CODEWITHSUNDEEP
iosswiftmultithreading
user14664032
user14664032

Reputation: 170

Understanding Swift thread safety

I have encountered a data race in my app using Xcode's Thread Sanitizer and I have a question on how to address it.

I have a var defined as:

var myDict = [Double : [Date:[String:Any]]]()

I have a thread setup where I call a setup() function:

let queue = DispatchQueue(label: "my-queue", qos: .utility)

queue.async {
    self.setup {                
    }
}

My setup() function essentially loops through tons of data and populates myDict. This can take a while, which is why we need to do it asynchronously.

On the main thread, my UI accesses myDict to display its data. In a cellForRow: method:

if !myDict.keys.contains(someObject) {
    //Do something
}

And that is where I get my data race alert and the subsequent crash.

Exception NSException * "-[_NSCoreDataTaggedObjectID objectForKey:]: unrecognized selector sent to instance 0x8000000000000000" 0x0000000283df6a60

Please kindly help me understand how to access a variable in a thread safe manner in Swift. I feel like I'm possibly half way there with setting, but I'm confused on how to approach getting on the main thread.

Upvotes: 1

Views: 2201

Answers (3)

Rob
Rob

Reputation: 437532

The basic pattern to achieve thread safety is to never mutate/access the same property from multiple threads at the same time. The simplest solution is to just never let any background queue interact with your property directly. So, create a local variable that the background queue will use, and then dispatch the updating of the property to the main queue.

Personally, I wouldn't have setup interact with myDict at all, but rather return the result via the completion handler, e.g.

// properties

var myDict = ...
private let queue = DispatchQueue(label: "my-queue", qos: .utility) // some background queue on which we'll recalculate what will eventually be used to update `myProperty`

// method doesn't reference `myDict` at all, but uses local var only

func setup(completion: @escaping (Foo) -> Void) {
    queue.async {
        var results = ...                                           // some local variable that we'll use as we're building up our results

        // do time-consuming population of `results` here;
        // do not touch `myDict` here, though;
        // when all done, dispatch update of `myDict` back to the main queue

        DispatchQueue.main.async {                                  // dispatch update of property back to the main queue
            completion(results)
        }
    }
}

Then the routine that calls setup can update the property (and trigger necessary UI update, too).

setup { results in
    self.myDict = results
    // also trigger UI update here, too
}

(Note, your closure parameter type (Foo in my example) would be whatever type myDict is. Maybe a typealias as advised elsewhere, or better, use custom types rather than dictionary within dictionary within dictionary. Use whatever type you’d prefer.)

By the way, your question’s title and preamble talks about TSAN and thread safety, but you then share a “unrecognized selector” exception, which is a completely different issue. So, you may well have two completely separate issues going on. A TSAN data race error would have produced a very different message. (Something like the error I show here.) Now, if setup is mutating myDict from a background thread, that undoubtedly will lead to thread-safety problems, but your reported exception suggests there might also be some other problem, too...

Upvotes: 1

Alexander
Alexander

Reputation: 63264

Preface: This will be a pretty long non-answer. I don't actually know what's wrong with your code, but I can share the things I do know that can help you troubleshoot it, and learn some interesting things along the way.

Understanding the error

Exception NSException * "-[_NSCoreDataTaggedObjectID objectForKey:]: unrecognized selector sent to instance 0x8000000000000000"

  • An Objective C exception was thrown (and not caught).
  • The exception happened when attempting to invoke -[_NSCoreDataTaggedObjectID objectForKey:]. This is a conventional way to refer to an Objective C method in writing. In this case, it's:
    • An instance method (hence the -, rather than a + that would be used for class methods)
    • On the class _NSCoreDataTaggedObjectID (more on this later)
    • On the method named objectForKey:
  • The object receiving this method invocation is the one with address 0x8000000000000000.

This is a pretty weird address. Something is up.

Another hint is the strange class name of _NSCoreDataTaggedObjectID. There's a few observations we can make about it:

Some back story

Objective C used message passing as its sole mechanism for method dispatch (unlike Swift which usually prefers static and v-table dispatch, depending on the context). Every method call you wrote was essentially syntactic sugar overtop of objc_msgSend (and its variants), passing to it the receiver object, the selector (the "name" of the method being invoked) and the arguments. This was a special function that would do the job of checking the class of the receiver object, and looking through that classes' hierarchy until it found a method implementation for the desired selector.

This was great, because it allows you to do a lot of cool runtime dynamic behaviour. For example, menu bar items on a macOS app would just define the method name they invoke. Clicking on them would "send that message" to the responder chain, which would invoke that method on the first object that had an implementation for it (the lingo is "the first object that answers to that message").

This works really well, but has several trade-offs. One of them was that everything had to be an object. And by object, we mean a heap-allocated memory region, whose first several words of memory stored meta-data for the object. This meta-data would contain a pointer to the class of the object, which was necessary for doing the method-loopup process in objc_msgSend as I just described.

The issue is, that for small objects, (particularly NSNumber values, small strings, empty arrays, etc.) the overhead of these several words of object meta-data might be several times bigger than the actual object data you're interested in. E.g. even though NSNumber(value: true /* or false */) stores a single bit of "useful" data, on 64 bit systems there would be 128 bits of object overhead. Add to that all the malloc/free and retain/release overhead associated with dealing with large numbers of tiny object, and you got a real performance issue.

"Tagged pointers" were a solution to this problem. The idea is that for small enough values of particular privileged classes, we won't allocate heap memory for their objects. Instead, we'll store their objects' data directly in their pointer representation. Of course, we would need a way to know if a given pointer is a real pointer (that points to a real heap-allocated object), or a "fake pointer" that encodes data inline.

