Thanks Jean-Paul,
Useful information, but then I am confused -
If I am running in a deferred how do I cause the errback to be called
instead of the normal (non-error) callback? How do I pass a failure
object to the error handler? How should I do the
self.process.errback(fail)?
All of this has to happen inside of the deferred doesn't it, because
it is the deferred that is doing the work and where the errors will
occur?
Conrad
On 28 Feb 2009, at 14:16, Jean-Paul Calderone wrote:
On Sat, 28 Feb 2009 13:21:06 +0000, Conrad Winchester <conradwinches...@me.com
> wrote:
Hi all,
I am writing a restful twisted (8.1.0) server and am trying to
return useful error statuses from requests. The problem is that
even following the documentation extremely carefully I am getting
an AlreadyCalled Error every time I generate a failure. Here is my
main class (leaving out code that I feel is not relevant to the
question:
from xml.dom import minidom
from twisted.web import server, resource
from FWGException import FWGException
class DeferredPostHandler(resource.Resource,minidom.Document):
NOT_IMPLEMENTED = 501
BAD_REQUEST = 400
def writeError(self, failure, request, *args, **kw):
failure.trap(FWGException)
request.setResponseCode(self.errorCode)
request.write("Error handling request: %s" %
failure.getErrorMessage())
request.finish()
def writeResult(self, result, request, *args, **kw):
self.writexml(request)
request.finish()
def render_GET(self, request):
self.process=self.db.runInteraction(self.processForm,request)
I think that self.db is a ConnectionPool instance. This means that
here,
you're starting self.processForm in a thread. I'm not sure if this is
the reason you're having difficulties, but it may be. See below.
self.process.addCallbacks(self.writeResult, self.writeError,
[request], {}, [request], {})
return server.NOT_DONE_YET
I have a handler that extends this
from DeferredPostHandler import DeferredPostHandler
from twisted.web import http
from twisted.python.failure import Failure
from FWGException import FWGException
class CollectionAdminHandler(DeferredPostHandler):
def __init__(self, task, db, config):
DeferredPostHandler.__init__(self,db)
self.config = config
self.task = task
self.errorCode = http.OK
def processForm(self, cursor, request):
if self.task == 'add':
self.addCollection(cursor, request)
elif self.task == 'delete':
self.deleteCollection(cursor, request)
else:
self.errorCode = DeferredPostHandler.NOT_IMPLEMENTED
fail = Failure(FWGException("Unsupported Task"))
self.process.errback(fail)
Here you're creating a Failure and errbacking a Deferred in a
thread, since
processForm is running in a thread. This isn't allowed. Twisted
APIs are
generally not safe to be called in any thread except the reactor
thread.
def addCollection(self, cursor, request):
fail = None
if self.isValidAddRequest(request):
pass
else:
self.errorCode = DeferredPostHandler.BAD_REQUEST
fail = Failure(FWGException("Missing Parameter"))
if fail:
self.process.errback(fail);
Likewise here.
def deleteCollection(self, cursor, request):
print 'Delete Collection'
def isValidAddRequest(self, request):
return 'title' in request.args and 'category' in
request.args and 'date' in request.args
Unfortunately whenever I call
self.process.errback(fail)
To remove the incorrect calls to Twisted APIs from a thread, you
should
probably just raise an exception where you're currently errbacking
that
Deferred. In render_GET, or in writeError, you can handle the failure
which will result from that.
Ah, and now I see your problem. self.process is the Deferred returned
by runInteraction. You're not supposed to errback that Deferred (or
callback it either). A pretty good rule of thumb is that if you
create
a Deferred, then you get to fire it. In this case, runInteraction is
creating it, so runInteraction gets to fire it. runInteraction will
fire it with the result of processForm, so that's why you get the
error.
You're firing that Deferred and runInteraction is firing it.
Jean-Paul
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