This method of app delegate is called when app starts to transition into inactive state: https://developer.apple.com/documentation/uikit/uiapplicationdelegate/1622950-applicationwillresignactive?language=objc. The docs state that it is the place to stop various on going tasks. Why do we need to do this? Those HTTP calls are executing on the background thread but we can't just let them run when application enters the background state (user swipes app) since iOS might kill our app for this. To execute code when application is in the background iOS provides specific APIs:

• We can ask for some additional time to finish work in the background with this method: https://developer.apple.com/documentation/uikit/uiapplication/1623051-beginbackgroundtaskwithname?language=objc - we could use it here but I just wasn't sure if we actually have to. If the user opens the app and starts using it the code in the method above will do what is necessary. However it the reviewers have different opinion we can use this API.

• Background Tasks API: https://developer.apple.com/documentation/backgroundtasks?language=objc This would also work but those are very undeterministic as to when the OS executes our code: https://developer.apple.com/forums/thread/131205.

• Background URL session: https://developer.apple.com/documentation/foundation/url_loading_system/downloading_files_in_the_background?language=objc - it would also work but I think it is an overkill for this feature. However it is a strong candidate for backup implementation on iOS. cc @michal @kamil

Both approaches has their pros and cons but I both cases they arise from the fact that our implementation of TemporaryMessageStorage supports only the following operations:

1. Write one
2. Read all
3. Delete all

(In fact reading all instantly deletes all but it is straightforward to refactor it to support those operations separately).
If we could delete the specific request we could apply the second approach where we read all at once, do the networking and delete only the successful ones. Theoretically we could achieve it by writing each holder to a separate file, but my intuition is that is not the right direction to follow.

I examined this API: https://developer.apple.com/documentation/foundation/url_loading_system/downloading_files_in_the_background?language=objc since at first sight it looked promising. However after studying it I came to the conclusion that it basically works like this:

1. Application creates the request and submits it to the background session object.
2. Request is executed in separate process at the time the OS finds most appropriate. Request execution is independent of the main app activity.
3. When request completes successfully or fails the OS wakes up our app (or not if it was active) and calls relevant method of AppDelegate where we can handle success or failure.

So basically the only benefit here is that the request is less likely to fail (since it is executed on a dedicated process) and we don't have to handle the app being put to background (which is not actually an issue as we do it with the applicationWillResignActive or we could do it with https://developer.apple.com/documentation/uikit/uiapplication/1623051-beginbackgroundtask). The second approach (read, send, delete) can still fail if NSE writes while we are executing requests. The first approach (read and delete, send, rewrite if send fails) can still fail due to exception being thrown since handling failure would take place in the main app code.

So the conclusion is that the problem is not that iOS offers limited API to do networking in the background since the API is apparently quite rich. The problem is that we don't have a storage layer that would support deletion of specific holder rather than all at once. With such a storage we could easily make the code in this differential fault tolerant.

The similar diff on Android (D9069) uses WorkManager API which uses its internal SQLite to persist failed requests so that they can be retried later.

The name of this method is probably confusing without the docs: https://developer.apple.com/documentation/foundation/nsurlsessiontask/1411121-resume.
Basically this resume means restart or start if it has not been started previously. In this case it starts the task.

I couldn't think of an alternative that wouldn't require ugly ternary operators.

One possible option would be to create a new variable in line 1105 - will that work?