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Fixed a race condition in Signal terminal event handling. #267

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merged 4 commits into from
Feb 18, 2017
Merged

Fixed a race condition in Signal terminal event handling. #267

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2dfa7cb
Fixed a race condition in the `Signal.observe`.
andersio Feb 17, 2017
34e3e5e
Enable Travis CI for all branches named `hotfix-SEMVER`.
andersio Feb 17, 2017
403e4c7
Added information regarding the fixed deadlock to the test case.
andersio Feb 17, 2017
b9b7ea7
Merge branch 'hotfix-1.0.1' into signal-deadlock-fix
andersio Feb 17, 2017
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17 changes: 14 additions & 3 deletions Sources/Signal.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -313,13 +313,24 @@ public final class Signal<Value, Error: Swift.Error> {
return ActionDisposable { [weak self] in
if let s = self {
s.updateLock.lock()

if case let .alive(snapshot) = s.state {
var observers = snapshot.observers
observers.remove(using: token)
s.state = .alive(AliveState(observers: observers,
retaining: observers.isEmpty ? nil : self))

// Ensure the old signal state snapshot does not deinitialize before
// `updateLock` is released. Otherwise, it might result in a
// deadlock in cases where a `Signal` legitimately receives terminal
// events recursively as a result of the deinitialization of the
// snapshot.
withExtendedLifetime(snapshot) {
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@andersio andersio Feb 17, 2017 •
edited
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This is not strictly necessary, since snapshot is guaranteed to live till the end of the scope. But it serves a documentation purpose, and would be optimised away by the compiler IIRC.

s.state = .alive(AliveState(observers: observers,
retaining: observers.isEmpty ? nil : self))
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Thoughts on extracting this to a local newState so we can remove the awkward formatting over two lines?

s.updateLock.unlock()
}
} else {
s.updateLock.unlock()
}
s.updateLock.unlock()
}
}
} else {
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17 changes: 16 additions & 1 deletion Tests/ReactiveSwiftTests/ActionSpec.swift
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Expand Up @@ -58,7 +58,7 @@ class ActionSpec: QuickSpec {
action.errors.observeValues { errors.append($0) }
action.completed.observeValues { completedCount += 1 }
}

it("should retain the state property") {
var property: MutableProperty<Bool>? = MutableProperty(false)
weak var weakProperty = property
Expand Down Expand Up @@ -114,6 +114,21 @@ class ActionSpec: QuickSpec {
expect(action.isExecuting.value) == false
}

it("should not deadlock") {
final class ViewModel {
let action2 = Action<(), (), NoError> { SignalProducer(value: ()) }
}

let action1 = Action<(), ViewModel, NoError> { SignalProducer(value: ViewModel()) }

action1.values
.flatMap(.latest) { viewModel in viewModel.action2.values.map { _ in viewModel } }
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@andersio andersio Feb 17, 2017 •
edited
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The deadlock happens as the observer disposable releases the closure { _ in viewModel } here without releasing the mapped signal's updateLock first. The deinitialization triggers the propagation of terminal event of the Action, which eventually hits the mapped signal.

.observeValues { _ in }

action1.apply().start()
action1.apply().start()
}

describe("completed") {
beforeEach {
enabled.value = true
Expand Down