet.getcurrent()
    print wait_one(inner())
    print "Here"

reactor.callLater(0, outer)
reactor.run()

There are 5 main parts:

A sleep function, that starts a timer, then switches back to the parent greenlet. When the timer goes off, it switches back to the greenlet that is sleeping.
A make_async decorator. This takes some synchronous looking code and runs it in a greenlet. IT also returns a deferred so the caller can register callbacks when the code completes.
A wait_one function, which blocks the greenlet until the deferred being waited on resolves.
The inner function, which (when wrapped) returns a deferred, sleeps for a random time, and then passes the time it slept for to the deferred.
The outer function, which calls inner(), waits for it to return, then prints the return value.

When I run this code I get this output (Note the error on the last two lines):

outer(): greenelet: <greenlet.greenlet object at 0xb729cc5c>
inner(): greenelet: <greenlet.greenlet object at 0xb729ce3c>
Sleeping for 0.545666723422 seconds...
sleep(): greenelet: <greenlet.greenlet object at 0xb729ce3c>
wait_one(): greenelet: <greenlet.greenlet object at 0xb729cc5c>
done
0.545666723422
Here
Exception twisted.python.failure.Failure: <twisted.python.failure.Failure <class 'greenlet.GreenletExit'>> in <greenlet.greenlet object at 0xb729ce3c> ignored
GreenletExit did not kill <greenlet.greenlet object at 0xb729ce3c>

Doing a bit of research I've found that:

The last line is logged by greenlet.c
The previous line is logged by python itself, as it's ignoring an exception raised in a del method.

I'm having real trouble debugging this as I can't access the GreenletExit or twisted.python.failure.Failure exceptions to get their stack traces.

Does anyone have any ideas what I'm doing wrong, or how I get debug the exceptions that are being thrown?

One other data point: If I hack wait_one() to just return immediately (and not to register anything on the deferred it is passed), the errors go away. :-/

This article explains how, once you're scheduling coroutines implicitly this way, you are not really "using Twisted" any more in the sense that you're not getting the benefits of its programming model: https://glyph.twistedmatrix.com/2014/02/unyielding.html - also, you may be interested in a library that already does this, rather than writing your own - https://github.com/radix/corotwine — Glyph, Jul 30 '14 at 05:52

score 2 · Accepted Answer · answered Oct 26 '13 at 11:40

Rewrite your error callback in wait_one like this:

  def errback(failure):
    ## new code
    if g.dead:
        return
    ##
    if not active:
        g.throw(failure)
    else:
        reactor.callLater(0, g.throw, failure)

If greenlet is dead (finished running), there is no point throwing exceptions in it.

score 0 · Answer 2 · answered Oct 26 '13 at 13:01

mguijarr's answer fixed the problem, but I wanted to write up how I got into this situation.

I have three greenlets:

{main} that's runing the reactor.
{outer} that's running outer().
{inner} that's rrunning inner().

When the sleep finishes the {main} switches to {inner} which switches to {outer}. Outer then returns and raises GreenletExit in {inner}. This propogates back to twisted. It sees an exception being raised from callback(), and so invokes errback(). This tries to throw the exception into {outer} (which has already exited), and I hit the error.

Error when using twisted and greenlets

2 Answers2