Open Radar

 
Summary:
If you instantiate many NSURLSessions with a background configuration while the app is running. Once backgrounded, when foregrounded again if another instance of NSURLSession with a background configuration is created, e.g.

NSURLSessionConfiguration *backgroundSessionConfiguration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:backgroundSessionIdentifier];
[NSURLSession sessionWithConfiguration:backgroundSessionConfiguration delegate:nil delegateQueue:nil]

it will hang for several seconds. Use of the NSURLSession after instantiation is not required for the problem to occur.

Steps to Reproduce:
1. Launch attached sample app on any device iOS 8 - 9. In -application:didFinishLaunchingWithOptions:, 100 instances of NSURLSession with a background configuration are created.
2. Lock the device.
3. Unlock the device, and immediately start pressing the button repeatedly. Notice that the main thread is blocked because on -applicationDidBecomeActive, the app tries to instantiate an NSURLSession with a background configuration, which takes several seconds to complete.

Expected Results:
The thread should not hang, and -sessionWithConfiguration: should return quickly. Its reasonable to expect that -sessionWithConfiguration: can be called on the main thread without hindering user experience.

Actual Results:
-sessionWithConfiguration: blocks the thread for several seconds, depending on the number of NSURLSessions instantiated while the app was previously in the foreground.

Version:
iOS 8 - iOS 9 Beta 5

Notes:
This does not occur with NSURLSessions that do not use a background session configuration.

Configuration:
Any iOS device

Attachments:
https://dl.dropboxusercontent.com/u/46435935/URLSessionLocking.zip