696519 - Unable to seek in a WebM file that does not have a Cues element

Status: RESOLVED WONTFIX

(Core :: Audio/Video, defect)

Description

[mshamraeva]

User Agent: Mozilla/5.0 (Windows NT 6.1; rv:5.0.1) Gecko/20100101 Firefox/5.0.1
Build ID: 20110707182747

Steps to reproduce:

1. Play a WebM video in Firefox.
2. Set playhead time for the video to 0.

Please check the demo page that shows this behaviour: https://dev.universalsubtitles.org/en/jsdemo/webmffdemo
(just follow the simple instructions on the demo page). Thank you!


Actual results:

After resetting playhead time, the video does not play anymore.


Expected results:

The video should still be playable (works in Chrome).

Comment 1

[Bobby Johnson [:rjohnson19]]

Issue occurs as described on:
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:10.0a1) Gecko/20111021 Firefox/10.0a1

Comment 2

[Matthew Gregan [:kinetik]]

The file has no Cues, despite claiming it does have one:

| + Seek entry at 90
|  + Seek ID: 0x1c 0x53 0xbb 0x6b (KaxCues) at 93
|  + Seek position: 39834097 at 100

But the file is only 39256064 long, meaning the Cues start 578033 bytes after the end of the file.

It looks like Chrome can seek without an index, possibly only in the already downloaded sections of the file.

Comment 3

[Matthew Gregan [:kinetik]]

To be clear: this file is invalid or corrupt.  The WebM container spec requires a Cues element for seeking in WebM files.  It also requires the SeekHead element indicate whether Cues are present.  This file's SeekHead claims Cues are present at a location beyond the end of the file.

Chrome can seek in a file without Cues, but only in the already downloaded portion (bug 575140 proposes adding similar functionality to Firefox).  Watching the network activity in developer tools, seeking to the end of the file results in Chrome waiting for the entire file to download before completing the seek (suggesting a simple linear scan, but that's unverified).

Opera 11.52 fails to seek anywhere in the file.

Even if bug 575140 was fixed, I think it's valid for a WebM player to rely on the Cues to be present if the SeekHead claims it is, so it's not guaranteed that this invalid file would play in all conforming WebM players.