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Re: HTTP/2 response completed before its request

From: Roberto Peon <grmocg@gmail.com>
Date: Tue, 1 Jul 2014 20:59:44 -0700
Message-ID: <CAP+FsNdNzgpv-0OtPxV88bFNcN0d=MHr9tG-tWZj6W7Vn4Xtjw@mail.gmail.com>
To: Jesse Wilson <jesse@swank.ca>
Cc: Willy Tarreau <w@1wt.eu>, Zhong Yu <zhong.j.yu@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>
I'd expect that OkHttp sends WINDOW_UPDATE for the connection-level flow
control, but doesn't send WINDOW_UPDATE for the receiving stream until the
application has consumed it.
If it works this way, then there is no deadlock.

If the implementation needs to be stingy with memory, then it should set a
small or zero default window size, and send a WINDOW_UPDATE at the end of
the request, allowing the server to respond then. This would force
half-duplex behavior on response entity-bodies for that stream.

-=R


On Tue, Jul 1, 2014 at 8:47 PM, Jesse Wilson <jesse@swank.ca> wrote:

> Although OkHttp's network layer is constantly reading from the socket, it
> won't acknowledge response body data until it's been consumed by the
> application layer. And the application layer won't consume the response
> until after it's done transmitting the request.
>
> So we're vulnerable to deadlock because our application layer is not
> concurrent and our network layer refuses to buffer an unbounded amount of
> data.
>  On Jul 1, 2014 9:07 PM, "Roberto Peon" <grmocg@gmail.com> wrote:
>
>> In HTTP2, however, it works differently since the browser must always
>> read, and both sides must respect flow control.
>>
>> You need to try pretty hard to get it into a pathological case that
>> deadlocks things (e.g. overly-large/infinite/non-existent flow control
>> window which the application is unable/unwilling to actually adhere to
>> coupled with more data sent than the application is willing to read).
>>
>> For my part, I would not change how the server works.
>> I'd have the server drop the connection to any endpoint for HTTP2 that
>> was not reading what the server was sending it.
>> Similarly, any client should drop the connection to any endpoint that was
>> not reading what it was sending it.
>>
>> -=R
>>
>>
>> On Tue, Jul 1, 2014 at 8:02 PM, Zhong Yu <zhong.j.yu@gmail.com> wrote:
>>
>>> On Tue, Jul 1, 2014 at 2:42 PM, Willy Tarreau <w@1wt.eu> wrote:
>>> > On Tue, Jul 01, 2014 at 02:21:07PM -0500, Zhong Yu wrote:
>>> >> On Tue, Jul 1, 2014 at 11:45 AM, Roberto Peon <grmocg@gmail.com>
>>> wrote:
>>> >> > Getting a response before the request has finished definitely
>>> happens
>>> >> > sometimes, even in HTTP/1.1
>>> >>
>>> >> A server should not do that, or it will cause deadlocks with most
>>> >> major browsers.
>>> >>
>>> >> 100-continue is supposed to be helpful in this case, but it's not
>>> >> really adopted in practice.
>>> >
>>> > I disagree, and there are a number of situations where it's quite
>>> desirable
>>> > to act like this. For example, imagine that I'm uploading a large
>>> image to
>>> > a site and my session has expired. I want the site to send the error
>>> as soon
>>> > as possible so that my browser stops emitting for nothing. I don't
>>> want it to
>>> > wait minutes just to know that I need to re-login first then try again.
>>> >
>>> > Browsers already handle this quite well in 1.1, and the real issue in
>>> fact
>>>
>>> All the browsers I tested (firefox/chrom/safari/IE) appear to be
>>> half-duplex - they will not read the response until the request body
>>> is completely sent. A server can send an immediate response before
>>> reading the request body, but the browser won't read the response
>>> immediately.
>>>
>>> Since sending the response before draining the request body carries
>>> the risk of deadlock, it's probably better to drain the request body
>>> before sending the response. That is, the server is forced to do
>>> half-duplex, because most clients do half-duplex.
>>>
>>> Zhong Yu
>>> bayou.io
>>>
>>> > tends to be on the server side where it's not always easy to drain all
>>> the
>>> > request from the client after the response was sent, which sometimes
>>> results
>>> > in a TCP RST which risks to clear the response before the client has a
>>> chance
>>> > to see it. But correctly done, it's a very useful feature.
>>> >
>>> > Willy
>>> >
>>>
>>
>>
Received on Wednesday, 2 July 2014 04:00:11 UTC

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