Why do I get a EXC_CRASH (SIGABRT) error on a for() line

Question

I got sent an error log for an app that I've been working on that is experiencing intermittent crashes, generally after working for a 30+ minutes. The exception received in the log is of the type EXC_CRASH (SIGABRT). I haven't experienced the crash myself, and don't have access to the offending device, so the log is all I have to go after when trying to replicate the problem.

After symbolicating the log, it appears that the culprit is the following bit of code (Thread 0 position 10 in the Error log)

if(oldStatus != _connected) {
    for(id<OnlineStatusSubscriber> subscriber in _subscribers) {
        [subscriber onlineStatus:self statusChangedTo:_connected];
    }
}

specifically

for(id<OnlineStatusSubscriber> subscriber in _subscribers) {

if the line number is to be believed.

And I can't figure out what might be going wrong here.

Subscribers is initialized in the init of the class

_subscribers = [NSMutableArray new];

and never thereafter set to nil.

Listeners get added to _subscribers through this method

- (void)subscribe:(id<OnlineStatusSubscriber>)subscriber {
    [_subscribers addObject:subscriber];
}

and every class that calls that method does use the OnlineStatusSubscriber interface and implement the onlineStatus:statusChangedTo: method.

I've added the Error Log to a pastebin: https://pastebin.com/Ux0XRm15

As well as the class in question: https://pastebin.com/NjGcaNxp (where the offending line is line 73)

Answer 1

Never mind. I somehow missed that I was unsubscribing from one of the onlineStatus:statusChangedTo: calls. It wasn't even multi-threading, just a direct path. So stupid.

Problem is fixed by iterating over [_subscribers copy] instead of _subscribers.