@Shared can trigger a view update from a thread other than the main thread, which sometimes causes crash

[larryonoff]

Description

There’s an issue where updating @Shared from a thread other than the main thread triggers a warning during debugging and sometimes causes crashes in the release configuration.

Store state was accessed on a non-main thread. …

The "Store" class is not thread-safe, and so all interactions with an instance of "Store" (including all of its scopes and derived view stores) must be done on the main thread

Checklist

• I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

Crashes or warnings do not occur

Actual behavior

Crashlytics reports occasional crashes. Xcode shows warnings during debugging.

Steps to reproduce

Refer to the sample project

The Composable Architecture version information

1.11.1

Destination operating system

iOS 17.5.1

Xcode version information

Version 15.4 (15F31d)

Swift Compiler version information

swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
Target: arm64-apple-macosx14.0

[mbrandonw]

Hi @larryonoff, this will be fixed in #3178 when we require that withLock be @MainActor. I am going to convert this to a discussion for now, so please feel free to continue the discussion over there!

[larryonoff]

Thanks! It was quick!

[larryonoff]

A new source of the issue has been identified.

A crash occurs in production when the PersistenceReaderKey triggers the didSet callback of the subscribe function from a non-main thread. This problem arises because SwiftUI updates its views from non-main threads.

The following entry was recorded in Crashlytics:

SwiftUI/ThreadUtils.swift:114: Fatal error: calling into SwiftUI on a non-main thread is not supported

sample from our code

  public func subscribe(
    initialValue: Purchases?,
    didSet: @escaping (Purchases?) -> Void
  ) -> Shared<Purchases>.Subscription {
    let task = Task {
      for await purchases in self.purchases.purchasesUpdates() {
        didSet(purchases)

       // looks that possible solution is wrapping didSet into MainActor
       // await MainActor.run { didSet(purchases) }
       // or Task { @MainActor in
      }
    }

    return Shared.Subscription {
      task.cancel()
    }
  }

Crashlytics stack trace

[larryonoff]

Warnings were also observed regarding SwiftUI being accessed from a non-main thread when using @Shared(.appStorage) NSUserDefaults was updated from a non-main thread.

[larryonoff]

@mbrandonw It seems that the issue described above deserves GitHub issue.

[mbrandonw]

Hi @larryonoff, this should be addressed here: #3206

[larryonoff]

@mbrandonw awesome! thanks for the update