NSInternalInconsistencyException when querying Core Data while iOS is migrating iCloud transaction logs -

i started seeing error reported on crashlytics after updating model in ios app (lightweight migration, since data store synced via icloud). issue seems affect ~2-3% of sessions , found difficult reproduce in test environment.

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here error (stack trace of crashing thread):

thread : crashed: com.twitter.crashlytics.ios.exception 0  webtv                          0x000000010031ad28 clsprocessrecordallthreads + 4298337576 1  webtv                          0x000000010031ad28 clsprocessrecordallthreads + 4298337576 2  webtv                          0x000000010031b1d8 clsprocessrecordallthreads + 4298338776 3  webtv                          0x0000000100305c08 clshandler + 4298251272 4  webtv                          0x0000000100318f20 __clsexceptionrecord_block_invoke + 4298329888 5  libdispatch.dylib              0x00000001994c9770 _dispatch_client_callout + 16 6  libdispatch.dylib              0x00000001994d49c4 _dispatch_barrier_sync_f_invoke + 100 7  webtv                          0x0000000100318ab8 clsexceptionrecord + 4298328760 8  webtv                          0x00000001003188d0 clsexceptionrecordnsexception + 4298328272 9  webtv                          0x0000000100318530 clsterminatehandler() + 4298327344 10 libc++abi.dylib                0x0000000198372f44 std::__terminate(void (*)()) + 16 11 libc++abi.dylib                0x0000000198372fd0 std::set_new_handler(void (*)()) + 90 12 libobjc.a.dylib                0x0000000198cdc1a4 _destroyalthandlerlist + 10 13 libdispatch.dylib              0x00000001994c9784 _dispatch_client_callout + 36 14 libdispatch.dylib              0x00000001994d49c4 _dispatch_barrier_sync_f_invoke + 100 15 coredata                       0x0000000183e53b08 -[nsmanagedobjectcontext performblockandwait:] + 252 16 webtv                          0x00000001001320ec wtvcoredatahelper.(executesync in _b3caa6076b08d24ea69ca58785dff9db)(nsfetchrequest, error : inout nserror?) -> [anyobject]? (wtvcoredatahelpers.swift:136) 17 webtv                          0x00000001001390f0 wtvcoredatahelper.defaultprofile() -> wtvprofile? (wtvcoredatahelpers.swift:529) 18 webtv                          0x00000001001335c0 wtvcoredatahelper.(defaultprofileifnil in _b3caa6076b08d24ea69ca58785dff9db)(wtvprofile?) -> wtvprofile? (wtvcoredatahelpers.swift:204) 19 webtv                          0x0000000100132de8 wtvcoredatahelper.(systemfolder in _b3caa6076b08d24ea69ca58785dff9db)(wtvprofile?) -> wtvfolder? (wtvcoredatahelpers.swift:179) 20 webtv                          0x00000001001358a8 wtvcoredatahelper.homepage(profile : wtvprofile?) -> wtvpage? (wtvcoredatahelpers.swift:345) 21 webtv                          0x0000000100273bec swwebbrowserviewcontroller.homepageurl.getter (swwebbrowserviewcontroller.swift:97) 22 webtv                          0x00000001002607f4 swbrowserhomebutton.evaluateforchangeofstate(nsurl?) -> () (swbrowsercontrolbuttons.swift:94) 23 webtv                          0x000000010025e5b8 swbrowsercontrolbutton.store(fasfluxstore!, didemitchangenotification : nsnotification!) -> () (swbrowsercontrolbuttons.swift:36) 24 webtv                          0x000000010025e630 @objc swbrowsercontrolbutton.store(fasfluxstore!, didemitchangenotification : nsnotification!) -> () (swbrowsercontrolbuttons.swift) [...] 49 graphicsservices               0x000000018ef98088 gseventrunmodal + 180 50 uikit                          0x00000001896caf60 uiapplicationmain + 204 51 webtv                          0x00000001002c0c60 main (appdelegate.swift:19) 52 libdyld.dylib                  0x00000001994fa8b8 start + 4 

