Open Radar

 
Summary:
I added state restoration to an app in the last update. I imediately started getting an incredibly high number of crash reports from all over the application. Each one crashed in a different thread, in different parts of the app.
 
Every report has two things in common. All of them are SIGSEGV errors with a SEGV_ACCERR error code. All of them reference the following calls in the main thread:

15  libsystem_c.dylib                    0x30b04eb9 exit + 10
16  UIKit                                0x257795e7 -[UIApplication _terminateWithStatus:] + 404
17  UIKit                                0x25964a83 -[UIApplication _updateSnapshotAndStateRestorationArchiveForBackgroundEvent:saveState:exitIfCouldNotRestoreState:] + 108

I did some digging and found that UIKit state restoration fails if the application is launched in the background to perform a background app refresh. The following messages are printed:

Error reading archived restorable state: Error Domain=NSCocoaErrorDomain Code=257 "The operation couldn’t be completed. (Cocoa error 257.)" UserInfo=0x174861dc0 {NSFilePath=/var/mobile/Containers/Data/Application/2AEAAA47-401A-4AD9-ACD4-157EB07E6A9F/Library/Saved Application State/com.north.watchville.savedState/data.data, NSUnderlyingError=0x17425ad90 "The operation couldn’t be completed. Operation not permitted"}

Application could not restore state when launched in background while locked, skipping snapshot/state save when asked to update snapshot in background, and terminating app instead
 
It seems as though the state restoration files are protected in a way that the rest of my application's data is not. The call to exit causes trouble on any background threads that are still running so I see lots of crash reports.
 
Interestingly enough, -[UIApplicationDelegate application:shouldSaveApplicationState:] and -[UIApplication application:shouldRestoreApplicationState:] are never called in this mode. So I never have an option to opt out of state restoration in the background and prevent this crash.

Additionally, while building the sample project for this report, I discovered that if I don't implement

func application(application: UIApplication, willFinishLaunchingWithOptions launchOptions: [NSObject : AnyObject]?) -> Bool {
    return true
}

the errors and crashes go away and state restoration works in the background as expected. If it is implemented, state restoration fails with the issues outlined above.

Steps to Reproduce:
In the attached sample application:

- Run the "Foreground" scheme on a device
- Close the application by pressing the home button so that UIKit has a chance to archive all application state
- Sop the application from Xcode
- Run the "Background" scheme on a device while the device is locked

If application:willFinishLaunchingWithOptions: (AppDelegate.swift line 25) is included in the build, you will see errors around state restoration. If it is not included, everything (including state restoration) works as expected.

Expected Results:
State restoration should work, or UIKit should not attempt to archive or restore state in cases where it could fail (background running on a locked device).

Actual Results:
State restoration fails which results in the application terminating itself, resulting in high crash volume.

It prints the following error messages:

Error reading archived restorable state: Error Domain=NSCocoaErrorDomain Code=257 "The operation couldn’t be completed. (Cocoa error 257.)" UserInfo=0x174861dc0 {NSFilePath=/var/mobile/Containers/Data/Application/2AEAAA47-401A-4AD9-ACD4-157EB07E6A9F/Library/Saved Application State/com.north.watchville.savedState/data.data, NSUnderlyingError=0x17425ad90 "The operation couldn’t be completed. Operation not permitted"}

Application could not restore state when launched in background while locked, skipping snapshot/state save when asked to update snapshot in background, and terminating app instead

Then UIApplication terminates itself.

Sample project: http://cl.ly/asp8