How to ensure at boot that all the mandatory data are available

[kelson42]

When user download and write the image, it happens often that he fails to get a fully working solution because:

• the image is corrupt
• the image is not properly written

As a result the hotspot is disfunctional... but the symptom is often not clear. That makes the diagnostic long and costly.

We have reliable tools to download, verify image integrity and write image on the mass storage. Unfortunately we can not control if user use these tools (appropriatly).

Therefore, I suspect that, if there is a check to do, it can only be done at the boot of the device.

An idea would be to check a few key files or key data written on the filesystem. Maybe by focusing to locations which are at the end of the mass storage considering that most weak download/writting systems work linearly.

[rgaudin]

the image is not properly written

This has become exceptional because we insist on completing check step during flashing.

the image is corrupt

While this is statistically rare, the fact that our images are very large makes it more likely to happen. Also it's hosted on a third party that might be far from users.
There cannot be clear symptoms so we need to rule out many things to fallback to a probable corrupt download. That's long and costly as you wrote. I'd like to mention that we are only aware of corruption that cause the whole system not to start but it's likely that there's corruption in ZIMs or other places that are less visible.

I don't like your suggestion though. It's too hacky, cannot be done right, based on false assumption that errors are at end of file (why would they?) and doesn't solve the problem. Even if we implement this and there's corruption on a file that wasn't specifically checked (which is almost as likely as current state), then the very same debug process needs to be taken because no assertion can be made.

A cheaper way would be to insist that all users downloading via HTTP run an integrity check. Effort on our hand can vary from finding a decent multi-platform existing tool (would require user to copy/paste hash) or make one which could by default request hash from imager-service. Even that would be very low effort.