...and here ios thread performing icloud transaction logs migration

thread : com.apple.coredata.ubiquity.entry.pq 0  libsystem_kernel.dylib         0x0000000199608c6c semaphore_wait_trap + 8 1  libdispatch.dylib              0x00000001994da684 _dispatch_semaphore_wait_slow + 244 2  foundation                     0x00000001850fed94 -[nsfilecoordinator(nsprivate) _blockonaccessclaim:] + 380 3  foundation                     0x00000001850ff8b0 -[nsfilecoordinator(nsprivate) _coordinatereadingitematurl:options:error:byaccessor:] + 440 4  foundation                     0x00000001850fd8fc -[nsfilecoordinator coordinatereadingitematurl:options:error:byaccessor:] + 72 5  coredata                       0x0000000183f23280 -[pfubiquityfilecoordinator coordinatereadingitematlocation:options:retryonerror:error:byaccessor:] + 140 6  coredata                       0x0000000183f3c27c -[pfubiquitylocation fileexistsatlocationwithlocalpeerid:error:] + 324 7  coredata                       0x0000000183f77790 -[pfubiquitysafesavefile existsincloud] + 88 8  coredata                       0x0000000183f4b958 -[pfubiquitymigrationmanager migratetransactionlogs:andbaselineifnecessaryforstorename:peerid:error:] + 1328 9  coredata                       0x0000000183f5db18 -[pfubiquitysetupassistant initializebaselineforstore:error:] + 2576 10 coredata                       0x0000000183f5a3b4 -[pfubiquitysetupassistant finishsetupforstore:error:] + 2324 11 coredata                       0x0000000183f59828 -[pfubiquitysetupassistant finishsetupwithretry:] + 152 12 coredata                       0x0000000183f222a8 __57-[pfubiquityswitchboardentry executeblockonprivatequeue:]_block_invoke + 64 13 libdispatch.dylib              0x00000001994c97b0 _dispatch_call_block_and_release + 24 14 libdispatch.dylib              0x00000001994c9770 _dispatch_client_callout + 16 15 libdispatch.dylib              0x00000001994d575c _dispatch_queue_drain + 864 16 libdispatch.dylib              0x00000001994cd274 _dispatch_queue_invoke + 464 17 libdispatch.dylib              0x00000001994c9770 _dispatch_client_callout + 16 18 libdispatch.dylib              0x00000001994d7bb0 _dispatch_root_queue_drain + 2140 19 libdispatch.dylib              0x00000001994d734c _dispatch_worker_thread3 + 112 20 libsystem_pthread.dylib        0x00000001996e9478 _pthread_wqthread + 1092 21 libsystem_pthread.dylib        0x00000001996e9028 start_wqthread + 4 

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i believe code follows best practices core data concurrency (see relevant extracts swift code below). in particular

• the managedobjectcontext configured privatequeueconcurrencytype
• all calls core data apis wrapped in managedobjectcontext.performblock()

• managedobjectcontext saved , reset in storewillchange

  what missing?

here code crashes

private func executesync(request: nsfetchrequest) -> [anyobject]? {             var results: [anyobject]?     if let moc = managedobjectcontext {       moc.performblockandwait({ () -> void in         {           try results = moc.executefetchrequest(request) // crashes here         } catch let error nserror {           error("error executefetchrequest of wtvcoredatahelper threw error \(error)")         }       })     }     return results   } 

setup of of persistentstorecoordinator, managedobjectcontext

lazy var persistentstorecoordinator: nspersistentstorecoordinator? = {     var coordinator: nspersistentstorecoordinator? = nspersistentstorecoordinator(managedobjectmodel: self.managedobjectmodel)     let url = self.applicationdocumentsdirectory.urlbyappendingpathcomponent("webvideocast.sqlite")     var options = [nsobject:anyobject]()     options[nspersistentstoreubiquitouscontentnamekey] = kconstants.ubiquitystoreidentifier     options[nsmigratepersistentstoresautomaticallyoption] = true     options[nsinfermappingmodelautomaticallyoption] = true     var error: nserror? = nil     var failurereason = "there error creating or loading application's saved data."     {       try coordinator!.addpersistentstorewithtype(nssqlitestoretype, configuration: nil, url: url, options: options)     } catch var error nserror {       coordinator = nil       var dict = [nsobject:anyobject]()       dict[nslocalizeddescriptionkey] = "failed initialize application's saved data"       dict[nslocalizedfailurereasonerrorkey] = failurereason       dict[nsunderlyingerrorkey] = error       error = nserror(domain: kconstants.errordomain, code: 9999, userinfo: dict)       error("unresolved error \(error), \(error.userinfo)")       abort()     } catch {       fatalerror()     }     return coordinator     }()    lazy var managedobjectcontext: nsmanagedobjectcontext? = {     // returns managed object context application (which bound persistent store coordinator application.) property optional since there legitimate error conditions cause creation of context fail.     let coordinator = self.persistentstorecoordinator     if coordinator == nil {       return nil     }      var managedobjectcontext = nsmanagedobjectcontext(concurrencytype: nsmanagedobjectcontextconcurrencytype.privatequeueconcurrencytype)     managedobjectcontext.persistentstorecoordinator = coordinator     // set mergepolicy prioritise external inputs     let mergepolicy = nsmergepolicy(mergetype:nsmergepolicytype.mergebypropertystoretrumpmergepolicytype )     managedobjectcontext.mergepolicy = mergepolicy      return managedobjectcontext     }() 

relevant delegate methods:

@objc func storewillchange(notification: nsnotification) {     if let moc = self.managedobjectcontext {       moc.performblockandwait({ () -> void in         if moc.haschanges {           {             try moc.save()           } catch let error nserror {             error("error unresolved error while saving context \(error), \(error.userinfo)")           } catch {             fatalerror()           }         }         moc.reset()       })     }   } 

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update: reproduced problem once on device model had been updated latest version - not sure lightweight migration contributing factor.