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!

https://stackoverflow.com/a/43548031/1265393 – pkamb May 13 '21 at 21:05 — pkamb, May 13 '21 at 21:05

score 28 · Accepted Answer · answered May 27 '15 at 06:03

28

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

answered May 27 '15 at 06:03

mustafa

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Interesting. I have an enter before the first closure is called. hmmm. – jestro May 27 '15 at 07:18
1

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! – jestro May 28 '15 at 00:23
5

@mustafa 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? – ofer2980 Oct 09 '16 at 13:06
@ofer2980 did you find a way? – hyouuu Jun 03 '18 at 19:22
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. – Mohd Haider May 01 '20 at 10:02

score 8 · Answer 2 · edited May 13 '21 at 21:15

8

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"

edited May 13 '21 at 21:15

pkamb

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answered Mar 04 '19 at 19:14

Bhavesh Sarwar

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What in tarnation – Munib May 22 '19 at 05:33
2

@PranoyC This should obviously not be used in the production code, but was really helpful for debugging. – Hannes Hertach May 12 '20 at 09:32

dispatch_group_leave crash in swift

2 Answers2

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