> On Oct 10, 2019, at 12:57 PM, Ryosuke Niwa <rn...@webkit.org> wrote:
> Hi Chris,
> I'm excited that you're working on OffscreenCanvas because I think it would 
> be a valuable feature

Me too!!!

> , and I'm confident we can come up with a strategy to limit its privacy & 
> security risk as we see fit.
> However, many of your patches seem to ignore the fact most of WebCore objects 
> aren't thread safe. For example, CSS parser and the entire CSS object model 
> aren't designed to used in non-main thread. Regardless of how ready Linux 
> ports might be, we can't land or enable this feature in WebKit until all 
> thread safety issues are sorted out.
> Unfortunately, I can't make a time commitment to review & find every thread 
> safety issue in your patches. Please work with relevant experts and go over 
> your code changes.

I’d be happy to work with you on this.

> For example, it's never safe to an object that's RefCounted in multiple 
> threads because RefCounted isn't thread safe. One would have to use 
> ThreadSafeRefCounted. It's never safe to use AtomString from one another in 
> another because AtomString has a pool of strings per thread. For that matter, 
> it's never safe to use a single String object from two or more threads 
> because String itself is RefCounted and isn't thread safe. It's not not okay 
> to do readonly access to basic container types like Vector, HashMap, etc... 
> because they don't guarantee atomic update of internal data structures and 
> doing so can result in UAF.
> I think the hardest part of this project is validating that enabling this 
> feature wouldn't introduce dozens of new thread safety issues and thereby 
> security vulnerabilities.

Sounds like this this is a good candidate for a feature flag.

> - R. Niwa
> On Thu, Oct 10, 2019 at 4:23 AM Chris Lord <cl...@igalia.com 
> <mailto:cl...@igalia.com>> wrote:
> I've spent the last month or so 'finishing' the implementation of
> OffscreenCanvas[1], based on Žan Doberšek's work from a year ago[2].
> OffscreenCanvas is an API for being able to use canvas drawing without a
> visible canvas, and from within Workers. It's supported by Blink and has
> partial support in Gecko.
> It's at the point now where I'd consider it a finished draft - it is
> almost fully implemented and passes the majority of relevant tests in a
> debug build without crashing, but has some areas that need completion on
> other platforms (async drawing on non-Linux) and some missing parts (Web
> Inspector, ImageBitmapRenderingContext). It almost certainly needs
> reworking in places.
> My work is on GitHub[3] - I'd like to solicit reviews and comment. Some
> of the bugs hanging off [2] have patches that need review and I think
> are near ready to being landable as the foundation of this work. It is
> broadly split up like so:
> - Refactor to move functionality from HTMLCanvasElement to CanvasBase
> - Refactor to not unnecessarily require HTMLCanvasElement in places
> - Implement OffscreenCanvas functionality
> - Make font loading/styling usable from a Worker and without a Document
> - Implement AnimationFrameProvider on DedicatedWorkerGlobalScope
> - Implement asynchronous drawing updates on placeholder canvases
> I expect the font-related stuff to be the most contentious, and my
> AnimationFrameProvider implementation may be too trivial (but might be
> ok for a first go?)
> All feedback appreciated. Best regards,
> Chris
> [1]
> https://html.spec.whatwg.org/multipage/canvas.html#the-offscreencanvas-interface
> <https://html.spec.whatwg.org/multipage/canvas.html#the-offscreencanvas-interface>
> [2] https://bugs.webkit.org/show_bug.cgi?id=183720 
> <https://bugs.webkit.org/show_bug.cgi?id=183720>
> [3] https://github.com/Cwiiis/webkit/tree/offscreen-canvas 
> <https://github.com/Cwiiis/webkit/tree/offscreen-canvas>
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