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Re: Bug in Acid3 test #72 (race condition when images decode asynchronously)

From: Daniel Holbert <dholbert@mozilla.com>
Date: Fri, 30 Dec 2016 16:12:12 -0800
To: Ian Hickson <ian@hixie.ch>, www-archive@w3.org
Message-ID: <d37ebf7f-4cca-9583-1e09-5281ae5193db@mozilla.com>
Thanks for clarifying that -- I wasn't familiar with that "update the
image data" spec section.

Based on that section, I think you're correct, and Acid3 test #72 is
indeed OK after all.  I've got a local patch to fix Firefox to perform
data-URI dimension-decodes synchronously, so that we'll go back to
satisfying this chunk of spec & passing that subtest.

~Daniel

On 12/23/2016 02:13 AM, Ian Hickson wrote:
> I believe this is non-conforming. data: URLs are defined to be fetched
> synchronously and image decoding to obtain dimensions is defined to
> happen instantaneously (for non-data: URLs, this is black-box
> indistinguishable from it being rolled into the network delay, of course).
> 
> Specifically, see step 14 of "update the image data", noting that
> everything up to step 13 happens synchronously from the parser, then the
> fetching happens as per the fetch spec in one task (because it's a data:
> URL, it's not asynchronous), and that triggers step 14 which requires
> the image to be decoded and the size determined synchronously in a
> single task.
> 
> -- 
> Ian Hickson
> 
> On Fri, Dec 23, 2016 at 2:03 AM Daniel Holbert <dholbert@mozilla.com
> <mailto:dholbert@mozilla.com>> wrote:
> 
>     On 12/23/2016 12:06 AM, Ian Hickson wrote:
>     > The image in question is a data: URL. There's a ten millisecond gap
>     > between the last test and this test. That guarantees that there's at
>     > least one opportunity to handle the microtask in which the data: URL
>     > gets loaded. As far as I can tell this means that by the time the
>     height
>     > property accessor is read, it's guaranteed that the image is loaded.
> 
>     Nah, it's actually not guaranteed to be loaded -- hence the bug that was
>     filed for Firefox failing this subtest. :)
> 
>     In current Firefox versions, image-decoding actually happens on a
>     *helper-thread* -- not in a main-thread microtask. So, depending on
>     thread-switching / signalling overhead, the image very well might not
>     have been decoded (& had its results reported back) when the 10
>     millisecond timeout expires. Hence, it may very well be in the
>     spec-defined "unavailable" state at that point, when the test reads the
>     <img> size.
> 
>     You might think that this is silly and we should redesign this to decode
>     image sizes faster, and you'd perhaps be right.
> 
>     But whether we redesign it or not, our current behavior *is* compliant
>     with the HTML spec.  And I don't see why the Acid3 test should
>     arbitrarily mandate a decode-in-10ms behavior.
> 
>     I linked to a patch in my initial post, which simply makes the test
>     check whether the image is ready before reading the size (& retry if
>     not).  Given the above explanation (and the spec discussion in my
>     initial post), does my suggested change seem reasonable?
> 
>     Thanks,
>     ~Daniel
> 
> -- 
> 
> -- 
> Ian Hickson
> 
> 😸
> 
Received on Saturday, 31 December 2016 00:12:46 UTC

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