Longer downloads fail silently when app is backgrounded on iOS

[bikubi]

Bug Report

Problem

Reproduce:

• Start a download that runs approx. longer than 30s.
• Background the app (i.e. home button, open other app)
• Be on battery (no cable) for quicker fails. Also iPhones fail quicker than iPads. YMMV.

What is expected to happen?

• The download continues until it's done

What does actually happen?

• The download fails
• The file is not created
• neither success nor error handlers are called

Information

• This seems to be a common issue for background tasks on cordova. Hence the existence of the background mode plugin, which doesn't work any more with modern OSes. See also Android doze.
• I thought it might not apply because i see several references to backgroundTaskID etc. in the code, which the general docs about background execution talk about. But I'm too unfamiliar with iOS dev to really tell.
• I thought the "background fetch" capability would solve this, but nope. Might need a specific implementation to work.
• This is somewhat related to There's any way to resume downloads? #213, essentially a duplicate of the issue described in this comment, but its own issue -- resumable downloads could mitigate / work around, but not solve the core issue.
• Seems not to affect Android, but I might have to test harder (longer downloads)
• I will give https://github.com/sgrebnov/cordova-plugin-background-download a try, but I am not getting my hopes up
• At the very least, the error callback should fire.
• For now I guess I'll have to add a hint: "Please don't close the app while download is running"... (or remember that I had started a download, an then onforeground check if it is still running, check if the file is there... oof, heuristic headaches)

Environment, Platform, Device

I think only iOS, Android needs more testing.

Version information

• cordova 11.0.0
• cordova-ios 6.2.0
• current dev version of this plugin 7ba6fa3 (but also 5ed73ae)
• cordova-plugin-file 7.0.0 (but also 5.0.0)
• Xcode 13.1
• iOS 15.4.1, 16(.1 iirc), 12.3.1 (I think. It looks like that one has just trained itself for longer exec times, sigh)

Checklist

• I searched for existing GitHub issues
• I updated all Cordova tooling to most recent version
• I included all the necessary information above

[bikubi]

Could be a duplicate of #321 but note that this is primarily not about request time-outs (in the http or network sense) and not related to file size, only execution time.