[SOLVED] EXC_BAD_ACCESS KERN_INVALID_ADDRESS on Struct Decodable.init(from:)

EXC_BAD_ACCESS KERN_INVALID_ADDRESS on Struct Decodable.init(from:)

I have a function that can be run up to 2 times simultaneously to fetch information from an API. In Google Crashlytics, I see quite a few users affected by this crash but I've never seen anything like it and am not sure where to go from here.

The meat of the method:

URLSession.shared.dataTask(with: urlRequest) { (data, res, err) in
    guard let data = data else { return }
    do {
        let decoder = JSONDecoder()
        decoder.dateDecodingStrategy = .formatted(self.dateFormatter)
        let json = try decoder.decode([String: TokenResponse].self, from: data)
    }
    catch {
        print(error.localizedDescription)
    }
}.resume()

TokenResponse:

struct TokenResponse: Decodable {
    var ticket : String
    var expiration : Date?
    var sessionId: String
}

Stack Trace:

Crashed: com.apple.NSURLSession-delegate
0  libicucore.A.dylib             0xe4874 icu::Calendar::clear() + 168
1  CoreFoundation                 0x57ab4 __cficu_ucal_clear + 28
2  CoreFoundation                 0x57ab4 __cficu_ucal_clear + 28
3  CoreFoundation                 0xca884 CFDateFormatterGetAbsoluteTimeFromString + 396
4  CoreFoundation                 0xe0490 CFDateFormatterCreateDateFromString + 108
5  Foundation                     0x1c2dc getObjectValue + 272
6  Foundation                     0x9f300 -[NSDateFormatter getObjectValue:forString:errorDescription:] + 200
7  Foundation                     0xbb5d0 -[NSDateFormatter dateFromString:] + 64
8  libswiftFoundation.dylib       0x37a58 __JSONDecoder.unbox(_:as:) + 592
9  libswiftFoundation.dylib       0x21064 __JSONDecoder.unbox_(_:as:) + 452
10 libswiftFoundation.dylib       0x1e040 _JSONKeyedDecodingContainer.decode<A>(_:forKey:) + 848
11 libswiftFoundation.dylib       0x27a48 protocol witness for KeyedDecodingContainerProtocol.decode<A>(_:forKey:) in conformance _JSONKeyedDecodingContainer<A> + 56
12 libswiftFoundation.dylib       0x258e0 protocol witness for KeyedDecodingContainerProtocol.decode<A>(_:forKey:) in conformance _JSONKeyedDecodingContainer<A> + 36
13 libswiftCore.dylib             0x549a8 KeyedDecodingContainerProtocol.decodeIfPresent<A>(_:forKey:) + 648
14 libswiftFoundation.dylib       0x231dc protocol witness for KeyedDecodingContainerProtocol.decodeIfPresent<A>(_:forKey:) in conformance _JSONKeyedDecodingContainer<A> + 36
15 libswiftCore.dylib             0x67dbc _KeyedDecodingContainerBox.decodeIfPresent<A, B>(_:forKey:) + 520
16 libswiftCore.dylib             0x55880 KeyedDecodingContainer.decodeIfPresent<A>(_:forKey:) + 76
17 MyApp                          0x1648dc MyClass.TokenResponse.init(from:) + 4344695004 (<compiler-generated>:4344695004)
18 MyApp                          0x164a78 protocol witness for Decodable.init(from:) in conformance MyClass.TokenResponse + 4344695416 (<compiler-generated>:4344695416)
19 libswiftCore.dylib             0x34c124 dispatch thunk of Decodable.init(from:) + 32
20 libswiftFoundation.dylib       0x51380 specialized __JSONDecoder.unbox<A>(_:as:) + 2540
21 libswiftFoundation.dylib       0x21048 __JSONDecoder.unbox_(_:as:) + 424
22 libswiftFoundation.dylib       0xcfc4 JSONDecoder.decode<A>(_:from:) + 1548
23 libswiftFoundation.dylib       0x44fc4 dispatch thunk of JSONDecoder.decode<A>(_:from:) + 56
24 MyApp                          0x12b84c closure #1 in MyClass.GetAllServerTokens() + 188 (MyClass.swift:188)
25 MyApp                          0x12add0 thunk for @escaping @callee_guaranteed (@guaranteed Data?, @guaranteed NSURLResponse?, @guaranteed Error?) -> () + 4344458704 (<compiler-generated>:4344458704)
26 CFNetwork                      0x22b34 CFURLRequestSetMainDocumentURL + 3028
27 CFNetwork                      0x33af8 _CFNetworkErrorCopyLocalizedDescriptionWithHostname + 11412
28 libdispatch.dylib              0x2914 _dispatch_call_block_and_release + 32
29 libdispatch.dylib              0x4660 _dispatch_client_callout + 20
30 libdispatch.dylib              0xbde4 _dispatch_lane_serial_drain + 672
31 libdispatch.dylib              0xc98c _dispatch_lane_invoke + 444
32 libdispatch.dylib              0x171a8 _dispatch_workloop_worker_thread + 656
33 libsystem_pthread.dylib        0x10f4 _pthread_wqthread + 288
34 libsystem_pthread.dylib        0xe94 start_wqthread + 8

EDIT: Example of the JSON that is fetched. I cannot post due to security concerns. It changes every 2 hours so I don't have the exact copy that caused the crashes but it should always follow this pattern. ticket and sessionId are never null:

{
    "1": {
        "ticket":"aRandomJSONWebTokenHere",
        "expiration":"2022-01-16T10:00:38.2775891Z",
        "sessionId":"aUuidHere"
    },
    "2": {
        "ticket":"aRandomJSONWebTokenHere",
        "expiration":"2022-01-16T10:00:38.2775891Z",
        "sessionId":"aUuidHere"
    }...all the way until 60
}

EDIT 2: self.dateFormatter:

let dateFormatter : DateFormatter = {
    let df = DateFormatter()
    df.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"     
    return df
}()

self.dateFormatter is not modified. It is used in other places in the same manner, as a dateDecodingStrategy. Maybe this crash happens when more than one URLSession read self.dateFormatter at the same time?

Solution

Maybe this crash happens when more than one URLSession read self.dateFormatter at the same time?

Sure, it's possible. Your code is certainly thread-unsafe! But it's obvious what to do; in your URLSession.shared.dataTask completion handler, get on the main thread and stay there.

URLSession.shared.dataTask(with: urlRequest) { (data, res, err) in
    guard let data = data else { return }
    DispatchQueue.main.async {
        do { // ...
        }
        catch { // ...
        }
    }
}.resume()