[webkit-dev] "Magic Iframe" removal proposed

[Dmitry Titov]

Hi,

There is a patch posted <https://bugs.webkit.org/show_bug.cgi?id=81590> for
removal of the 'magic iframe' feature. This is the ability to move 'live'
iframe from one page to another w/o unloading it.
If you have interest or ideas about this feature, please reply.

HISTORY
This feature was added 2 years ago (bug
here<https://bugs.webkit.org/show_bug.cgi?id=32848>).
It was intended as a shared app context for complex apps that span several
pages. In case when random set of pages is closed, the surviving iframe
could be passed between remaining pages and serve as 'app state'.
This behavior is somewhat described in HTML5
spec<http://www.whatwg.org/specs/web-apps/current-work/multipage/the-iframe-element.html#the-iframe-element>:
"Removing an iframe from a Document does not cause its browsing context to
be discarded. Indeed, an iframe's browsing context can survive its original
parent Document if its iframe is moved to another Document."
All non-WebKit browsers don't have this and always unload the iframe when
it is disconnected form the document.

PROBLEMS
We did have quite a few issues with this mechanism. The root of the problem
seems to be that traditionally, multiple 'permissions' and 'live objects'
are associated with a top-level page, or a top frame of some kind, instead
of being associated with each Frame. Even HTML specifications often
formulate the scope of things like permissions in terms of a page - for
example, geo permissions are computed based on the origin of the page. When
an iframe is transferred from one page to another, it may enter a different
set of permissions while already performing operations authorized
before. Association with the top-level page is also used to determine which
one should show modal UI for HTTP Auth, per-origin permissions, or
certificate issues for example.
As a result, we had quite a few bugs popping up (and fixed).

WHY REMOVE
This is somewhat obscure feature of the platform. Only a few apps that we
knew used the feature. Most developers, both app and webkit kind, don't
even know about it. When new mechanisms/APIs are implemented, a lot of
objects get associated with Page (WebView) level and they are almost
'automatically' broken in case of live iframe transfer because once old
page closes, it destroys the objects with lifetimes scoped by it. This
makes it somewhat dangerous and difficult to support. The benefits that it
gives to the big multi-page applications do not seem to warrant the actual
complexity of maintaining this feature.
Other browsers never implemented the feature, siting difficult design due
to various thorny security issues as well.

This is potentially a compatibility issue for sites that use
document.adoptNode(iframe) to ensure live transfer of an iframe from one
page to another.
In the future, if there will be sufficient need, it is possible to design a
'shared module' feature that would explicitly deal with various
security/lifetime boundaries.

Please let us know what you think.

Thanks,
Dmitry
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