Update "Entire source account" deletion to complete when server reports success

[eloquence]

#1311 updates "Files and messages" deletion to complete when the server reports success, instead of waiting for the next sync. This dramatically improves deletion performance, especially on a server with many sources where a sync takes a long time. We should apply the same performance improvement to "Entire source account" deletion.

[eloquence]

(Proposing this as a post-0.5.0 enhancement assuming we discover no novel issues with this approach.)

[eloquence]

Reopened via revert in #1426. To fully resolve, we'll want to make sure we don't observe the behavior noted in #1420.

[sssoleileraaa]

#1420 is not specific to account deletion, rather it has to do with how we switch between online and offline modes in the client. So those issues should be resolved separately, I believe.

[sssoleileraaa]

Also, #1420 is higher priority than speeding up source deletion right now.

[eloquence]

I do think the two issues should ideally be resolved together. Deletion is already a major pain point for our users, and we don't want to push out a release that makes things slower. And slowness is also a security issue: if you are in an emergency situation, having to wait for an entire sync duration to delete sources could prevent you from deleting things you need to delete, and you may be just as likely to close the laptop while the on-disk deletion is not complete.

[sssoleileraaa]

Good point, let's prioritize adding this into the next release as well, but it is technically a separate issue. Right now, I'm seeing three issues that we'll need to work on: this one, #1420, and #1429. Is that your understanding?

[eloquence]

Yes, that makes sense - #1429 could be resolved by ensuring that files and database updates are performed the moment the server notifies us the deletion is successful (instead of waiting for a sync).

[sssoleileraaa]

Yes, that is a potential solution that we need to explore more, and going back to slow deletion is not ideal. Since syncs can take a long time, because of the way we have to retrieve all data from the server to find out what we need to delete or add (this won't be true once we implement API v2 as suggested here freedomofpress/securedrop#5104), forcing an immediate sync is not ideal. It's just the old, simple way of doing things. Since the sync happens on a separate thread, we can hit a situation where the deletion will have to wait until an ongoing sync is finished, but this would be faster than forcing an end sync after deletion. I'll write up some notes in a doc and share here later.