Regression: Concurrent read from __SwiftDeferredNSArray crashing in Swift 5.7

[matttodd]

Describe the bug
In my app, we experienced a new EXC_BAD_ACCESS crash from users on iOS 16 (beta 4 - build 20A5328h) when reading from a swift Array bridged to NSArray on multiple threads. The crash can be reproduced by running the function provided below.

Steps To Reproduce

func crash() {
    let arraySize = 10_000 // large size appears to matter, maybe takes longer to load backing buffer for __SwiftDeferredNSArray?
    let array = Array(repeating: 1, count: arraySize)
    let nsArray = array as NSArray
    DispatchQueue.concurrentPerform(iterations: 10) { _ in
        _ = nsArray.object(at: 0)
    }
}

Expected behavior
Reading from an NSArray should be thread safe.

Environment (please fill out the following information)

• OS: iOS 16
• Build: Beta build 4 - 20A5328h
• Xcode Version/Tag/Branch: xcode 14 beta
• Swift: 5.7

Additional context
My theory is it has to do with recent changes to _swift_setClassMetadata in this commit

[technikyle]

Experiencing same crash! Thanks for reporting. Note this affects all Swift arrays accessed in ObjC. Would think that is common.

[tbkka]

CC: @aschwaighofer @Catfish-Man

[technikyle]

I just want to point that for the provided example, the array size required to produce the crash varies by device. For my iPad is around 10,000 and for iOS simulator is about 10,000,000.

In the real world, this our our top crash for users on an iOS 16 device.

[aschwaighofer]

This example involves types that are not bridged verbatim (Array<Int>, deferred array storage). The _swift_setClassMetadata path is not implicated by this because it happens only in the verbatim case.

[technikyle]

If this issue is unrelated to the suggested commit, just wondering if this is being investigated to determine the cause.

[brototyp]

Did anyone figure out something about this? Found this issue on our App in the wild. I am a bit concerned that there will not be a fix before iOS 16 is released and our App is crashing massively. How did you go about this? Is there information somewhere other than this issue? Did any of you file a radar/feedback? Did you implement a workaround instead of waiting? I am curious 😊

[matttodd]

Did anyone figure out something about this? Found this issue on our App in the wild. I am a bit concerned that there will not be a fix before iOS 16 is released and our App is crashing massively. How did you go about this? Is there information somewhere other than this issue? Did any of you file a radar/feedback? Did you implement a workaround instead of waiting? I am curious 😊

We were able to get a quick-and-dirty workaround by doing an initial read from the array on a single thread. We didn't file on the Feedback Assistant but made a Swift Forums post here.

[brototyp]

Thanks for the update @matttodd. I guess we'll go for a workaround as well.

[suttiwat]

Me too. I encounter with this some sort of bug about SwiftArray too. But in my situation, I just use

try? JSONSerialization.data(withJSONObject: containerModel, options: .fragmentsAllowed)

It's crash inside this JSONSerialization.data method even I use optional try.
But this is not happen on all user devices. Just for some specific condition (on some device) which I can't identity that condition exactly.

Attached is stack trace.