Postmortem for a recent video upload issue

A user recently notified us of an issue causing a video to not be decrypted. We have identified a potential issue and are rolling out a fix, while continuing to investigate. We'll update this document as we have more details.

Details

Since the initial report, we have been combing through the code that deals with uploads to find where the issue might lie.

1. In the web or desktop code, we haven't found any issue so far. Still, we've added some more assertions just as an extra safety check.
2. We found a scenario with partial chunked reads, however based on the evidence it seems unlikely that this was the cause in practice. In any case, we've reworked that code, and have also added extra logging.
3. In the mobile code, we found a potential issue in the auto-resumable multipart background upload…