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[SR-15227] MainActor static variable initialization not enforced on main thread. #57549

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bradleymackey opened this issue Sep 22, 2021 · 7 comments · Fixed by #40652
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[SR-15227] MainActor static variable initialization not enforced on main thread. #57549

bradleymackey opened this issue Sep 22, 2021 · 7 comments · Fixed by #40652
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@kavon
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actor isolation Feature → concurrency: Actor isolation bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself concurrency Feature: umbrella label for concurrency language features let & var Feature: constant and variable declarations SILGen Area → compiler: The SIL generation stage static declarations Feature → declarations: Static declarations swift 5.5 unexpected behavior Bug: Unexpected behavior or incorrect output

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@bradleymackey
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bradleymackey commented Sep 22, 2021 •
edited by AnthonyLatsis

Previous ID SR-15227
Radar rdar://problem/83411416
Original Reporter @bradleymackey
Type Bug
Status Closed
Resolution Done
Environment

swift-driver version: 1.26.9 Apple Swift version 5.5 (swiftlang-1300.0.31.1 clang-1300.0.29.1)

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, Concurrency
Assignee @kavon
Priority Medium

md5: 084c5699ba4bca8acdf7ced11d06439a

cloned to:

Issue Description:

@MainActor 
struct Bar {
  init() { 
    print("bar init is main", Thread.isMainThread) 
  } 
  func barCall() { 
    print("bar call is main", Thread.isMainThread) 
  } 
} 

@MainActor 
struct Foo { 
  @MainActor 
  static let global = Foo() 

  init() { 
    print("foo init is main", Thread.isMainThread) 
    let b = Bar() 
    b.barCall() 
  } 

  func fooCall() { 
    print("foo call is main", Thread.isMainThread) 
  } 

} 

Task.detached { 
  await Foo.global.fooCall() 
} 
// prints: 
// foo init is main false 
// bar init is main false 
// bar call is main false 
// foo call is main true

While fooCall() is enforced on the main thread, the initialization of the static variable is not, which breaks the contract of @MainActor.
@typesanitizer
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@swift-ci create

@kavon
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kavon commented Jan 20, 2022

Resolved in #40652

@bradleymackey
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This issue is still present in Swift 5.6, as per the example above. Curiously, if `global` is changed to a `static var`, the issue goes away and everything runs on the main thread as expected. `static let` causes the non-MainActor behaviour.

@bradleymackey
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See last comment

@kavon
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kavon commented Mar 17, 2022

Thanks for verifying that it is still broken for the `let` case. I believe the reason why it makes a difference is that we typically don't apply isolation to the access of an isolated `let` within the same module, but because it's static, all accesses must be considered isolated, because the first access will trigger the initialization.

@kavon
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kavon commented Mar 17, 2022

I've cloned this issue over to specifically handle the static-let here: https://bugs.swift.org/browse/SR-16009

@kavon
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kavon commented Mar 17, 2022

So we'll consider this SR to be about the case where it was a var, which was also broken but was fixed. Since the static-let case is going to require a fix in a different part of the compiler, I decided to clone.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This was referenced Apr 22, 2023
Closed
Merged
@AnthonyLatsis AnthonyLatsis added static declarations Feature → declarations: Static declarations actor isolation Feature → concurrency: Actor isolation unexpected behavior Bug: Unexpected behavior or incorrect output let & var Feature: constant and variable declarations SILGen Area → compiler: The SIL generation stage swift 5.5 labels Apr 22, 2023
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@kavon kavon

Labels
actor isolation Feature → concurrency: Actor isolation bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself concurrency Feature: umbrella label for concurrency language features let & var Feature: constant and variable declarations SILGen Area → compiler: The SIL generation stage static declarations Feature → declarations: Static declarations swift 5.5 unexpected behavior Bug: Unexpected behavior or incorrect output
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Fixes for stored property initializers kavon/swift
4 participants
@kavon @typesanitizer @bradleymackey @AnthonyLatsis