iOS Crash during background data processing when adding NSOperations to NSOperationQueue - ios

Question

My app is getting a 'random' crash when doing some background data processing. I am adding NSOperations (subclassed) to an array, then adding that array of NSOperations to a NSOperationQueue.
I am seeing 3-4 errors which all point to the this line in the stack trace:
[_operationQueue addOperations:_operationsArray waitUntilFinished:YES];
Here is how I am creating my NSOperations (LibSyncOperation) and adding them to _operationsArray, which eventually gets added to the _operationQueue
LibSyncOperation *libSyncOperation = [[LibSyncOperation alloc] initWithURL:path];
if(!libSyncOperation.isExecuting && !libSyncOperation.isFinished)
{
[_operationsArray addObject:libSyncOperation];
TFLog(#"Operation is *NOT* Executing, add to OperationQueue");
}
else
{
TFLog(#"Operation is already Executing, do not add to OperationQueue");
}
Here are some of the stack traces:
*** -[NSOperationQueue addOperations:waitUntilFinished:]: 3 (of 4) operations are finished, executing, or already in a queue, and cannot be enqueued
CoreFoundation-[NSException initWithCoder:]
0 CoreFoundation 0x18835f09c __exceptionPreprocess
1 libobjc.A.dylib 0x1947b5d78 objc_exception_throw
2 CoreFoundation 0x18835efdc -[NSException initWithCoder:]
3 Foundation 0x188ec1864 __addOperations
4 Foundation 0x188ec1bfc -[NSOperationQueue addOperations:waitUntilFinished:]
5 Tower-iSales-Tab 0x100223b54 __70-[NetworkManager checkForDataFilesShouldEmptyQueue:isInitialDownload:]_block_invoke_2183 in NetworkManager.m on Line 437
6 libdispatch.dylib 0x194d84420 _dispatch_call_block_and_release
7 libdispatch.dylib 0x194d843e0 _dispatch_client_callout
8 libdispatch.dylib 0x194d8b3fc _dispatch_root_queue_drain
9 libdispatch.dylib 0x194d8b638 _dispatch_worker_thread2
10 libsystem_pthread.dylib 0x194f19918 _pthread_wqthread
11 libsystem_pthread.dylib 0x194f197a8 start_wqthread
Another:
*** Collection <__NSMallocBlock__: 0x1784402a0> was mutated while being enumerated.
CoreFoundation
0 CoreFoundation 0x186226f50
1 libobjc.A.dylib 0x192c041fc objc_exception_throw
2 CoreFoundation 0x186226984
3 Foundation 0x186d91928 -[NSOperationQueue addOperations:waitUntilFinished:]
4 Tower-iSales-Tab 0x1002bbb54 __70-[NetworkManager checkForDataFilesShouldEmptyQueue:isInitialDownload:]_block_invoke_2183 in NetworkManager.m on Line 437
5 libdispatch.dylib 0x1931dc014 _dispatch_call_block_and_release
6 libdispatch.dylib 0x1931dbfd4 _dispatch_client_callout
7 libdispatch.dylib 0x1931e32b8 _dispatch_root_queue_drain
8 libdispatch.dylib 0x1931e34fc _dispatch_worker_thread2
9 libsystem_pthread.dylib 0x1933716bc _pthread_wqthread
10 libsystem_pthread.dylib 0x19337154c start_wqthread
Last One:
-[__NSBlockVariable__ countByEnumeratingWithState:objects:count:]: unrecognized selector sent to instance 0x1782499c0
CoreFoundation__methodDescriptionForSelector
0 CoreFoundation 0x18835f09c __exceptionPreprocess
1 libobjc.A.dylib 0x1947b5d78 objc_exception_throw
2 CoreFoundation 0x188363d14 __methodDescriptionForSelector
3 CoreFoundation 0x188361a7c ___forwarding___
4 CoreFoundation 0x1882814ac __forwarding_prep_0___
5 Foundation 0x188ec1ca0 -[NSOperationQueue addOperations:waitUntilFinished:]
6 Tower-iSales-Tab 0x1002afb54 __70-[NetworkManager checkForDataFilesShouldEmptyQueue:isInitialDownload:]_block_invoke_2183 in NetworkManager.m on Line 437
7 libdispatch.dylib 0x194d84420 _dispatch_call_block_and_release
8 libdispatch.dylib 0x194d843e0 _dispatch_client_callout
9 libdispatch.dylib 0x194d8b3fc _dispatch_root_queue_drain
10 libdispatch.dylib 0x194d8b638 _dispatch_worker_thread2
11 libsystem_pthread.dylib 0x194f19918 _pthread_wqthread
12 libsystem_pthread.dylib 0x194f197a8 start_wqthread
Is there some obvious reason this crash would occur? I understand what the error message is saying, that NSOperations that are executing or finished cannot be enqueued again, the thing is, my _operationsArray is re-allocating anytime the update process runs, to ensure there are no NSOperations that were already enqueued hanging around.
One thing I am not doing, is re-allocating the NSOperationQueue each time, I figure that could be the cause, but since I cannot reproduce the error on my end (the users are experiencing this), I would like to know for sure. Heres the code for my NSOperationQueue (note this is all in a singleton class)
if(!_operationQueue)
_operationQueue = [[NSOperationQueue alloc]init];
[_operationQueue setMaxConcurrentOperationCount:1];
Could this be the culprit?
Any advice is appreciated. Thanks.
EDIT
As requested, heres the main for my LibSyncOperation:
-(void)main
{
if (![self isCancelled])
{
[self willChangeValueForKey:#"isExecuting"];
executing = YES;
NSURLRequest *downloadRequest = [NSURLRequest requestWithURL:downloadURL cachePolicy:nil timeoutInterval:9999999];
NSURLConnection *downloadConnection = [[NSURLConnection alloc] initWithRequest:downloadRequest delegate:self];
[downloadConnection scheduleInRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];
[downloadConnection start];
if (downloadConnection)
{
do
{
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
} while (!downloadDone);
[self terminateOperation];
}
else
{
[self terminateOperation];
}
}
else
{
[self terminateOperation];
}
}