[webkit-dev] Use of Frame by ResourceHandle

David Levin levin at chromium.org
Sun Sep 12 12:33:53 PDT 2010

On Sat, Sep 11, 2010 at 11:07 PM, Adam Barth <abarth at webkit.org> wrote:

> On Sat, Sep 11, 2010 at 10:52 PM, Darin Fisher <darin at chromium.org> wrote:
> > On Sat, Sep 11, 2010 at 10:42 PM, Adam Barth <abarth at webkit.org> wrote:
> >> On Sat, Sep 11, 2010 at 10:02 PM, Darin Fisher <darin at chromium.org>
> wrote:
> >> > I don't understand.  WebWorkers use ThreadableLoader, which routes the
> >> > network request back to the main thread where there is an associated
> >> > Frame.
> >> >  (SharedWorkers have a dummy frame associated with them.)
> >>
> >> See.  The dummy frame sounds unfortunate.
> >
> > It solved/avoided a load of problems/complexity.  What are your concerns?
> Having fake versions of objects add complexity to all the code that
> expects to talk to real versions of those objects.  For example,
> SVG-in-<img> creates a ton of fake objects and has been the source of
> a lot of bugs (including security bugs).  It seems like having a
> notion of a networking context makes more sense than pretending shared
> workers are associated with a rectangular region of a screen
> somewhere.

A clarification:
The "fake" frame only happens in Chromium. It is due to the fact that
workers are in a different process from the real frame.

In !chromium platforms, the real frame is used to send the request for both
dedicated and shared workers. (It is a bit unfortunate in the shared worker
case because closing that frame will kill the xhr request but the reasoning
has been that code should be resilient to xhr failures as they can happen
for a number of reasons.)


> >>  In general, there are also
> >> situations on the main thread where we'd like to perform a load
> >> without a Frame.  I'd have to look at the details, but there are
> >> long-standing bugs about applying XSLT to Frame-less documents.  Also,
> >> the PingLoader doesn't have a Frame available (it's job is to make
> >> image requests that outlive the Frame).
> >
> > PingLoader has an associated Frame when it kicks off the load.  That is
> the
> > critical time when Frame association is usually needed.
> What happens when code later in the loading cycle assumes this Frame
> is still present?  To avoid exploding, that code needs to understand
> that in this tiny corner of the loader, life is different, which is a
> big testing and maintenance burden.
> > For example, you
> > cannot load any network requests in Chromium unless you know what Page
> (you
> > need to know the routing ID of the tab) is requesting the resource.  I
> > assume PingLoader still generates the
> > FrameLoaderClient::dispatchWillSendRequest notification, right?
> I don't think so.  PingLoader talks directly to ResourceHandle.
> PingLoader knows about the Frame, but it looks like it only uses it to
> determine the outgoing referrer, to
> addExtraFieldsToSubresourceRequest, and to grab the networking
> context.
> > How do you get a frame-less document?  Via XMLHttpRequest.responseXML?
> > Perhaps it could use the Frame of the script execution context?  (Which
> > script execution context is a good question.)
> There are are lots of ways to get a Frameless document.  For example,
> JavaScript can call document.implementation.createDocument.  Also, the
> DOMParser will given you a document.  XMLHttpRequest will give you
> one.  You can get one by having an XSLT.  The PageCache has some.
> There was a patch that someone was pushing at some point to chain
> these documents back to a "master" document that has a frame.  That's
> certainly one approach, but I don't think it should be necessary.
> >> In general, there is no necessary connection between network requests
> >> made by WebCore and Frames.  Techniques that aim to associate a frame
> >> with every network request won't work in some cases because such a
> >> Frame might not exist.
> >
> > There always has been such an association.
> Right, and there are bugs we've never been able to fix because of that
> coupling.
> > I would like to understand the
> > concerns better.  I guess it means that I need to understand the
> frame-less
> > document issue and why you think having a dummy frame associated with
> shared
> > workers is a problem.
> Here's an example bug from 2006 that's marked Critical:
> https://bugs.webkit.org/show_bug.cgi?id=10313
> The patch attached to that bug is a giant workaround for the fact that
> the loader is too dependent on Frame.
> Adam
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