[Question] Extremely corrupted backup file

[strugee]

I have a Signal backup that cannot be restored in the Signal app. I've built signalbackup-tools and tried to use it to repair the broken backup, but the tool seems to have hit a lot of corruption. In https://github.com/bepaald/signalbackup-tools/issues/17#issuecomment-724243153, this description:

It's from a phone that initially had not enough space to perform a backup but at some point it seemed that enough space was available and the backup finished correctly.

sounds exactly like me. So, my question is twofold: am I encountering the same problem as #17, where somehow part of the backup file is a copy of the earlier part, and if not, is there a way to restore my data? If the answer to the first question is yes, I'll be reporting that bug to Signal upstream as that really seems related to the disk space issue.

I'm on revision f3db27438a3b442dc9bbce43455eec457599bb6e. I don't have access to the original device (I'm in this situation because I accidentally factory reset my phone while flashing Google factory images today), but I do have access to several years worth of Signal backups. At least some of these are known-good, although the known-good ones are probably pretty old. I also have access to a Signal Desktop installation that was originally linked to the installation that generated the backup. I did not test this backup in any way prior to today, a mistake which I will not be making in the future ;)

Examination of server backups show that the file has not been corrupted since being uploaded to my personal Nextcloud. I believe but am not sure that Nextcloud mobile verifies checksums when uploading, so likely the file was corrupted before it ever made it onto the network.

The VM I was running it in seems to have crashed just before the tool finished, but I already have 12 GB of logs... I'm wondering if that will be enough to determine whether the repeated data problem has occurred :sweat_smile: if so, I could email you a link. Or if you want to give me a script or something to just run on it locally I can make that happen too.