dispatch_group_leave crash in swift

Question

This happens very rarely. Here is the last line of the stack trace:

0  libdispatch.dylib              0x0000000197a85a9c dispatch_group_leave + 48

dispatch_group_leave is called in a complete closure which is invoked like this:

  dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
                { () -> Void in
                    let query = HKStatisticsCollectionQuery(quantityType: quantityType,
                        quantitySamplePredicate: nil,
                        options: statisticOptions,
                        anchorDate: anchorDate,
                        intervalComponents: interval)
                    query.initialResultsHandler = {

                        complete()

So we dispatch to a background thread, run a HKStatisticsCollectionQuery, and then call a function parameter closure called complete. Inside complete is where the dispatch_group_leave is called and the crash happens.

Any ideas are most appreciated! Thanks!

Gamogenesis

https://mcmap.net/q/361512/-exc_bad_instruction-code-exc_i386_invop-subcode-0x0-on-dispatch_semaphore_dispose – Gur

Answer 1

If dispatch_group_leave call isn't balanced with dispatch_group_enter then crash may happen.

Germinant

Interesting. I have an enter before the first closure is called. hmmm. – Gamogenesis

I think this was it. Certain leaves were getting called too fast. Moved all the enters and the notify to before the leaves can be called. Thanks! – Gamogenesis

@Germinant is there a way to prevent the case of calling dispatch_group_leave more than it should? i.e., for cases that I can't tell for sure that completion block - of which in it the dispatch_group_leave is called - will not be called more than once? – Intrigant

@Intrigant did you find a way? – Defloration

We can add a little bit sleep thread call after dispatch_group_leave. It will help dispatch_group to apply concurrency rules. It saved my day. – Misguide

Answer 2

In the worst case, you can check the discount group's count via its debugDescription String:

let count = self.groupExecuting.debugDescription.components(separatedBy: ",").filter({$0.contains("count")}).first!.components(separatedBy: CharacterSet.decimalDigits.inverted).filter({Int($0) != nil})
assert(count.first != "0")

This strategy is further explored in this question: DispatchGroup: check how many "entered"

Huntingdon

What in tarnation – Moy

@PranoyC This should obviously not be used in the production code, but was really helpful for debugging. – Isfahan