8000 Actor-isolated property wrapper leads to compiling code that crashes at runtime · Issue #77604 · swiftlang/swift · GitHub
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stephencelis opened this issue Nov 14, 2024 · 4 comments
Open

Actor-isolated property wrapper leads to compiling code that crashes at runtime #77604

stephencelis opened this issue Nov 14, 2024 · 4 comments
Labels< 8000 /span>
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. crash Bug: A crash, i.e., an abnormal termination of software property wrappers Feature: property wrappers triage needed This issue needs more specific labels

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@stephencelis
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stephencelis commented Nov 14, 2024 •
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Description

It is possible to actor isolate the wrappedValue of a property wrapper and interact with it in a nonisolated way, leading to a crash.

Reproduction

@propertyWrapper
struct Count {
  @MainActor var value = 0

  @MainActor
  var wrappedValue: Int {
    get { value }
    set { value = newValue }
  }
}

@Test func f() async {
  @Count var count
  count += 1  // 💥
}

Stack dump

N/A. Produces code that crashes at runtime. Doesn't crash the compiler.

Expected behavior

I expect the line that crashes to not compile with a message such as:

Main actor-isolated property 'wrappedValue' can not be mutated from a nonisolated context

Environment

swift-driver version: 1.115 Apple Swift version 6.0.2 (swiftlang-6.0.2.1.2 clang-1600.0.26.4)
Target: arm64-apple-macosx15.0

Additional information

No response
@stephencelis stephencelis added bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. crash Bug: A crash, i.e., an abnormal termination of software triage needed This issue needs more specific labels labels Nov 14, 2024
@stephencelis
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(@tshortli Related to some of the other issues we've been talking about.)

@tshortli tshortli added the property wrappers Feature: property wrappers label Nov 14, 2024
@ArihantMarwaha
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Proposed Solution:

Modify the Swift compiler to:

  1. Detect actor-isolated property wrapper properties
  2. Prevent nonisolated access/mutation
  3. Emit a compile-time error instead of a runtime crash

Recommended Fix Prototype:

@propertyWrapper
struct Count {
  @MainActor private var value = 0

  @MainActor
  var wrappedValue: Int {
    get { value }
    set { value = newValue }
  }
}

// Compiler should now prevent this
@Test func f() async {
  @Count var count
  count += 1  // ❌ Compile-time error
}

Potential Compiler Error Message

Error: Cannot mutate main actor-isolated property 'wrappedValue' from a nonisolated context
Suggestion: Use '@MainActor' or transfer the mutation to the main actor

@ArihantMarwaha
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would love to work on this fix if given a go ahead

@marieandromeda
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I just ran into this myself, and luckily I searched before posting a new issue. Here was my test case, if it's useful:

@MainActor @propertyWrapper class FootGun {
    var wrappedValue = 42
}

await Task.detached {
    @FootGun var x
    print(x)    //    <-- Crashes here
}.value

Context: I was trying to come up with a way to ensure that property wrapper access is isolated to the main actor.

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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. crash Bug: A crash, i.e., an abnormal termination of software property wrappers Feature: property wrappers triage needed This issue needs more specific labels
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@stephencelis @marieandromeda @tshortli @ArihantMarwaha
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