[MSE] Appending URLs

Hi,

I wanted to start a discussion about changing the sourceAppend() signature
to take URLs instead of a Uint8Array (Bug
16998<https://www.w3.org/Bugs/Public/show_bug.cgi?id=16998>).
The primary motivation here is to avoid pulling the media data up into the
JavaScript layer unnecessarily. Changing to a signature like this also
provides a way for the user agent to control the rate of appending and
potentially improves how effectively it can use its cache.

There has been a little bit of discussion in the bug comments and the
current signatures being proposed are shown below.

void sourceAppend(in DOMString id, in DOMString url, optional long long
start, optional long long length);

[TreatNonCallableAsNull] attribute Function? onappenddone;
[TreatNonCallableAsNull] attribute Function? onappenderror;

If we decide to go the more object oriented path in Bug
17082<https://www.w3.org/Bugs/Public/show_bug.cgi?id=17082>,
I'd expect the id parameter to be dropped from the signature.

The application calls sourceAppend() with the URL and optional range
parameters to append data and waits for either the appenddone or
appenderror events to fire before attempting another append. appenddone
signals that the data was successfully appended and appenderror indicates
that the append failed for some reason.

For applications that want to generate media data on the fly or modify it
before appending, they can use XHR to fetch the data and BlobBuilders to
stage the modified data. Blob URLs can then be generated and passed to
sourceAppend(). Blob URLs could also be used to handle data received as
Blobs from the WebSockets or File APIs.

Here are some open questions I still have about this approach:
1. What information should we report via the appenddone event? bytes
transferred? duration of transfer ?
2. What information should we report via the appenderror event? HTTP status
code? HTTP reason string? # of bytes appended?
3. Should we add a appendprogress event that periodically fires, like the
HTMLMediaElement progress event, that conveys how much data has been
downloaded so far and how much time it has spent actually downloading?
4. The user agent may want to delay or slow down the download because of
memory or bandwidth constraints. Do we want to convey this to the web
application? If so, how?

Please let me know what you think.

Aaron