[Development] Proposal for allowing handling of HTTP redirects in QNAM

Richard Moore rich at kde.org
Tue Dec 24 11:59:56 CET 2013

On 24 December 2013 07:57, Mandeep Sandhu <mandeepsandhu.chd at gmail.com> wrote:
> Hi All,
> Few days back I stumbled upon this task:
> https://bugreports.qt-project.org/browse/QTBUG-8232
> "QNetworkAccessManager should support redirection"
> I think this is a useful feature that can be added to QNAM as it makes
> the life of a developer easy.
> I went through all the comments in the task page, libcurl
> documentation and also saw how some other libraries, like Python's
> urllib, handle redirection.
> I'm collating all that info here as a proposal for new functionality
> to QNAM and friends:
> 1. Allow handling of redirects at both global (QNAM) and per request
> (QNetworkRequest) level. Per request setting overrides global setting.
> Default will be to NOT handle redirects.
> 2. New "redirected(QUrl)" signal to be emitted in case auto-redirects
> are enabled and a redirection occurs.

This interface wouldn't be enough to let you distinguish between the
different types of redirect, though tbh I'm not sure that matters.

> 3. QNetworkReply stores both, the original as well as the final url.

What about the intermediate ones in a chain of redirects?

> 4. Allow setting of "max redirect" to avoid infinite redirections.
> 5. Allow a way to specify which protocols are allowed on redirects.
> Eg: curl by default does not redirect to "file" and "scp" protocols.

I don't think that should be offered. We pretty much concluded that
same-protocol redirects were okay, as were http -> https but others
were a mine field. https->http is one that I'm still unsure of since
it's one we see in real life, but has potential risks. If an app needs
detailed control (eg. as a browser does) then it should implement the
handling itself. QNAM should only handle the simple case.

> 6. Allow a way to specify redirect behaviour for POST requests. Eg: if
> a 301 is returned, the RFC states that the user agent MUST NOT change
> it to a GET request automatically.

IIRC the RFC is wrong or at least incomplete here and does not
describe actual behaviour of browsers. See
http://en.wikipedia.org/wiki/Post/Redirect/Get for more details.

> 7. New error code in QNetworkReply to indicate "too many redirects" error.
> The last update on this task was more than a year back. So I'm not
> sure if someone's already worked on it or if it's been abandoned due
> to some issue.
> I was wondering if a patch for it will be acceptable?

Sure, however it's quite involved. From inside the http backend you
need to start a new request meaning there's not a 1:1 mapping between
user requests and the internal ones. This is similar to what happens
with proxy auth etc. so you should look at the code that implements
that for inspiration.



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