Crash on iOS 12 or lower when Objective-C app uses a Swift framework that tries to decode a missing JSON property

[felipe-saez]

Hello team,

Hope you are all doing well.

There seems to be a weird crash which happens under the following conditions:

• The app integrates ATInternet (either through CocoaPods or manually)
• The app uses Objective-C as main language
• The app is running on iOS 12 or lower
• The app integrates a framework that attempts to parse JSON data into a Swift custom type that has properties that the JSON data doesn't (or are set to null).

More specifically, the crash occurs when you call the decode method of the JSONDecoder class. It also happens when calling the decode or decodeIfPresent methods of the KeyedDecodingContainer struct, see the example below:

@objc public class TestClass: NSObject {
    
    struct TestType: Decodable {
        let name: String
        let description: String
        let nonExistingProperty: String // we won't provide this property as part of the data
    }
    
    @objc public func process() {
        let data = """
        {
            "name": "A name",
            "description": "A description"
        }
        """.data(using: .utf8) // <- we don't provide `nonExistingProperty`
        
        let jsonDecoder = JSONDecoder()
        
        if let data = data, let decoded = try? jsonDecoder.decode(TestType.self, from: data) { // <-- This is where the app crashes
            print("Name: \(decoded.name)")
        }
    }
}

This is the crash message, which doesn't give much details: Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)

Please find this project that meets all the conditions mentioned above where it's possible to reproduce the crash (I'm currently using Xcode 12.5):

ATInternetSampleApp.zip

Please find also the stack trace I get when the crash happens:

Stack trace crash AT Internet.txt

I noticed that this crash is similar to what is described here: https://stackoverflow.com/questions/58694448/ios-12-error-swift-getassociatedtypewitnessslowimpl-exc-bad-access-in-assembly-c/59338381#59338381 Basically, they mention that without explanation, once they removed an unused protocol, in their case Identifiable, they managed to remove the issue. By looking into the ATInternet code, more specifically here https://github.com/at-internet/atinternet-apple-sdk/blob/master/ATInternetTracker/Sources/Crypt.swift#L171, I noticed that the CustomStringConvertible protocol is not used and if I remove it, the crash disappears. Neither the issue nor the fix make sense, because one would think that because of the @available(iOS 13.0, *) check this code would not be visible to iOS 12 but removing the unused protocol just magically works, maybe an Xcode compiler bug?

Please let me know if you think more information is required.

Kind regards,

Felipe

[nsagnett]

Hello,

Many thanks for the PR (and thanks XCode to give us some dream every day !)

I just released 2.23.5 version which embed your fix, thanks again !

Regards,

[felipe-saez]

thanks for deploying the fix so quickly!