Open Radar

 
Summary:
AFNetworking 2, a very popular networking library for iOS and OS X, watches each of its NSURLSessionTasks' states, in order to activate the network activity indicator on iOS, among other reasons. It does so with Key-Value Observing, which makes perfect sense.

Here's the problem: When you watch `state`, there is an occasional crash when the state changes.

Related:

- Possibly rdar://problem/15513481 (I can't tell, because I can't read the bug.)
- https://devforums.apple.com/message/920269#920269
- https://github.com/AFNetworking/AFNetworking/issues/1477
- https://github.com/AFNetworking/AFNetworking/blob/5732fab409a85ca1f4dc55f013b4d5522d92eed2/AFNetworking/AFURLSessionManager.m#L351

Steps to Reproduce:
Use the following code:

static void * AFTaskStateChangedContext = &AFTaskStateChangedContext;
NSURLSessionTask *task;

[task addObserver:self forKeyPath:NSStringFromSelector(@selector(state)) options:(NSKeyValueObservingOptions)(NSKeyValueObservingOptionOld |NSKeyValueObservingOptionNew) context:AFTaskStateChangedContext];

Then, release it in a widely-used production app.
Then, receive crash reports.

Expected Results:
KVO should not generate a crash.

Actual Results:
KVO crashes.

Version:
iOS 7.0.4, 7.1.1, 7.1.2, 8.0

Notes:
Attached are four partial crash reports. Crashlytics won't give us full crash reports, or I'd send them up too.

Configuration:
iPhone 4S, 5, 5c, 5s; iPad 2, 3, mini, mini w/Retina display, Air

Attachments:
'four-crash-reports.txt' was successfully uploaded.