Issue: Upgrader seems to hang on a step but in reality is processing next steps

This might happen if the XHR process returns a value that is not recognized (such as an echo or var_dump). Without knowing the output of the XHR, there's not much that can be done about this.

This also might occur if your PHP process times out, or if your PHP process runs out of memory. We don't really have much control over these situations. In these cases, you probably won't notice any problems if you have php.ini's display_errors disabled, and even if it's enabled, it's unlikely to be passed to the XHR process in a way that it understands.

And without knowing the process block contents, we don't know what needs to be tweaked to avoid these situations on your installation. When that happens, you'll have to restart the process.

It keeps happening on this step:
Updating data in table: xf_user_alert_optout(399636)
It quickly processes a number of rows. Then gets stuck on one. Until I refresh and restart the upgrader. Im not certain if its actually processing further or if its really stuck. It takes really long. Maybe because I have a huge userbase? It has been processing this for more than an hour now.

Now there no longer is a process bar or process block. The process information is displayed in the xf page. But also stops after some entries. It seems the process has restarted. It was at 96% and then went back up from 3% several times. It looks like the same steps are repeated if I restart the process. Until it gets stuck again. There are no errors.

It processes xf_user_alert_optout entries in batches of 10000. If it's peak time, perhaps that's too much and it gets stuck waiting for rows to unlock (maybe if multiple users in the same batch are active at the same time). We could lower it to something more like 2000, but it would take more batches to complete. However, each query would be less expensive so maybe it would be quicker in the long run.

You seem to be past this step already, but I'd be interested to hear your thoughts.

CPU and load were very low. If thats irrelevant then I think lowering batch quantities seems a good idea.
But other than that issue, it seems the layout of the upgrader is broken.

I was unable to upload a screenshot to the broken vbulletin attachment system, but here is a screenshot:
https://imgur.com/a/bUiF574

The upgrade is complete now.

You shouldn't encounter the broken layout again. It looks like it's limited to the following situation:
- Started upgrading from 4.0.x, AND
- Finished the upgrade steps for 4.1.0 Alpha 1, AND
- Restarted the upgrader during some other upgrade step, AND
- The templates for 4.1.x were never imported yet.

I've made some tweaks to try to prevent this, since we should have access to the template-version in this scope.

Also reduced the batch size on xf_user_alert_optout for the future as discussed.