The key realization that malloc only ever returns memory aligned to 16-byte boundaries. This means that 4 bits of every memory address were always 0 (if they weren't, then it wouldn't have been 16-byte aligned). These "unused" 4 bits could be employed to discriminate real pointers from tagged pointers. Exactly which bits are used and how differs between process architectures and runtime versions, but the general idea is the same.

If a pointer value had 0000 for those 4 bits then the system would know it's a real object pointer that points to a real heap-allocated object. All other possible values of those 4-bit values could be used to signal what kind of data is stored in the remaining bits. The Objective C runtime is actually opensource, so you can actually see the tagged pointer classes and their tags:

{
    // 60-bit payloads
    OBJC_TAG_NSAtom            = 0, 
    OBJC_TAG_1                 = 1, 
    OBJC_TAG_NSString          = 2, 
    OBJC_TAG_NSNumber          = 3, 
    OBJC_TAG_NSIndexPath       = 4, 
    OBJC_TAG_NSManagedObjectID = 5, 
    OBJC_TAG_NSDate            = 6,

    // 60-bit reserved
    OBJC_TAG_RESERVED_7        = 7, 

    // 52-bit payloads
    OBJC_TAG_Photos_1          = 8,
    OBJC_TAG_Photos_2          = 9,
    OBJC_TAG_Photos_3          = 10,
    OBJC_TAG_Photos_4          = 11,
    OBJC_TAG_XPC_1             = 12,
    OBJC_TAG_XPC_2             = 13,
    OBJC_TAG_XPC_3             = 14,
    OBJC_TAG_XPC_4             = 15,
    OBJC_TAG_NSColor           = 16,
    OBJC_TAG_UIColor           = 17,
    OBJC_TAG_CGColor           = 18,
    OBJC_TAG_NSIndexSet        = 19,
    OBJC_TAG_NSMethodSignature = 20,
    OBJC_TAG_UTTypeRecord      = 21,

    // When using the split tagged pointer representation
    // (OBJC_SPLIT_TAGGED_POINTERS), this is the first tag where
    // the tag and payload are unobfuscated. All tags from here to
    // OBJC_TAG_Last52BitPayload are unobfuscated. The shared cache
    // builder is able to construct these as long as the low bit is
    // not set (i.e. even-numbered tags).
    OBJC_TAG_FirstUnobfuscatedSplitTag = 136, // 128 + 8, first ext tag with high bit set

    OBJC_TAG_Constant_CFString = 136,

    OBJC_TAG_First60BitPayload = 0, 
    OBJC_TAG_Last60BitPayload  = 6, 
    OBJC_TAG_First52BitPayload = 8, 
    OBJC_TAG_Last52BitPayload  = 263,

    OBJC_TAG_RESERVED_264      = 264

You can see, strings, index paths, dates, and other similar "small and numerous" classes all have reserved pointer tag values. For each of these "normal classes" (NSString, NSDate, NSNumber, etc.), there's a special internal subclass which implements all the same public API, but using a tagged pointer instead of a regular object.

As you can see, there's a value for OBJC_TAG_NSManagedObjectID. It turns out, that NSManagedObjectID objects were numerous and small enough that they would benefit greatly for this tagged-pointer representation. After all, the value of NSManagedObjectID might be a single integer, much like NSNumber, which would be wasteful to heap-allocate.

If you'd like to learn more about tagged pointers, I'd recommend Mike Ash's writings, such as https://www.mikeash.com/pyblog/friday-qa-2012-07-27-lets-build-tagged-pointers.html

There was also a recent WWDC talk on the subject: WWDC 2020 - Advancements in the Objective-C runtime

The strange address

So in the previous section we found out that _NSCoreDataTaggedObjectID is the tagged-pointer subclass of NSManagedObjectID. Now we can notice something else that's strange, the pointer value we saw had a lot of zeros: 0x8000000000000000. So what we're dealing with is probably some kind of uninitialized-state of an object.

Conclusion

The call stack can shed further light on where this happens exactly, but what we know is that somewhere in your program, the objectForKey: method is being invoked on an uninitialized value of NSManagedObjectID.

You're probably accessing a value too-early, before it's properly initialized.

To work around this you can take one of several approaches:

  1. A future ideal world, use would just use the structured concurrency of Swift 5.5 (once that's available on enough devices) and async/await to push the work on the background and await the result.
  2. Use a completion handler to invoke your value-consuming code only after the value is ready. This is most immediately-easy, but will blow up your code base with completion handler boilerplate and bugs.
  3. Use a concurrency abstraction library, like Combine, RxSwift, or PromiseKit. This will be a bit more work to set up, but usually leads to much clearer/safer code than throwing completion handlers in everywhere.

Upvotes: 2

CouchDeveloper
CouchDeveloper

Reputation: 19106

One way to access it asynchronously:

typealias Dict = [Double : [Date:[String:Any]]]
var myDict = Dict()
func getMyDict(f: @escaping (Dict) -> ()) {
    queue.async {
        DispatchQueue.main.async {
            f(myDict)
        }
    }
}

getMyDict { dict in
    assert(Thread.isMainThread)
}

Making the assumption, that queue possibly schedules long lasting closures.

How it works?

You can only access myDict from within queue. In the above function, myDict will be accessed on this queue, and a copy of it gets imported to the main queue. While you are showing the copy of myDict in a UI, you can simultaneously mutate the original myDict. "Copy on write" semantics on Dictionary ensures that copies are cheap.

You can call getMyDict from any thread and it will always call the closure on the main thread (in this implementation).

Caveat:

getMyDict is an async function. Which shouldn't be a caveat at all nowadays, but I just want to emphasise this ;)

Alternatives:

  • Swift Combine. Make myDict a published Value from some Publisher which implements your logic.

  • later, you may also consider to use async & await when it is available.

Upvotes: 1

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