> On 5 May 2017, at 10:41, Nagy, Attila <b...@fsn.hu> wrote:
>
> On 05/05/2017 10:26 AM, Phil Mayers wrote:
>> On 04/05/17 16:30, Nagy, Attila wrote:
>>
>>> I would like to use the simplest (and correct of course) solution.
>>> Juggling with buffering/data by hand seems even more risky to me.
>>
>> The problem with the approach you've outlined is that it treats the
>> transport (a private member) in ways that I suspect are invalid. In
>> particular there's no handling of the length of the object or chunked
>> encodings - I suspect what you're doing will only work on simple HTTP
>> requests with no connection reuse.
> What possible side effects do you see here? What problems could it cause?
The first is that Twisted will break your code eventually. Private member
attributes are not covered by Twisted’s deprecation policy, and they can be
changed without warning for any reason. So you’ll need to pin your Twisted
version.
As a second note, you may lock yourself out of HTTP/2. HTTP/2 is not guaranteed
to give you access to a raw transport object (though it might), because in
HTTP/2 the protocol is not a dumb byte pipe like it is in HTTP/1.1. Code like
this forces Twisted devs who want to add HTTP/2 support (like myself) to
implement HTTP/2 as a multiple-object abstraction to allow each
request/response pair’s underlying “transport” member to act like a dumb
byte-pipe transport, when we’d much rather use a less complex abstraction (as
an example you should look at the HTTP/2 server code in twisted.web, which has
multiple classes to maintain this fiction that you can just call
“transport.write” and expect that to work).
As a third note, your code does not handle the possibility that
original._transport may not implement IPushProducer. While *in practice* it
tends to, it needn’t. On top of that, it is not forbidden for an IPushProducer
implementation to call write() even when paused, and code that wants to be
correct in the face of all situations will need to be able to buffer anyway.
However, you’re right that this is not ideal. I think the best solution would
be an enhancement to twisted.web that updates the default Response object to
accept an IConsumer as the protocol argument of deliverBody. This would allow
t.w._newclient.Response to be the arbiter of what it means to “pause”
production, and allow you to continue to proxy between the two but without
accessing a private member (you’d get given the producer you need to pause in
registerProducer).
If that’s an enhancement you’d be interested in, I can work with you to get
that patch in place. Then your code would change a bit (note that this code
won’t work right now):
class UploadProducer(protocol.Protocol):
implements(IBodyProducer)
implements(IConsumer)
def __init__(self, get_resp):
self.length = get_resp.length
self.producing = False
self._producer = None
self._consumer = None
self._completed = Deferred()
# IConsumer
def registerProducer(self, producer, streaming):
assert streaming
self._producer = producer
if self._consumer is None:
self._producer.pauseProducing()
def unregisterProducer(self):
# Raise an error or something
pass
def write(self, data):
self._consumer.write(data)
# IProtocol
def connectionLost(self, reason):
self._completed.callback(reason)
# IBodyProducer
def startProducing(self, consumer):
if self._producer is not None:
self._producer.resumeProducing()
self._consumer = consumer
return completed
def resumeProducing(self):
self._producer.resumeProducing()
def pauseProducing(self):
self._producer.pauseProducing()
def stopProducing(self):
self._producer.stopProducing()
@inlineCallbacks
def copy(src, dst):
get_resp = yield treq.get(src, unbuffered=True)
print "GET", get_resp.code, get_resp.original
producer = UploadProducer(get_resp)
get_resp.deliverBody(producer)
put_resp = yield treq.put(dst,data=producer)
print "PUT", put_resp, put_resp.code
With this arrangement as well it’d potentially be possible to use something
like tubes, or at least get closer to using tubes for this use case. Right now
it’s a bit of an annoyance that t.w._newclient doesn’t allow the body receiving
protocol to exert backpressure on the data.
Anyway, just a thought.
Cory
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