Fix a race condition bug involving timers and async tasks

[buggmagnet]

This PR fixes a subtle bug where a DispatchSourceTimer is used alongside swift async code in an unsafe way.

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This change is 

[linear]

IOS-582 Fix async API misuse in timer source usage

[pinkisemils]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @buggmagnet)

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ios/PacketTunnelCore/Actor/Task+Duration.swift line 53 at r1 (raw file):

                var hasResumed = false
                timer.setEventHandler {
                    hasResumed = true

Is it never possible for this event handler to be invoked after the the continuation has already been cancelled?

[buggmagnet]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @pinkisemils)

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ios/PacketTunnelCore/Actor/Task+Duration.swift line 53 at r1 (raw file):

Previously, pinkisemils (Emīls Piņķis) wrote…

Is it never possible for this event handler to be invoked after the the continuation has already been cancelled?

The default repeating interval is .never so the timer won't fire its event handler more than once unless explicitly specified.

[buggmagnet]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @pinkisemils)

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ios/PacketTunnelCore/Actor/Task+Duration.swift line 53 at r1 (raw file):

Previously, buggmagnet wrote…

The default repeating interval is .never so the timer won't fire its event handler more than once unless explicitly specified.

If the continuation is cancelled before the timer fires, then it gets cancelled and its event handler shouldn't be scheduled anymore

[rablador]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @buggmagnet and @pinkisemils)

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ios/PacketTunnelCoreTests/TaskSleepTests.swift line 35 at r1 (raw file):

            var task = recoveryTask()
            try await Task.sleep(duration: .milliseconds(10))
            task.doAnythingToSilenceAWarning()

What does this do?

[buggmagnet]

Reviewable status: 1 of 2 files reviewed, 2 unresolved discussions (waiting on @pinkisemils and @rablador)

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ios/PacketTunnelCoreTests/TaskSleepTests.swift line 35 at r1 (raw file):

Previously, rablador (Jon Petersson) wrote…

What does this do?

What it says on the tin !
Just kidding, I've added a comment and renamed the function to make it more explicit why we do this here.

[pinkisemils]

Reviewed 1 of 2 files at r1.
Reviewable status: 1 of 2 files reviewed, 2 unresolved discussions (waiting on @rablador)

[pinkisemils]

Reviewed 1 of 2 files at r1.
Reviewable status: 1 of 2 files reviewed, 2 unresolved discussions (waiting on @rablador)

[pinkisemils]

Reviewable status: 1 of 2 files reviewed, 2 unresolved discussions (waiting on @buggmagnet and @rablador)

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ios/PacketTunnelCore/Actor/Task+Duration.swift line 53 at r1 (raw file):

Previously, buggmagnet wrote…

If the continuation is cancelled before the timer fires, then it gets cancelled and its event handler shouldn't be scheduled anymore

And the event handler and cancel handler are guaranteed to never be executed concurrently?

[pinkisemils]

Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on @rablador)

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ios/PacketTunnelCore/Actor/Task+Duration.swift line 53 at r1 (raw file):

Previously, pinkisemils (Emīls Piņķis) wrote…

And the event handler and cancel handler are guaranteed to never be executed concurrently?

The docs answer my question.

[pinkisemils]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @rablador)

[rablador]

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

[rablador]

Reviewable status: complete! all files reviewed, all discussions resolved

[pinkisemils]

Reviewable status: complete! all files reviewed, all discussions resolved

[pinkisemils]

Reviewed all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved