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[whatwg] <video> resource selection algorithm and NETWORK_NO_SOURCE

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Mon, 26 Jul 2010 22:53:50 +1000
Message-ID: <AANLkTi=R=3YBXLfcVbC7vMokxQzw+fY=w7pYbx-8v9vw@mail.gmail.com>
On Mon, Jul 26, 2010 at 7:41 PM, Philip J?genstedt <philipj at opera.com>wrote:

> I actually don't think there's anything wrong about the spec as it is.
>
> NETWORK_NO_SOURCE is not a state of failure, it is a waiting state. Here's
> what happens as the parser inserts elements into the DOM:
>
> <video controls width="400px">
> <!-- video has been inserted but resource selection hasn't run -->
>  <source type="video/mp4">
> <!-- resource selection is started, ends on step 21 with NETWORK_NO_SOURCE
> and pointer pointing to just after the only source element -->
>  <source type="video/webm">
> <!-- resource selection continues at step 22, going another loop and ending
> at step 21, but with pointer pointer to after the second source element -->
>  <source type="video/ogg">
> <-- same as above, but pointing to after the third source element -->
> </video>
>
> If nothing is done, the resource selection algorithm will wait forever at
> step 21. To get out of that state you have to either call load() to restart
> resource selection, or append another source element.
>
> If we should let networkState be NETWORK_EMPTY after inserting the dummy
> <source> elements, then resource selection would be run from the beginning
> each time, which means that during parsing, the first <source> element will
> be considered 3 times, the second 2 times and the last 1 time. In your
> example this doesn't matter, but if the reason a <source> failed was because
> of network errors, unsupported Content-Type or an unsupported file format,
> then that will also happen too many times, adding useless network traffic
> (unless e.g. 404 results are cached).
>
> So, I think the algorithm should stay as it is and that the other browsers
> should change their implementations. In your original example you used
> <source> elements with no src attribute. This is invalid, and I think the
> proper solution would be to create each source element, set the src
> attribute and append it to the video element.
>

Ok, I re-read the spec about <source> and you are correct: it's invalid to
create <source> elements without a @src attribute. It could, however, still
be created without a URI, in which case the resulting state would indeed be
a NETWORK_NO_SOURCE, since it fails to find a valid @currentSrc .

I still think, though, that NETWORK_NO_SOURCE is a failure state, since it
is specified as
NETWORK_NO_SOURCE (numeric value 3):
    The element's resource selection algorithm is active, but it has failed
to find a resource to use.


I now wonder about the intention of play() (and also of pause()). As I
understood it, they are both meant to reload the media resource if
@currentSrc has changed, similar to what load() is supposed to do.

Also, I wonder what NETWORK_EMPTY is used for when not at the beginning of
parsing the media resource. I haven't managed to catch Opera in that state.

Cheers,
Silvia.
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