8000 A simple concurrency safety breach of a var being captured by reference · Issue #76929 · swiftlang/swift · GitHub
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CrystDragon opened this issue Oct 9, 2024 · 6 comments
Open

A simple concurrency safety breach of a var being captured by reference #76929

CrystDragon opened this issue Oct 9, 2024 · 6 comments
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8000 bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. concurrency Feature: umbrella label for concurrency language features

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@CrystDragon
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CrystDragon commented Oct 9, 2024 •
edited by YOCKOW
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Description

The following code compiles without warnings.

nonisolated  func f() async -> Int {
    var value = 0
    let task1 = Task { @MainActor in
        for _ in 0..<100000 {
            value += 1
        }
    }
    for _ in 0..<100000 {
        value += 1
    }
    await _ = task1.value
    return value
}

print(await f())

But it cannot produce 200000 because of the apparent race on value.

Also, for the following modified code, the compiler will complain "Pattern that the region based isolation checker does not understand how to check".

nonisolated func f() async -> Int {
    var value = 0
    let task1 = Task { @MainActor in
        for _ in 0..<100000 {
            value += 1
        }
    }
    let task2 = Task { @MainActor in
        for _ in 0..<100000 {
            value += 1
        }
    }
    await _ = task1.value
    await _ = task2.value
    return value
}

Reproduction

See above.

Expected behavior

  • There should be warnings/errors for the first example.
  • Solve "Pattern that the region based isolation checker does not understand how to check" for the second example.

Environment

  • Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
  • Target: arm64-apple-macosx14.0
  • Version 16.0 (16A242d)

Additional information

No response
@CrystDragon CrystDragon added bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. triage needed This issue needs more specific labels labels Oct 9, 2024
@NSFatalError
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This issue seems to be related to the problem I've described on Swift Forums

@ktoso ktoso added concurrency Feature: umbrella label for concurrency language features and removed triage needed This issue needs more specific labels labels Oct 13, 2024
@CrystDragon CrystDragon changed the title Simple concurrency safety breach. A simple concurrency safety breach of a var being captured by reference Oct 17, 2024
@CrystDragon
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Just tested this using the latest release/6.1 snapshot. There's still no diagnostics.
Isn't this one a crucial regression? @hborla

@CrystDragon
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CrystDragon commented Jan 14, 2025 •
edited
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Status update after tested against the release/6.1 nightly build.

For the second code snippet, now the compiler can emit a error saying "Sending 'value' risks causing data races"

But for the first code snippet, the compiler still fails to produce any error.
Note that if compiled using Swift 5.9, there will be an error.
in Swift 5.9.

@ktoso Forgive my ignorance, but don't you agree this a particular naive bad case?
Are there any underlying complexities that make this one so hard to address?

@CrystDragon
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I would argue that in most Swift Concurrency cases, from a developer's view, "a false positive" could be a better option compared to "a false negative".
Personally I'd rather choose to be limited while free-of-error than to be uncertain of correctness.

@hborla
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hborla commented Mar 27, 2025
8234

Yes, this is a data-race safety hole that needs to be fixed. It's a bug in region based isolation.

@CrystDragon
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The nightly build can now correctly emit diagnostics for the 2 code snippets in OP. Good work!

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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. concurrency Feature: umbrella label for concurrency language features
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