Fix crash in dispatch queue assertion with background tasks

[buggmagnet]

This PR should fix the crash we've been encountering in the latest beta version of the app.
It does so by forcing the registered tasks completion handlers to run on the main thread of the application.
It also removes some of the nonisolated(unsafe) markers that were added in those completion handlers.

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This change is 

[linear]

IOS-1033 Investigate crash in test flight for MullvadVPN app

[pinkisemils]

Reviewed 6 of 7 files at r1.
Reviewable status: 6 of 7 files reviewed, 2 unresolved discussions (waiting on @buggmagnet)

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ios/MullvadVPN/TunnelManager/TunnelManager.swift line 228 at r1 (raw file):

            completionHandler: { [weak self] result in
                guard let self else { return }
                DispatchQueue.main.async {

I know these are my changes, but what ensures that this completion handler will be executed on the main actor here?

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ios/MullvadVPN/TunnelManager/TunnelManager.swift line 503 at r1 (raw file):

            }

            completionHandler(error)

Should this really be removed?

[buggmagnet]

Reviewable status: 6 of 7 files reviewed, 2 unresolved discussions (waiting on @pinkisemils)

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ios/MullvadVPN/TunnelManager/TunnelManager.swift line 228 at r1 (raw file):

Previously, pinkisemils (Emīls Piņķis) wrote…

I know these are my changes, but what ensures that this completion handler will be executed on the main actor here?

It doesn't need to be, also I think we don't use it anywhere AFAiCT 🤔

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ios/MullvadVPN/TunnelManager/TunnelManager.swift line 503 at r1 (raw file):

Previously, pinkisemils (Emīls Piņķis) wrote…

Should this really be removed?

It should not, thanks for catching this !

[pinkisemils]

Reviewed 1 of 1 files at r2.
Reviewable status: 6 of 7 files reviewed, 1 unresolved discussion (waiting on @buggmagnet)

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ios/Operations/ResultOperation.swift line 134 at r3 (raw file):

    }

    private func dispatchAsyncOn(_ queue: DispatchQueue?, _ block: @escaping @Sendable () -> Void) {

Does this block have to be Sendable? It could just be sending IMO.
Isn't it the case that anything that is Sendable can be trivially sent without causing any extra headaches for the caller?

[buggmagnet]

Reviewable status: 6 of 7 files reviewed, 1 unresolved discussion (waiting on @pinkisemils)

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ios/Operations/ResultOperation.swift line 134 at r3 (raw file):

Previously, pinkisemils (Emīls Piņķis) wrote…

Does this block have to be Sendable? It could just be sending IMO.
Isn't it the case that anything that is Sendable can be trivially sent without causing any extra headaches for the caller?

yes, DispatchQueue.async requires the block to be @Sendable

[pinkisemils]

Reviewed 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

[SteffenErn]

Doesn't crash anymore on iOS 18.1.1

Reviewed 6 of 7 files at r1, 1 of 1 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved